Building soil for better crops

Building Soils for Better CropsSecond Edition

Practical information for farmers, ranchers, educators, and gardeners —presented in an engaging, easy-to-read style.

Building Soils explains how ecological soil management boosts fertility and yieldswhile reducing pest pressures and environmental impacts. Ecological managementworks with the built-in strengths of your plant/soil system. 240 pages.

Soil samples taken from two treatments ofan experiment at the Rodale Research Cen-ter show the effect of conventional soil man-agement (left) compared to ecologicallybased management using cover crops andmanure (right).

“This is the most practical guide I have read about how whole soil systems work.Written in clear, illustrative language, it makes the world of soils accessible tofarmers and beginning students, as well as soil science professionals.”

—FRED KIRSCHENMANN

NORTH DAKOTA GRAIN AND LIVESTOCK FARMER

“Authors Magdoff and van Es have greatly expanded the scope of “Building Soils”while keeping the language straightforward and farmer-friendly. Readers willespecially like the new, improved diagrams. I would recommend this book to peoplewho enjoy getting their hands dirty and want to know how to build better soil.”

— RAY R. WEIL,PROFESSOR OF SOIL SCIENCE

AT UNIVERSITY OF MARYLAND AND CO-AUTHOR OF

THE NATURE AND PROPERTIES OF SOILS

A SAN publication, produced withfunding by the Sustainable AgricultureResearch and Education(SARE) program of CSREES,U.S. Department of Agriculture.

SECOND EDITION

HANDBOOKSERIESBOOK 4

Sustain

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Building Soilsfor Better CropsBuilding Soils

for Better Crops

Fred Magdoff and Harold van Es

Building So

ils for Better Cro

ps

Building So

ils for Better Cro

ps SEC

ON

D ED

ITION


SUSTAINABLEAGRICULTURE NETWORK

THE NATIONAL OUTREACH ARM OF SARE

SUSTAINABLEAGRICULTURE NETWORK

THE NATIONAL OUTREACH ARM OF SARE

Building Soils for Better Crops2ND EDITION


Copyright© 2000 by the Sustainable AgricultureNetwork (SAN), with funding from the SustainableAgriculture Research and Education (SARE) programof the CSREES, U.S. Department of Agriculture.

SAN, the national outreach partner of USDA’s SAREprogram, is a consortium of individuals, universitiesand government, business and nonprofit organizationsdedicated to the exchange of information on sustain-able agricultural systems.

For more information about the Sustainable Agricul-ture Network or its other publications seewww.sare.org or contact:

Andy ClarkSAN Coordinatorc/o AFSIC, Room 304National Agricultural Library10301 Baltimore Ave.Beltsville, MD 20705-2351(301) 504-6425; (301) 504-6409 (fax)[email protected]

SARE is a competitive grants program that providesfunding for research and education projects thatpromote agricultural systems that are profitable,environmentally sound and enhance the viability ofrural communities nationwide.

For more information about the SARE program andSARE grants contact:Office of Sustainable Agriculture ProgramsU.S. Department of Agriculture1400 Independence Ave., S.W., Stop 2223Washington, D.C. 20250-2223(301) 405-3186; (301) 314-7373 (fax)[email protected]

To order copies of this book, send a check or purchaseorder for $19.95 + $3.95 s/h to:

Sustainable Agriculture PublicationsHills Building, Room 10University of VermontBurlington VT 05405-0082

To order SAN publications by credit card (VISA, MC),please call (802) 656-0484.

For first book sent outside of North America, pleaseadd $6. Add $2.50 for each additional book. Pleaseinclude your mailing address and telephone number.

Previous titles in SAN’s handbook series include 1:Managing Cover Crops Profitably, 1st Edition,edited by thestaff of Rodale Institute; 2: Steel in the Field: A Farmer’sGuide to Weed Management Tools, 1997, edited by GregBowman; 3. Managing Cover Crops Profitably, 2nd Editionwritten by Greg Bowman, Craig Cramer and ChrisShirley and edited by Andy Clark.

Library of Congress Cataloging-in-Publication Data

Magdoff, Fred, 1942-Building soils for better crops / by Fred Magdoff and Harold van Es.--2nd ed.

p. cm. -- (Sustainable Agriculture Network handbook series ; bk. 4)Includes bibliographical references.ISBN 1-888626-05-4

1. Humus. 2. Soil management. I. Van Es, Harold, 1958- II.Sustainable Agriculture Network. III. Title. IV. Series.

S592.8 .M34 2000631.4’17--dc21

00-029695

Printed in the United States of America on recycledpaper

The U.S. Department of Agriculture (USDA) prohibitsdiscrimination in all its programs and activities on thebasis of race, color, national origin, gender, religion,age, disability, political beliefs, sexual orientation, andmarital or family status. (Not all prohibited basesapply to all programs). Persons with disabilities whorequire alternative means for communication ofprogram information (Braille, large print, audiotape,etc.) should contact USDA’s TARGET Center at 202-720-2600 (voice and TDD).

To file a complaint of discrimination, write USDA,Director, Office of Civil Rights, Room 326-W, WhittenBuilding, 14th and Independence Avenue, SW,Washington, DC 20250-9410 or call (202) 720-5964(voice or TDD). USDA is an equal opportunityprovider and employer.

Every effort has been made to make this book ascomplete and as accurate as possible. This text is onlya guide, however, and should be used in conjunctionwith other information sources on crop, soil and farmmanagement. The editors, authors and publisherdisclaim any liability, loss, or risk, personal orotherwise, which is incurred as a consequence,directly or indirectly, of the use and application of anyof the contents of this book.

Mention, visual representation or inferred reference ofa product, service, manufacturer or organization inthis publication does not imply endorsement by theUSDA, the SARE program or the authors. Exclusiondoes not imply a negative evaluation.

Graphic design, interior layout and cover design byAndrea Gray. Cover illustration by Frank Fretz. Someinterior illustrations by Bonnie Acker and ElayneSears. Copyediting by Tawna Mertz, Valerie Berton andAndy Clark. Indexing by Peggy Holloway. Printing byJarboe Printing, Washington, DC.

Contents

Preface v

Introduction vii

PART ONE

THE BASICS OF SOIL ORGANIC MATTER,PHYSICAL PROPERTIES, AND NUTRIENTS

1 Healthy Soils 3

2 What is Soil Organic Matter? 9

3 The Living Soil 13

4 Why is Organic Matter So Important? 21

5 Amount of Organic Matter in Soils 33

6 Let’s Get Physical: Soil Tilth, Aeration, and Water 41

7 Nutrient Cycles and Flows 55

PART TWO

ECOLOGICAL SOIL & CROP MANAGEMENT

8 Managing for High Quality Soils 63

9 Animal Manures 77

10 Cover Crops 87

11 Crop Rotations 99

12 Making and Using Composts 109

13 Reducing Soil Erosion 119

14 Preventing and Lessening Compaction 125

15 Reducing Tillage 135

16 Nutrient Management: An Introduction 147

17 Management of Nitrogen and Phosphorus 157

18 Other Fertility Issues: Nutrients, CEC, Acidity and Alkalinity 167

19 Getting the Most from Soil Tests 177

PART THREE

PUTTING IT ALL TOGETHER

20 How Good are Your Soils? On-Farm Soil Health Evaluation 203

21 Putting it All Together 209

Glossary 215

Resources 221

Index 225

v

�Preface

To use the land without abusing it.

— J. OTIS HUMPHRY, EARLY 1900S

We have written this book with farmers,extension agents, students, and garden-

ers in mind. Building Soils for Better Crops is apractical guide to ecological soil managementthat provides background informationas well as details of soil-improvingpractices. This book is meant to givethe reader an appreciation of the im-portance of soil health and to suggestecologically sound practices that helpto develop and maintain healthy soils.

The first edition of Building Soils for BetterCrops focused exclusively on soil organic mat-ter management. If you follow practices thatbuild and maintain good levels of soil organicmatter, you will find it easier to grow healthyand high-yielding crops. Plants can withstanddroughty conditions better and won’t be as both-ered by insects and diseases. By maintainingadequate levels of organic matter in soil, there

is less reason to use as much commercial fertil-izer and lime as many farmers now purchase.Soil organic matter is that important!

Although organic matter management is theheart of the second edition, we decidedto write a more comprehensive guidethat includes the other essential aspectsof building healthy soils. This editioncontains four chapters, two new andtwo completely rewritten, on manag-ing soil physical properties. We also in-

cluded four new chapters on nutrient manage-ment and one on evaluating soil health. In ad-dition, farmer profiles describe a number of keypractices that enhance the health of their soils.

A book like this one cannot give exact answersto problems on specific farms. There are just toomany differences from one field to another, andone farm to another, to warrant blanket recom-mendations. To make specific suggestions, it is

vi BUILDING SOILS FOR BETTER CROPS

necessary to know the details of the soil, crop, cli-mate, machinery, human considerations, and othervariable factors. Good soil management is betterachieved through education and understandingthan with blanket recommendations.

Over many centuries, people have struggledwith the same issues we struggle with today. Wequote some of these persons in epigraphs at thebeginning of each chapter in appreciation forthose who have come before. Vermont Agricul-tural Experiment Station Bulletin No. 35, pub-lished in 1908, is especially fascinating. It con-tains an article by three scientists about the im-portance of soil organic matter that is strikinglymodern in many ways. Another example frommore than a half century ago: The message ofEdward Faulkner’s Plowman’s Folly, that reducedtillage and increased use of organic residues areessential to improving soil, is as valid today asin 1943 when it was first published. The sayingis right — what goes around comes around.Sources cited at the end of chapters are thosewe referred to during writing. They are not acomprehensive list of references on the subject.

Many people reviewed individual chapters orthe entire manuscript at one stage or anotherand made very useful suggestions. We wouldlike to thank: Jim Bauder, Douglas Beegle, KeithCassel, Andy Clark, Steve Diver, John Doran,Tim Griffin, Vern Grubinger, Wendy Sue Harper,John Hall, John Hart, Bill Jokela, Keith Kelling,Fred Kirschenmann, Shane LaBrake, Bill Lieb-hardt, Birl Lowery, Charles Mitchell, Paul Mugge,Cass Peterson, George Rehm, Joel Rissman, EricSideman, Ev Thomas, Michelle Wander, and RayWeil. Special thanks to Valerie Berton, SAREcommunications specialist, who wrote the farmprofiles, copyedited the manuscript and over-saw production. Any mistakes are, of course,ours alone.

— Fred MagdoffDepartment of Plant & Soil Science

University of Vermont

& Harold van EsDepartment of Crop & Soil Sciences

Cornell University

May 2000

vii

One of our truly modern miracles is our ag-ricultural system, which produces abun-

dant, affordable food. High yields come fromthe use of improved crop varieties, fertilizers,pest control products, and irrigation.At the same time, mechanization andthe ever-increasing capacity of fieldequipment allows farmers to work in-creasing acreage.

Despite the high productivity peracre and per person, many farmers, agriculturalscientists, and extension specialists see severeproblems associated with our intensive agricul-tural production systems. Examples abound:

� Too much nitrogen fertilizer or animalmanure sometimes causes high nitrateconcentrations in groundwater. Theseconcentrations can become high enough topose a human health hazard.

�Introduction

Used to be anybody could farm. All you needed was

a strong back. . . but nowadays you need a good

education to understand all the advice you get so

you can pick out what’ll do you the least harm.

—VERMONT SAYING, MID-1900S

� Phosphate in runoff water enters waterbodies and degrades their waters by stimu-lating algae growth.

� Antibiotics used to fight diseases in farmanimals can enter the food chainand may be found in the meat weeat. Their overuse has resulted inoutbreaks of human illness fromstrains of disease-causing bacteriathat have become resistant to

antibiotics.� Erosion associated with conventional tillage

and lack of good rotations degrades ourprecious soil and, at the same time, causesthe silting up of reservoirs, ponds, andlakes.

The food we eat and our surface and groundwaters are sometimes contaminated with disease-causing organisms and chemicals used in agri-

viii BUILDING SOILS FOR BETTER CROPS

culture. Pesticides used to control insects andplant diseases can be found in foods, animalfeeds, groundwater, and in surface water run-ning off agricultural fields. Farmers and farmworkers are at special risk. Studies have shownhigher cancer rates among those who work withor near certain pesticides. The general public isincreasingly demanding safe, high quality foodthat is produced without excessive damage tothe environment — and many are willing to paya premium to obtain it.

Farmers are also in a perpetual struggle tomaintain a decent standard of living. As con-solidations and other changes occur in the agri-culture input, food processing, and marketingsectors, the farmer’s bargaining position weak-ens. The high cost of purchased inputs and thelow prices of many agricultural commodities,such as wheat, corn, cotton, and milk, havecaught farmers in a cost-price squeeze thatmakes it hard to run a profitable farm.

Given these problems, you might wonder ifwe should continue to farm in the same way. Amajor effort is underway to develop and imple-ment practices that are both more environmen-tally sound than conventional practices and atthe same time more economically rewarding forfarmers. As farmers use management skills andbetter knowledge to work more closely with thebiological world, they frequently find that thereare ways to decrease use of products purchasedoff the farm.

With the new emphasis on sustainable agri-culture comes a reawakening of interest in soilhealth. Early scientists, farmers, and gardenerswere well aware of the importance of soil qual-ity and organic matter to the productivity of soil.The significance of soil organic matter, includ-ing living organisms in the soil, was understood

by scientists at least as far back as the 17th cen-tury. John Evelyn, writing in England during the1670s, described the importance of topsoil andexplained that the productivity of soils tendedto be lost with time. He noted that their fertilitycould be maintained by adding organic residues.Charles Darwin, the great natural scientist of the19th century who developed the modern theoryof evolution, studied and wrote about the im-portance of earthworms to the cycling of nutri-ents and the general fertility of the soil.

. . . organic matter was

“once extolled as the essential

soil ingredient, the bright particular

star in the firmament of the

plant grower . . .”

Around the turn of the 20th century, therewas again an appreciation of the importance ofsoil health. Scientists had realized that “wornout” soils, where productivity had drastically de-clined, resulted mainly from the depletion ofsoil organic matter. At the same time, they couldsee a transformation coming: although organicmatter was “once extolled as the essential soilingredient, the bright particular star in the fir-mament of the plant grower, it fell like Lucifer”under the weight of “modern” agricultural ideas(Hills, Jones, and Cutler, 1908). With the avail-ability of inexpensive fertilizers and larger farmequipment after World War II, and of cheapwater for irrigation in some parts of the westernUnited States, many people working with soilsforgot or ignored the importance of organicmatter in promoting high quality soils.

INTRODUCTION ix

As farmers and scientists placed less empha-sis on soil organic matter during the last half ofthe 20th century, farm machinery was gettinglarger. More horse power for tractors allowedmore land to be worked by fewer people. Large4-wheel drive tractors allowed farmers to dofield work when the soil was wet, creating se-vere compaction and sometimes leaving the soilin a cloddy condition, requiring more harrow-ing than otherwise would be needed. The useof the moldboard plow, followed by harrowing,broke down soil structure and left no residueson the surface. Soils were left bare and very sus-ceptible to wind and water erosion. New har-vesting machinery was developed, replacinghand-harvesting of crops. As dairy herd size in-creased, farmers needed bigger spreaders tohandle the manure. The use of larger equipmentcreated new problems. Making many passesthrough the field with heavy equipment forspreading fertilizer and manure, preparing aseedbed, planting, spraying pesticides, and har-vesting created the potential for significantamounts of soil compaction.

What many people think are

individual problems…may just be

symptoms of a degraded,

poor quality soil.

A new logic developed that most soil-relatedproblems could be dealt with by increasing ex-ternal inputs. This is a reactive way of dealingwith soil issues — you react after seeing a “prob-lem” in the field. If a soil is deficient in somenutrient, you buy a fertilizer and spread it on

the soil. If a soil doesn’t store enough rainfall,all you need is irrigation. If a soil becomes toocompacted and water or roots can’t easily pen-etrate, you use an implement, such as a subsoiler,to tear it open. If a plant disease or insect infes-tation occurs, you apply a pesticide.

Are low nutrient status, poor water-holdingcapacity, soil compaction, susceptibility to ero-sion, and disease, nematode, or insect damagereally individual and unrelated problems? Per-haps they are better viewed as only symptomsof a deeper, underlying problem. The ability totell the difference between what is the underly-ing problem and what is only a symptom of aproblem is essential to deciding on the bestcourse of action. For example, if you are hittingyour head against a wall and you get a head-ache — is the problem the headache, and aspi-rin the best remedy? Clearly, the real problem isyour behavior and not the headache, and thebest solution is to stop banging your head onthe wall!

What many people think are individual prob-lems may just be symptoms of a degraded, poorquality soil. These symptoms are usually directlyrelated to depletion of soil organic matter, lackof a thriving and diverse population of soil or-ganisms, and compaction caused by use of heavyfield equipment. Farmers have been encouragedto react to individual symptoms instead of fo-cusing their attention on general soil healthmanagement. A new approach is needed to helpdevelop farming practices that take advantageof the inherent strengths of natural systems. Inthis way, we can prevent the many symptoms ofunhealthy soils from developing instead of re-acting after they develop. If we are to work to-gether with nature, instead of attempting to over-whelm and dominate it, the buildup and main-

x BUILDING SOILS FOR BETTER CROPS

tenance of good levels of organic matter in oursoils is as critical as management of physicalconditions, pH and nutrient levels.

This book has three parts. Part One providesbackground information about soil health andorganic matter: what it is, why it is so importantto general soil health and why some soils are ofhigher quality than others. Also included are dis-cussions of soil physical properties, soil waterstorage, and nutrient cycles and flows. Part Twodeals with practices that promote building bet-ter soils — with a lot of emphasis on promotingorganic matter buildup and maintenance. Fol-lowing practices that build and maintain organicmatter may be the key to soil fertility and mayhelp solve many problems. However, other soil-management practices also are needed to supple-ment soil organic matter management. Practices

for enhancing soil quality include the use of ani-mal manures and cover crops; good residuemanagement; appropriate selection of rotationcrops; use of composts; reduced tillage; mini-mizing soil compaction and enhancing aeration;better nutrient and amendment management;and adapting specific conservation practices forerosion control. Part Three deals with how tocombine soil-building management strategiesthat actually work on the farm and how to evalu-ate soil to tell if its health is improving.

SOURCE

Hills, J.L., C.H. Jones, and C. Cutler. 1908. Soildeterioration and soil humus. pp. 142–177.In Vermont Agricultural Experiment Station Bul-letin 135. College of Agriculture, Universityof Vermont. Burlington, Vermont.

THE LIVING SOIL 1

P A R T O N E

The Basics ofSoil Organic Matter,Physical Properties,

and Nutrients

3

1�

Healthy Soils

All over the country [some soils are]

worn out, depleted, exhausted, almost dead.

But here is comfort: These soils possess possibilities

and may be restored to high productive power,

provided you do a few simple things.

—C.W. BURKETT, 1907

It should come as no surprise that many cul-tures have considered soil central to their lives.

After all, people were aware that the food theyate grew from the soil. Our ancestors who firstpracticed agriculture must have beenamazed to see life reborn each yearwhen seeds placed in the ground ger-minated and then grew to maturity. Inthe Hebrew bible, the name given to thefirst man (Adam) is the masculine ver-sion of the word earth or soil (adama).The name for the first woman (Eve, or Hava inHebrew) comes from the word for living. Soil andhuman life were considered to be intertwined. Aparticular reverence for the soil has been an im-portant part of the cultures of many other civili-zations, including American Indian tribes.

Although we focus on the critical role soilsplay in growing crops, it’s important to keep inmind that soils also serve other important pur-

poses. Soils govern what percent of the rainfallruns off the field, as compared to the percentthat enters the soil and eventually helps rechargeunderground aquifers. When a soil is denuded

of vegetation and it starts to degrade,excessive runoff and flooding are morecommon. Soils also absorb, release, andtransform many different chemicalcompounds. For example, they help topurify wastes flowing from the septicsystem drain in your back yard. Soils

also provide habitats for a diverse group of or-ganisms, some of which are very important —such as with those bacteria that produce antibi-otics. Soil organic matter stores a huge amountof atmospheric carbon. Carbon, in the form ofcarbon dioxide, is a greenhouse gas associatedwith global warming. We also build roads andbuildings on soils; some are definitely better thanothers for this purpose.

4 BUILDING SOILS FOR BETTER CROPS

WHAT KIND OF SOILDO YOU WANT?

Farmers sometimes use the term soil health todescribe the condition of the soil. Scientists usu-ally use the term soil quality, but both refer tothe same idea — how good is the soil in its roleof supporting the growth of high yielding,healthy crops?

How would you know a high quality soil froma lower quality soil? Most farmers or gardenerswould say that they know one when they seeone. Farmers can certainly tell you which of thesoils on their farms are of low, medium, or highquality. They know high quality soil because itgenerates higher yields with less effort. Less rain-water runs off and fewer visible signs of erosionare seen on the better quality soils. Less poweris needed to operate machinery on a healthy soilthan on poorer, compacted soils. Soil scientistsare working together with farmers and agricul-tural extension personnel to try to come up witha widely accepted definition of soil health andto determine what factors (pH, bulk density,aggregate stability, etc.) need to be measured toestimate a soil’s quality.

The first thing many might think of is thatthe soil should have a sufficient supply of nutri-ents throughout the growing season. But don’tforget, at the end of the season there shouldn’tbe too much nitrogen and phosphorus left inhighly soluble forms or enriching the soil’s sur-face. Leaching and runoff of nutrients are mostlikely to occur after crops are harvested and be-fore the following year’s crops are well established.

We also want the soil to have good tilth sothat plant roots can fully develop with the leastamount of effort. A soil with good tilth is morespongy and less compact than a soil with poortilth. A soil that has a favorable and stable soil

For soil thou art…

—BOOK OF GENESIS

structure also promotes rainfall infiltration andwater storage for plants to use later. For good rootgrowth and drainage, we also want a soil withsufficient depth before there’s a restricting layer.

We want a soil to be well drained, so it driesenough to permit timely field operations. Also,it’s essential that oxygen is able to reach the rootzone to promote optimal root health — and thathappens best in a soil without a drainage prob-lem. (Keep in mind that these general charac-teristics do not hold for all crops. For example,flooded soils are important for paddy rice pro-duction.)

We want the soil to have low populations ofplant disease and parasitic organisms so plantsgrow better. Certainly, there should also be alow weed pressure, especially of aggressive andhard-to-control weeds. Most soil organisms arebeneficial and we certainly want high amountsof organisms that help plant growth, such asearthworms and many bacteria and fungi.

A high quality soil is free of chemicals thatmight harm the plant. These can occur natu-rally, such as soluble aluminum in very acid soilsor excess salts in arid region soils. Potentiallyharmful chemicals also are introduced by hu-man activity, such as fuel oil spills or applica-tion of sewage sludge with high concentrationsof toxic elements.

A high quality soil should resist being de-graded. It also should be resilient, recoveringquickly after unfavorable changes like compac-tion.

HEALTHY SOILS 5

THE NATURE AND NURTUREOF SOILS

Some soils are exceptionally good for growingcrops and others are inherently unsuitable; mostare in between. Many soils also have limitations,such as low organic matter content, texture ex-tremes (coarse sand or heavy clay), poor drain-age, and layers that restrict root growth. Iowa’sloess-derived prairie soils are naturally blessedwith a combination of silt loam texture and highorganic matter contents. By every standard forassessing soil health, these soils — in their vir-gin state — would rate very high. We can com-pare them with a person who is naturally veryhealthy and has great athletic abilities. Many ofus are not quite so lucky and Nature has givenus qualities that may never make us great base-ball players, swimmers, or marathon runners,even if we tried very hard.

ductive if they are managed well. However, theywill probably never reach parity, because somelimitations simply cannot be completely over-come. The key idea, however, is the same thatwe wish for our children — we want our soilsto reach their fullest potential.

HOW DO YOUBUILD A HEALTHY,

HIGH QUALITY SOIL?

Some characteristics of healthy soils are relativelyeasy to achieve — for example, an applicationof limestone will make a soil less acid and in-crease availability of many nutrients to plants.But what if the soil is only a few inches deep?There is little that can be done within economicreason, except on a very small garden-size plot.If the soil is poorly drained because of a restrict-ing subsoil layer of clay, tile drainage can be in-stalled, but at a significant cost.

We use the term building soils to emphasizethat the nurturing process of converting a de-graded or low quality soil into a truly high qual-ity one requires understanding, thought, andsignificant actions. This is also true for main-taining or improving already healthy soils. Soilorganic matter influences almost all of the char-acteristics we’ve just discussed. For soil tilth,organic matter is one of the main influences.Organic matter is even critical for managingpests — and good management of this resourceshould be the starting point for a pest manage-ment program on every farm. Good organicmatter management is, therefore, the founda-tion for high quality, healthy soils. Practices thatpromote good soil organic matter managementare, thus, the very foundation for a more sus-tainable and thriving agriculture. It is for this

Good soil organic matter

management is … the very

foundation for a more sustainable

and thriving agriculture.

The way we care for, or nurture, a soil modi-fies its inherent nature. A good soil can beabused through years of poor management andturn into one with poor health, although it gen-erally takes a lot of mistreatment to reach thatpoint. On the other hand, an innately challeng-ing soil may be very “unforgiving” of poor man-agement and quickly become even worse. Forexample, a heavy clay loam soil can be easilycompacted and turn into a dense mass. Boththe naturally good and poor soils can be pro-


reason that so much space is devoted to organicmatter in this book. However, we cannot forgetother critical aspects of management — such astrying to lessen compaction by heavy field equip-ment and good nutrient management.

Although the details of how best to createhigh quality soils differ from farm to farm andeven field to field, the general approaches arethe same:

� Use a number of practices that add organicmaterials to the soil.

� Use diverse sources of organic materials.� Reduce unneeded losses of native soil organic

matter.� Use practices that leave the soil surface pro-

tected from raindrops and temperature ex-tremes.

� Whenever traveling on the soil with fieldequipment, use practices that help developand maintain good soil structure.

� Manage soil fertility status to maintain opti-mal pH levels for your crops and a sufficient

supply of nutrients for plants without result-ing in water pollution.

� In arid regions, a combination of gypsum andleaching may be needed to reduce the amountof sodium or salt in the soil.

HOW DO SOILSBECOME DEGRADED?

Although we want to emphasize healthy, highquality soils, it is also crucial to recognize thatmany soils in the U.S. and around the worldhave become degraded — what many used tocall “worn out” soils. Degradation most com-monly occurs when erosion and decreased soilorganic matter levels initiate a downward spiral(figure 1.1). Soils become compact, making ithard for water to infiltrate and roots to developproperly. Erosion continues and nutrients de-cline to levels too low for good crop growth.The development of saline (too salty) soils un-der irrigation in arid regions is another cause of

Figure 1.1 The downward spiral of soil degradation. Modified from Topp et al., 1995.

Intensivetillage, soil

erosion andinsufficient

addedresidues

Soil organic matterdecreases

Surfacebecomes

compacted,crust forms More soil

organicmatteris lost Crop yields

decline

Aggregatesbreak down

Erosion by windand waterincreases

Less soil water storage,less diversity of soilorganism, fewernutrients for plants

HEALTHY SOILS 7

reduced soil health. (Salts added in the irriga-tion water need to be leached beneath the rootzone to avoid the problem.)

Historically, soil degradation has caused sig-nificant harm to many early civilizations, includ-ing the drastic loss of productivity resulting fromsoil erosion in Greece and many locations in theMiddle East (such as Israel, Jordan, and Leba-non). This led to either colonial ventures to helpfeed the citizenry or to the decline of the earlycultures.

Tropical rainforest conditions (high tempera-ture and rainfall, with most of the organic mat-ter near the soil surface) may cause significantsoil degradation within two or three years of con-version to cropland. This is the reason that the

Evaluating Your Soils

Score cards have been developed to helpfarmers assess their soils. They use a simplescale to rate the health of soils. You evaluatethe presence of earthworms, severity of ero-sion, ease of tillage, soil structure, color, de-gree of compaction, water infiltration rate, anddrainage status. Then you rate crops growingon the soils by such characteristics as theirgeneral appearance, growth rates, root health,degree of resistance to drought, and yield. It’sa good idea for every farmer to fill out such ascore card for every major field or soil typeon their farm every few years. But even with-out doing so, you probably already knowwhat a really high quality and healthy soilwould be like — one that would consistentlyproduce good yields of high quality cropswith minimal negative environmental impact.More on evaluating soil health in Chapter 20.

“slash and burn” system, with people moving toa new patch of forest every few years, developedin the tropics. After farmers depleted the soilsin a field, they would cut down and burn thetrees in the new patch, allowing the forest andsoil to regenerate in previously cropped areas.

The westward push of U.S. agriculture wasstimulated by rapid soil degradation in the East,originally a zone of temperate forest. Under theconditions of the humid portion of the GreatPlains (moderate rainfall and temperature, withorganic matter distributed deeper in the soil), ittook many decades for the effects of soil degra-dation to become evident.

SOURCES

Doran, J.W., M. Sarrantonio, and M.A. Liebig.1996. Soil heath and sustainability. pp. 1–54In Advances in Agronomy Vol. 56. AcademicPress, Inc. San Diego, CA.

Hillel, D. 1991. Out of the Earth: Civilization andthe Life of the Soil. University of CaliforniaPress. Berkeley, CA.

Spillman, W. J. 1906. Renovation of Worn-outSoils. Farmers’ Bulletin No. 245. USDA, Gov-ernment Printing Office. Washington, D.C.

Topp, G.C., K.C. Wires, D.A. Angers, M.R. Car-ter, J.L.B. Culley, D.A. Holmstrom, B.D. Kay,G.P. Lafond, D.R. Langille, R.A. McBride, G.T.Patterson, E. Perfect, V. Rasiah, A.V. Rodd,K.T. Webb. 1995. Changes in soil structure.Chapter 6. In The Health of Our Soils: TowardSustainable Agriculture in Canada (Acton, D.F.and L.J. Gregorich (eds.). Centre for Land andBiological Resources Research. ResearchBranch, Agriculture and Agri-Food Canada.Publication 1906/E. http://res.agr.ca/CANSIS/PUBLICATIONS/HEALTH/_overview.html

9

2�

What is Soil Organic Matter?

Follow the appropriateness of the season,

consider well the nature and conditions of the soil,

then and only then least labor will bring best success.

Rely on one’s own idea and not on the orders of nature,

then every effort will be futile.

—JIA SI XIE, 6TH CENTURY, CHINA

Soil consists of four important parts: mineral solids, water, air, and organic matter. Min-

eral solids are sand, silt, and clay. Sand has thelargest particle size; clay has the smallest. Theminerals mainly consist of silicon, oxy-gen, aluminum, potassium, calcium,and magnesium. The soil water, alsocalled the soil solution, contains dis-solved nutrients and is the main sourceof water for plants. Essential nutrientsare made available to the roots of plants throughthe soil solution. The air in the soil, which is incontact with the air above ground, provides rootswith oxygen and helps remove excess carbondioxide from respiring root cells. The clumpingtogether of mineral and organic particles to formaggregates of various sizes is a very importantproperty of soils. Compared to poorly aggre-gated soils, those with good aggregation usu-ally have better tilth and contain more spaces,

or pores, for storing water and allowing gas ex-change.

Organic matter has an overwhelming effecton almost all soil properties, although it is gener-

ally present in relatively small amounts.A typical agricultural soil has 1 to 6percent organic matter. It consists ofthree distinctly different parts — liv-ing organisms, fresh residues, andwell-decomposed residues. These

three parts of soil organic matter have been de-scribed as the living, the dead, and the very dead.This three-way classification may seem simpleand unscientific, but it is very useful.

The living part of soil organic matter includesa wide variety of microorganisms, such as bac-teria, viruses, fungi, protozoa, and algae. It evenincludes plant roots and the insects, earthworms,and larger animals, such as moles, woodchucks,and rabbits, that spend some of their time in


Soil Organic Matter—

the living

the dead

the very dead

the soil. The living portion represents about 15percent of the total soil organic matter. Micro-organisms, earthworms, and insects help breakdown crop residues and manures and, as theyuse the energy of these materials, mix them withthe minerals in the soil. In the process, they re-cycle plant nutrients. Sticky substances on theskin of earthworms and those produced by fungihelp bind particles together. This helps to sta-bilize the soil aggregates, clumps of particles thatmake up good soil structure. Organisms suchas earthworms and some fungi also help to sta-bilize the soil’s structure (for example, by pro-ducing channels that allow water to infiltrate)and, thereby, improve soil water status and aera-tion. A good soil structure increases water fil-tering into the soil and decreases erosion. Plantroots also interact in significant ways with thevarious microorganisms and animals living in

the soil. Another important aspect of soil organ-isms is that they are in a constant struggle witheach other (figure 2.1). Further discussion ofthe interactions between soil organisms androots, and among the various soil organisms, isprovided in chapter 3.

The fresh residues, or “dead” organic matter,consist of recently deceased microorganisms,insects, earthworms, old plant roots, crop resi-dues, and recently added manures. In somecases, just looking at them is enough to identifythe origin of the fresh residues (figure 2.2). Thispart of soil organic matter is the active, or easilydecomposed, fraction. This active fraction of soilorganic matter is the main supply of food forvarious organisms living in the soil — microor-ganisms, insects, and earthworms. As organicmaterials decompose, they release many of thenutrients needed by plants. Organic chemicalcompounds produced during the decompositionof fresh residues also help to bind soil particlestogether and give the soil a good structure.

Organic molecules directly released from cellsof fresh residues, such as proteins, amino acids,sugars, and starches, are also considered part ofthis fresh organic matter. These molecules gener-ally do not last long in the soil because so manymicroorganisms use them as food.

The well-decomposed organic material insoil, the “very dead,” is called humus. Humus isa term sometimes used to describe all soil or-ganic matter. Some use it to describe just thepart you can’t see without a microscope. We’lluse the term to refer only to the well-decom-posed part of soil organic matter. The alreadywell-decomposed humus is not a food for or-ganisms, but its very small size and chemicalproperties make it an important part of the soil.Humus holds on to some essential nutrients,storing them for slow release to plants. Humus

Figure 2.1 A nematode feeds on a fungus, part of aliving system of checks and balances. Photo byHarold Jensen.

WHAT IS SOIL ORGANIC MATTER? 11

Figure 2.2 Partially decomposed fresh residues (the“dead”) removed from soil. Fragments of stems, roots,fungal hyphae, are all readily used by soil organisms.

also can surround certain potentially harmfulchemicals and prevent them from causing dam-age to plants. Good amounts of soil humus canboth lessen drainage or compaction problemsthat occur in clay soils and improve water re-tention in sandy soils.

Organic matter decomposition is a processthat is similar to the burning of wood in a stove.When burning wood reaches a certain tempera-ture, the carbon in the wood combines withoxygen from the air and forms carbon dioxide.As this occurs, the energy stored in the carbon-containing chemicals in the wood is released asheat in a process called oxidation. The biologi-cal world, including humans, animals, and mi-croorganisms, also makes use of energy insidecarbon-containing molecules. This process ofconverting sugars, starches, and other com-pounds into a directly usable form of energy isalso a type of oxidation. We usually call it respi-ration. Oxygen is used and carbon dioxide andheat are given off in this process.

A multitude of microorganisms, earthworms,and insects get their energy and nutrients bybreaking down organic residues in soils. At the

same time, much of the energy stored in residuesis used by organisms to make new chemicals aswell as new cells. How does energy get storedinside organic residues in the first place? Greenplants use the energy of sunlight to link carbonatoms together into larger molecules. This pro-cess, known as photosynthesis, is used by plantsto store energy for respiration and growth.

Soil carbon is sometimes used as a synonymfor organic matter. Because carbon is the mainbuilding block of all organic molecules, theamount in a soil is very strongly related to the

Soil organic carbon is another way

of referring to organic matter.

total amount of all the organic matter — theliving organisms plus fresh residues plus welldecomposed residues. However, under semiaridconditions, it is common to also have anotherform of carbon in soils — limestone either asround concretions or dispersed evenly through-out the soil. Lime is calcium carbonate, whichcontains calcium, carbon, and oxygen. This isan inorganic carbon form. Even in humid cli-mates, when limestone is found very close tothe surface, some may be present in the soil.So, when people talk about soil carbon insteadof organic matter, they are usually referring toorganic carbon. The amount of organic matterin soils is about twice the organic carbon level.

SOURCE

Brady, N.C., and R.R. Weil. 1999. The Natureand Properties of Soils. 12th ed. MacmillanPublishing Co. New York, NY.

13

3�

The Living Soil

The plow is one of the most ancient and most valuable

of man’s inventions; but long before he existed the

land was in fact regularly ploughed, and continues to

be thus ploughed by earthworms.

—CHARLES DARWIN, 1881

W hen soil organisms and roots go abouttheir normal functions of getting energy

for growth from organic molecules they “respire”— using oxygen and releasing carbon dioxideto the atmosphere. (Of course, as wetake our essential breaths of air, we dothe same.) An entire field can beviewed as breathing as if it is one largeorganism. The soil is like an organismin another way too — a field also mayget “sick” in the sense that it becomesincapable of supporting healthy plants.

The organisms living in the soil, both largeand small, play a significant role in maintaininga healthy soil system and healthy plants. One ofthe main reasons we are interested in these or-ganisms is because of their role in breaking downorganic residues and incorporating them intothe soil. Soil organisms influence every aspectof decomposition and nutrient availability. As

organic materials are decomposed, nutrientsbecome available to plants, humus is produced,soil aggregates are formed, channels are createdfor water infiltration and better aeration, and

those residues originally on the sur-face are brought deeper into the soil.

We classify soil organisms in sev-eral different ways. Each organism canbe discussed separately or all organ-isms that do the same types of thingscan be discussed as a group. We also

can look at soil organisms according to their rolein the decomposition of organic materials. Forexample, organisms that use fresh residues astheir source of food are called primary (1°), orfirst-level, consumers of organic materials (seefigure 3.1). Many of these primary consumersbreak down large pieces of residues into smallerfragments. Secondary (2°) consumers are organ-isms that feed on the primary consumers them-


selves or their waste products. Tertiary (3°) con-sumers then feed on the secondary consumers.Another way to treat organisms is by generalsize, such as very small, small, medium, large,and very large. This is how we will discuss soilorganisms in this chapter.

There is constant interaction among the or-ganisms living in the soil. Some organisms helpother organisms, as when bacteria that live in-side the earthworm’s digestive system help de-compose organic matter. Although there aremany examples of such mutually beneficial sym-biotic relationships, an intense competition oc-curs among most of the diverse organisms in

healthy soils. Organisms may directly competewith each other for the same food. Some organ-isms naturally feed on others — nematodes mayfeed on fungi, bacteria, or other nematodes, andsome fungi trap and kill nematodes.

Some soil organisms can harm plants eitherby causing disease or by being parasites. In otherwords, there are “good” as well as “bad” bacte-ria, fungi, nematodes, and insects. One of thegoals of agricultural production systems shouldbe to create conditions that enhance the growthof beneficial organisms, which are the vast ma-jority, while decreasing populations of those fewthat are potentially harmful.

Figure 3.1 Soil organisms and their role in decomposing residues. Modified from D.L.Dindal, 1978.

2°–3°

energy flows in direction of arrows

ground beetles8–20 mm

pseudo-scorpian1–2 mm

springtails.5–3 mm

actinomycetes

centipedes50 mm

predatory mite.5–1 mm

mold mitebeetle mites

1mm

bacteria

rove beetles10mm

feather-wingedbeetle

1–2mm

nematodes

ant5–10 mm

flatworms70–150 mm

protozoa. .01–.5 mm

rotifera.1–.5 mm

beetlemites 1mm

whiteworms

10–25 mm

fly1–2 mm

sowbug10 mm

millipedes20–80 mm

earthworms 50–150 mm

nematodes1mm

land slugs & snails2–25 mm

organic residues

lengths of organisms given in millimeters (25 mm = 1 in)

1° = first level consumers2° = second level consumers3° = third level consumers

2° 2°

2°

2°

1°1°

1°

1°

fungi

THE LIVING SOIL 15

SOIL MICROORGANISMS

Microorganisms are very small forms of life thatcan sometimes live as single cells, although manyalso form colonies of cells. A microscope is usu-ally needed to see individual cells of these or-ganisms. Many more microorganisms exist intopsoil, where food sources are plentiful, thanin subsoil. They are especially abundant imme-diately next to plant roots, where sloughed offcells and chemicals released by roots provideready food sources. These organisms are impor-tant primary decomposers of organic matter, butthey do other things, such as providing nitro-gen through fixation to help growing plants. Soilmicroorganisms have had another direct impor-tance for humans — they are the origin of mostof the antibiotic medicines we use to fight vari-ous diseases.

Bacteria

Bacteria live in almost any habitat. They arefound inside the digestive system of animals, inthe ocean and fresh water, in compost piles (evenat temperatures over 130°F), and in soils. Theyare very plentiful in soils; a single teaspoon oftopsoil may contain more than 50 million bac-teria. Although some kinds of bacteria live inflooded soils without oxygen, most require well-aerated soils. In general, bacteria tend to dobetter in neutral soils than in acid soils.

In addition to being among the first organ-isms to begin decomposing residues in the soil,bacteria benefit plants by increasing nutrientavailability. For example, many bacteria dissolvephosphorus, making it more available for plantsto use.

Bacteria are also very helpful in providingnitrogen to plants. Although nitrogen is needed

in large amounts by plants, it is often deficientin agricultural soils. You may wonder how soilscan be deficient in nitrogen when we are sur-rounded by it — 78 percent of the air we breatheis composed of nitrogen gas. Yet plants as wellas animals face the dilemma of the Ancient Mari-ner, who was adrift at sea without fresh water:“Water, water, everywhere nor any drop todrink.” Unfortunately, neither animals nor plantscan use nitrogen gas (N2) for their nutrition.However, some types of bacteria are able to takenitrogen gas from the atmosphere and convertit into a form that plants can use to make aminoacids and proteins. This conversion process isknown as nitrogen fixation.

The microscopic flora and fauna

in our soils give soil its fertility,

otherwise it is just dirt.

—JOHN MALCOM, VERMONT DAIRY FARMER

Some nitrogen-fixing bacteria form mutuallybeneficial associations with plants. One suchsymbiotic relationship that is very important toagriculture is the nitrogen-fixing rhizobia groupof bacteria that live inside nodules formed onthe roots of legumes. These bacteria providenitrogen in a form that leguminous plants canuse, while the legume provides the bacteria withsugars for energy.

People eat some legumes or their products,such as peas, dry beans, and tofu made fromsoybeans. Soybeans, alfalfa, and clover are usedfor animal feed. Clovers and hairy vetch aregrown as cover crops to enrich the soil with or-ganic matter, as well as nitrogen, for the follow-


ing crop. In an alfalfa field, the bacteria may fixhundreds of pounds of nitrogen per acre eachyear. With peas, the amount of nitrogen fixed ismuch lower, around 30 to 50 pounds per acre.

The actinomycetes, another group of bacteria,break large lignin molecules into smaller sizes.Lignin is a large and complex molecule foundin plant tissue, especially stems, that is difficultfor most organisms to break down. Lignin alsofrequently protects other molecules like cellu-lose from decomposition. Actinomycetes havesome characteristics similar to fungi, but aresometimes grouped by themselves and givenequal billing with bacteria and fungi.

Fungi

Fungi are another important type of soil micro-organism. Yeast is a fungus used in baking andin the production of alcohol. A number of anti-biotics are produced by other fungi. We haveall probably let a loaf of bread sit around toolong only to find fungus growing on it. We haveseen or eaten mushrooms, the fruiting structureof some fungi. Farmers know that many plantdiseases, such as downy mildew, damping-off,various types of root rot, and apple scab, arecaused by fungi. Fungi are also important forstarting the decomposition of fresh organic resi-dues. They help get things going by softeningorganic debris and making it easier for otherorganisms to join in the decomposition process.Fungi are also the main decomposers of lignin.Fungi are less sensitive to acid-soil conditionsthan are bacteria. None are able to function with-out oxygen.

Many plants develop a beneficial relationshipwith fungi that increases the contact of roots withthe soil. Fungi infect the roots and send out root-like structures called hyphae (see figure 3.2). The

hyphae of these mycorrhizal fungi take up waterand nutrients that can then feed the plant. Thisis especially important for phosphorus nutritionof plants in low-phosphorus soils. The hyphaehelp the plant absorb water and nutrients andin return the fungi receive energy in the form ofsugars, which the plant produces in its leavesand sends down to the roots. This symbioticinterdependency between fungi and roots is

Mycorrhizal Fungi

Mycorrhizal fungi help plants take up nutrients,improve nitrogen fixation by legumes, andhelp to form and stabilize soil aggregates.Crop rotations select for more types and bet-ter performing fungi than does mono-crop-ping. Some studies indicate that using covercrops, especially legumes, between maincrops helps maintain high levels of spores andpromotes good mycorrhizal development inthe next crop. Roots that have lots of mycor-rhizae are better able to resist fungal diseases,parasitic nematodes, and drought.

Figure 3.2 Root heavily infected with mycorrhizalfungi (note round spores at the end of some hyphae).Photo by Sara Wright.

THE LIVING SOIL 17

called a mycorrhizal relationship. All things con-sidered, it’s a pretty good deal for both the plantand the fungus. The hyphae of these fungi helpdevelop and stabilize soil aggregates by secret-ing a sticky gel that glues mineral and organicparticles together.

SMALL AND MEDIUM-SIZEDSOIL ANIMALS

Nematodes

Nematodes are simple soil animals that resembletiny worms. They tend to live in the water filmsaround soil aggregates. Some types of nematodesfeed on plant roots and are well known plantpests. Diseases such as pythium and fusarium,which enter feeding wounds on the root, some-times cause more damage than the feeding it-self. However, most nematodes help in thebreakdown of organic residues and feed onfungi, bacteria, and protozoa as secondary con-sumers. In fact, as with the protozoa, nematodesfeeding on fungi and bacteria also helps con-vert nitrogen into forms for plants to use. Asmuch as 50 percent or more of mineralized ni-trogen comes from nematode feeding.

Earthworms

Earthworms are every bit as important asCharles Darwin believed more than a centuryago. They are keepers and restorers of soil fer-tility. Different types of earthworms, includingthe night-crawler, field (garden) worm, andmanure (red) worm, have different feeding hab-its. Some feed on plant residues that remainon the soil surface, while other types tend tofeed on organic matter that is already mixedwith the soil.

The surface-feeding nightcrawlers fragmentand mix fresh residues with soil mineral par-ticles, bacteria, and enzymes in their digestivesystem. The resulting material is given off asworm casts. Worm casts are generally higher inavailable plant nutrients, such as nitrogen, cal-cium, magnesium, and phosphorus, than thesurrounding soil and, therefore, make an im-

Algae

Algae, like crop plants, convert sunlight intocomplex molecules like sugars, which they canuse for energy and to help build the other mol-ecules they need. Algae are found in abundancein the flooded soils of swamps and rice pad-dies. They also can be found on the surface ofpoorly drained soils or in wet depressions. Al-gae also occur in relatively dry soils, and theyform mutually beneficial relationships withother organisms. Lichens found on rocks are anassociation between a fungus and an alga.

Protozoa

Protozoa are single-celled animals that use avariety of means to move about in the soil. Likebacteria and many fungi, they can be seen onlywith the help of a microscope. They are mainlysecondary consumers of organic materials, feed-ing on bacteria, fungi, other protozoa, and or-ganic molecules dissolved in the soil water. Pro-tozoa — through their grazing on nitrogen-richorganisms and excreting wastes — are believedresponsible for mineralizing much of the nitro-gen (released from organic molecules) in agri-cultural soils.

Fungi help to start the decomposition

of organic residues.


portant contribution to the nutrient needs ofplants. They also bring food down into their bur-rows, thereby mixing organic matter deep intothe soil. Earthworms feeding on debris alreadybelow the surface continue to decompose organicmaterials and mix them with the soil minerals.

A number of types of earthworms, includingthe surface-feeding nightcrawler, make burrowsthat allow rainfall to easily infiltrate into the soil.These worms usually burrow to three feet ormore under dry conditions. Even those types ofworms that don’t normally produce channels tothe surface help loosen the soil, creating chan-nels and cracks below the surface that help aera-tion and root growth. The number of earth-worms in the soil ranges from close to zero toover a million per acre. Just imagine, if you cre-ate the proper conditions for earthworms, youcan have 800,000 small channels per acre thatconduct water into your soil during downpours.

Earthworms do some unbelievable work.They move a lot of soil from below up to thesurface — from about 1 to 100 tons per acreeach year. One acre of soil 6 inches deep weighsabout 2 million pounds, or 1,000 tons. So 1 to100 tons is the equivalent of about .006 of aninch to about half an inch of soil. Agriculturalsoils that use conservation practices may stillerode, though at low annual rates of 1 to 4 tons/acre. A healthy earthworm population can coun-teract some of the effects of erosion — creatingtopsoil by bringing up subsoil and mixing it withorganic residues.

Earthworms do best in well-aerated soils thatare supplied with plentiful amounts of organicmatter. A study in Georgia showed that soils withhigher amounts of organic matter containedhigher numbers of earthworms. Surface feed-ers, a type we would especially like to encour-

age, need residues left on the surface. They areharmed by plowing or disking, which disturbstheir burrows and buries their food supplies.Worms are usually more plentiful under no-tillpractices than under conventional tillage systems.Although many pesticides have little effect onworms, others, such as aldicarb, parathion, andheptachlor, are very harmful to earthworms.

Diseases or insects that overwinter on leavesof crops can sometimes be partially controlledby high earthworm populations. The apple scabfungus — a major pest of apples in humid re-gions — and some leaf miner insects can bepartly controlled when worms eat the leaves andincorporate the residues deeper into the soil.

Insects and OtherSmall to Medium-Sized Animals

Insects are another group of animals that inhabitsoils. Common types of soil insects include ter-mites, springtails, ants, fly larvae, and beetles.Many insects are secondary and tertiary consum-ers. Springtails feed on fungi and animal re-mains. Many beetles, in particular, eat othertypes of soil animals. Some beetles feed on weedseeds in the soil. Termites, well-known feedersof woody material, also consume decomposedorganic residues in the soil.

Other medium- to large-sized soil animals in-clude millipedes, centipedes, mites, slugs, snails,and spiders. Millipedes are primary consumersof plant residues, whereas centipedes tend tofeed on other organisms. Mites may feed on foodsources like fungi, other mites, and insect eggs,although some feed directly on residues. Spi-ders feed mainly on insects and their role inkeeping insect pests from developing largepopulations can be important.

THE LIVING SOIL 19

VERY LARGE ANIMALS

Very large animals, such as moles, rabbits, wood-chucks, snakes, prairie dogs, and badgers, bur-row in the soil and spend at least some of theirlives below ground. Moles are secondary con-sumers, with their diet consisting mainly ofearthworms. Most of the other animals exist onvegetation. In many cases, their presence is con-sidered a nuisance for agricultural productionor lawns and gardens. Nevertheless, their bur-rows may help conduct water away from thesurface during downpours and thus decrease

erosion. In the South, the burrowing action ofcrawfish, abundant in many of the somewhatpoorly drained soils, can have a large effect onsoil structure. (In Texas and Louisiana, some ricefields are “rotated” with crawfish production.)

PLANT ROOTS

Healthy plant roots are essential for good cropyields. Roots are clearly influenced by the soilin which they live. If the soil is compact, low innutrients or water, or has other problems, plantswill not grow well. On the other hand, plants

Figure 3.3 Close-up view of a plant root. A) The mucigel layer containing some bacteria and clay particles on theoutside of the root. Also shown is a mycorrhizal fungus sending out its rootlike hyphae into the soil. B) Soilaggregates surrounded by thin films of water. Plant roots take water and nutrients from these films. Also shown isa larger aggregate made up of smaller aggregates pressed together and held in place by the root and hyphae.

rootinterior

hyphae of mycorrhizalfungi

root hair

mucigel layer

cell on root surface

A

soil aggregate

water film

larger aggregate

air spaceB

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also influence the soil in which they grow. Thephysical pressure of roots growing through soilhelps form aggregates by bringing particlescloser together. Small roots also help bind par-ticles together. In addition, many organic com-pounds are given off, or exuded, by plant rootsand provide nourishment for soil organisms liv-ing on or near the roots. A sticky layer surround-ing roots, called the mucigel, provides very closecontact between microorganisms, soil minerals,and the plant (figure 3.3).

For plants with extensive root systems, suchas grasses, the amount of living tissue below-ground may actually weigh more than theamount of leaves and stems we see above ground.

BIOLOGICAL DIVERSITYAND BALANCE

A diverse biological community in soils isessential to maintaining a healthy environmentfor plants. There may be over 100,000 differenttypes of organisms living in soils. Of those, onlya small number of bacteria, fungi, insects, andnematodes might harm plants in any given year.Diverse populations of soil organisms maintaina system of checks and balances that can keep

disease organisms or parasites from becomingmajor plant problems. Some fungi kill nema-todes and others kill insects. Still others pro-duce antibiotics that kill bacteria. Protozoa feedon bacteria. Some bacteria kill harmful insects.Many protozoa, springtails, and mites feed ondisease-causing fungi and bacteria. Beneficialorganisms, such as the fungus Trichoderma andthe bacteria Pseudemonas fluorescens, colonizeplant roots and protect them from attack byharmful organisms. Some of these organisms,isolated from soils, are now sold commerciallyas biological control agents.

SOURCES

Alexander, M. 1977. Introduction to Soil Microbiol-ogy. 2d ed. John Wiley & Sons. New York, NY.

Hendrix, P.F., M.H. Beare, W.X. Cheng, D.C.Coleman, D.A. Crossley, Jr., and R.R. Bruce.1990. Earthworm effects on soil organic mat-ter dynamics in aggrading and degradingagroecosystems on the Georgia Piedmont.Agronomy Abstracts, p.250, American Societyof Agronomy, Madison, WI.

Paul, E.A., and F.E. Clark. 1996. Soil Microbiol-ogy and Biochemistry. 2d ed Academic Press.San Diego, CA.

21

Organic matter functions in a number of keyroles to promote crop growth. It also is a

critical part of a number of global and regionalcycles.

A fertile soil is the basis for healthyplants, animals, and humans. Soil or-ganic matter is the very foundation forhealthy and productive soils. Under-standing the role of organic matter inmaintaining a healthy soil is essentialfor developing ecologically sound agriculturalpractices. It’s true that you can grow plants onsoils with little organic matter. In fact, you don’tneed any soil at all! [Although gravel or sandhydroponic systems without soil can grow ex-cellent crops, large-scale systems of this type areusually neither economically or ecologicallysound.] It’s also true that there are other impor-tant issues aside from organic matter when con-sidering the quality of a soil. However, as soil

4�

Why is Organic MatterSo Important?

Why are soils which in our father’s hands

were productive now relatively impoverished?

—J. L. HILLS, C. H. JONES, AND C. CUTLER, 1908

organic matter decreases, it becomes increasinglydifficult to grow plants, because problems withfertility, water availability, compaction, erosion,parasites, diseases, and insects become more

common. Ever higher levels of inputs— fertilizers, irrigation water, pesti-cides, and machinery — are requiredto maintain yields in the face of organicmatter depletion. But if attention ispaid to proper organic matter manage-

ment, the soil can support a good crop withoutthe need for expensive fixes.

The organic matter content of agriculturaltopsoil is usually in the range of 1 to 6 percent.A study of soils in Michigan demonstrated po-tential crop-yield increases of about 12 percentfor every 1 percent organic matter. In a Mary-land experiment, researchers saw an increase ofapproximately 80 bushels of corn per acre whenorganic matter increased from 0.8 to 2 percent.


You might wonder how something that’s only asmall part of the soil can be so important forgrowing healthy and high-yielding crops. Theenormous influence of organic matter on somany of the soil’s properties — biological,chemical, and physical — makes it of criticalimportance to healthy soils (figure 4.1). Part ofthe explanation for this influence is the smallparticle size of the well-decomposed portion oforganic matter — the humus. Its large surfacearea-to-volume ratio means that humus is incontact with a considerable portion of the soil.The intimate contact of humus with the rest ofthe soil allows many reactions, such as the re-lease of available nutrients into the soil water,

What Makes Topsoil?

Having a good amount of topsoil is important.But what gives topsoil its beneficial charac-teristics? Is it because it’s on TOP? If we bringin a bulldozer and scrape off one foot of soil,will the exposed subsoil now be topsoil be-cause it’s on the surface? Of course, every-one knows that there’s more to topsoil thanits location on the soil surface. Most of theproperties we associate with topsoil — goodnutrient supply, tilth, drainage, aeration, wa-ter storage, etc. — are there because topsoilis rich in organic matter and contains a hugediversity of life.

Figure 4.1 Adding organic matter results in many changes. Modified from Oshins, 1999.

addorganicmatter

increasedbiological

activity(& diversity)

reducedsoil-borne diseases,parasitic nematodes

pore structureimproved

improved tilthand water

storage

decomposition

aggregationincreased

humus and other growthpromoting substances

nutrientsreleased

harmfulsubstancesdetoxified

HEALTHY PLANTS

WHY IS ORGANIC MATTER SO IMPORTANT? 23

to occur rapidly. However, the many roles of liv-ing organisms make soil life an essential part ofthe organic matter story.

Plant Nutrition

Plants need 18 chemical elements for theirgrowth — carbon (C), hydrogen (H), oxygen(O), nitrogen (N), phosphorus (P), potassium(K), sulfur (S), calcium (Ca), magnesium (Mg),iron (Fe), manganese (Mn), boron (B), zinc (Zn),molybdenum (Mo), nickel (Ni), copper (Cu),cobalt (Co), and chlorine (Cl). Plants obtain car-bon as carbon dioxide (CO2) and oxygen par-tially as oxygen gas (O2) from the air. The re-maining essential elements are obtained mainlyfrom the soil. The availability of these nutrientsis influenced either directly or indirectly by the

presence of organic matter. The elements neededin large amounts — carbon, hydrogen, oxygen,nitrogen, phosphorus, potassium, calcium, mag-nesium, sulfur — are called macronutrients. Theother elements, called micronutrients, are es-sential elements needed in small amounts.

Nutrients from decomposing organic matter.Most of the nutrients in soil organic matter can’tbe used by plants as long as they exist as part oflarge organic molecules. As soil organisms de-compose organic matter, nutrients are convertedinto simpler, inorganic, or mineral forms thatplants can easily use. This process, called min-eralization, provides much of the nitrogen thatplants need by converting it from organic forms.For example, proteins are converted to ammo-

Figure 4.2 The cycle of plant nutrients.

animals

growing cropscrop residues

andanimal manures

soil organicmatter


nium (NH4+) and then to nitrate (NO3

-). Mostplants will take up the majority of their nitro-gen from soils in the form of nitrate. The miner-alization of organic matter is also an importantmechanism for supplying plants with such nu-trients as phosphorus and sulfur, and most ofthe micronutrients. This release of nutrients fromorganic matter by mineralization is part of alarger agricultural nutrient cycle (see figure 4.2).For a more detailed discussion of nutrient cyclesand how they function in various cropping sys-tems, see chapter 7.

Addition of nitrogen. Bacteria living in nod-ules on legume roots convert nitrogen from at-mospheric gas (N2) to forms that the plant canuse directly. There are a number of free-livingbacteria that also fix nitrogen.

Storage of nutrients on soil organic matter.Decomposing organic matter can feed plantsdirectly, but it also can indirectly benefit thenutrition of the plant. A number of essentialnutrients occur in soils as positively chargedmolecules called cations (pronounced cat-eye-ons). The ability of organic matter to hold ontocations in a way that keeps them available to

plants is known as cation exchange capacity(CEC). Humus has many negative charges. Be-cause opposite charges attract, humus is able tohold onto positively charged nutrients, such ascalcium (Ca++), potassium (K+), and magnesium(Mg++) (see figure 4.3a). This keeps them fromleaching deep into the subsoil when water movesthrough the topsoil. Nutrients held in this waycan be gradually released into the soil solutionand made available to plants throughout thegrowing season. However, keep in mind that notall plant nutrients occur as cations. For example,the nitrate form of nitrogen is negatively charged(NO3

-) and is actually repelled by the negativelycharged CEC. Therefore, nitrate leaches easilyas water moves down through the soil and be-yond the root zone.

Clay particles also have negative charges ontheir surfaces (figure 4.3b), but organic mattermay be the major source of negative charges forcoarse and medium textured soils. Some types ofclays, such as those found in the southeasternUnited States and in the tropics, tend to have lowamounts of negative charge. When these claysare present, organic matter may be the majorsource of negative charges that bind nutrients,even for fine textured (high clay content) soils.

Figure 4.3 Cations held on organic matter and clay.

-

-

Ca++Ca++

Ca++

Ca++

Ca++Mg++

K+ Zn++

a) cations held on humus b) cations held on clayparticle

c) cations held by organicchelate

K+ Mg++


Protection of nutrients by chelation. Organicmolecules in the soil may also hold onto andprotect certain nutrients. These particles, called“chelates” (pronounced key-lates) are byproductsof active decomposition of organic materials andare smaller than those that make up humus. Ingeneral, elements are held more strongly by che-lates than by binding of positive and negativecharges. Chelates work well because they bindthe nutrient at more than one location on theorganic molecule (figure 4.3c). In some soils,trace elements, such as iron, zinc, and manga-nese, would be converted to unavailable formsif they were not bound by chelates. It is not un-common to find low organic matter soils or ex-posed subsoils deficient in these micronutrients.

Other ways of maintaining available nutri-ents. There is some evidence that organic mat-ter in the soil can inhibit the conversion of avail-able phosphorus to forms that are unavailableto plants. One explanation is that organic mat-ter coats the surfaces of minerals that can bondtightly to phosphorus. Once these surfaces arecovered, available forms of phosphorus are lesslikely to react with them. In addition, humicsubstances may chelate aluminum and iron,both of which can react with phosphorus in thesoil solution. When they are held as chelates,these metals are unable to form an insolublemineral with phosphorus.

Beneficial Effects of Soil Organisms

Soil organisms are essential for keeping plantswell supplied with nutrients because they breakdown organic matter. These organisms makenutrients available by freeing them from organicmolecules. Some bacteria fix nitrogen gas fromthe atmosphere, making it available to plants.

Other organisms dissolve minerals and makephosphorus more available. If soil organismsaren’t present and active, more fertilizers willbe needed to supply plant nutrients.

A varied community of organisms is your bestprotection against major pest outbreaks and soilfertility problems. A soil rich in organic matterand continually supplied with different types offresh residues is home to a much more diversegroup of organisms than soil depleted of organicmatter. This greater diversity of organisms helpsinsure that fewer potentially harmful organismswill be able to develop sufficient populations toreduce crops yields.

Soil Tilth

When soil has a favorable physical conditionfor growing plants, it is said to have good tilth.Such a soil is porous and allows water to entereasily, instead of running off the surface. Morewater is stored in the soil for plants to use be-tween rains and less soil erosion occurs. Goodtilth also means that the soil is well aerated.Roots can easily obtain oxygen and get rid ofcarbon dioxide. A porous soil does not restrictroot development and exploration. When a soilhas poor tilth, the soil’s structure deterioratesand soil aggregates break down, causing in-creased compaction and decreased aeration andwater storage. A soil layer can become so com-pacted that roots can’t grow. A soil with excel-lent physical properties will have numerouschannels and pores of many different sizes.

Studies on both undisturbed and agriculturalsoils show that as organic matter increases, soilstend to be less compact and have more spacefor air passage and water storage. Sticky sub-stances are produced during the decompositionof plant residues. Along with plant roots and


fungal hyphae, they bind mineral particles to-gether into clumps, or aggregates. In addition,the sticky secretions of mycorrhizal fungi —those that infect roots and help plants get morewater and nutrients — are important bindingmaterial in soils. The arrangement and collec-tion of minerals as aggregates and the degree ofsoil compaction have huge effects on plantgrowth (see chapter 6). The development of ag-gregates is desirable in all types of soils becauseit promotes better drainage, aeration, and waterstorage. The one exception is for wetland crops,such as rice, when you want a dense, puddledsoil to keep it flooded.

Organic matter, as residue on the soil surfaceor as a binding agent for aggregates near the sur-face, plays an important role in decreasing soilerosion. Surface residues intercept raindrops anddecrease their potential to detach soil particles.These surface residues also slow water as it flowsacross the field, giving it a better chance to in-filtrate into the soil. Aggregates and large chan-nels greatly enhance the ability of soil to con-duct water from the surface into the subsoil.

Most farmers can tell that one soil is betterthan another by looking at them, touching them,how they work up when tilled, or even by sens-ing how they feel when walked on. What theyare seeing or sensing is really good tilth. For anexample, see the photo on the back cover ofthis book. It shows that soil differences can becreated by different management strategies.Farmers and gardeners would certainly rathergrow their crops on the more porous soil de-picted in the photo on the right.

Since erosion tends to remove the most fer-tile part of the soil, it can cause a significantreduction in crop yields. In some soils, the lossof just a few inches of topsoil may result in ayield reduction of 50 percent. The surface ofsome soils low in organic matter may seal over,or crust, as rainfall breaks down aggregates, andpores near the surface fill with solids. When thishappens, water that can’t infiltrate into the soilruns off the field, carrying valuable topsoil (fig-ure 4.4).

Large soil pores, or channels, are very im-portant because of their ability to allow a lot of

Figure 4.4 Changes in soil surface and water-flow pattern due to soil crusting.

infiltration

runoff

a) aggregated soil b) soil crusts after aggregates break down


water to flow rapidly into the soil. Larger poresare formed a number of ways. Old root chan-nels may remain open for some time after theroot decomposes. Larger soil organisms, suchas insects and earthworms, create channels asthey move through the soil. The mucus thatearthworms secrete to keep their skin from dry-ing out also helps to keep their channels openfor a long time.

Figure 4.5 Corn grown in nutrient solution with(right) and without (left) humic acids. In thisexperiment by Rich Bartlett and Yong Lee, addinghumic acids to a nutrient solution increased thegrowth of tomatoes and corn and increased thenumber and branching of roots. Photo by R. Bartlett.

Protection of the Soil AgainstRapid Changes in Acidity

Acids and bases are released as minerals dissolveand organisms go about their normal functionsof decomposing organic materials or fixing ni-trogen. Acids or bases are excreted by the rootsof plants, and acids form in the soil from the useof nitrogen fertilizers. It is best for plants if thesoil acidity status, referred to as pH, does notswing too wildly during the season. The pH scaleis a way of expressing the amount of free hydro-gen (H+) in the soil water. More acidic conditions,with greater amounts of hydrogen, are indicatedby lower numbers. A soil at pH 4 is very acid. Itssolution is 10 times more acid than a soil at pH5. A soil at pH 7 is neutral — there is just asmuch base in the water as there is acid. Mostcrops do best when the soil is slightly acid andthe pH is around 6 to 7. Essential nutrients aremore available to plants in this pH range thanwhen soils are either more acidic or more basic.Soil organic matter is able to slow down, or buffer,changes in pH by taking free hydrogen out ofsolution as acids are produced or by giving offhydrogen as bases are produced. (For discussionabout management of acidic soils, see chapter 18.)

Stimulation of Root Development

Microorganisms in soils produce numerous sub-stances that stimulate plant growth. Humus it-self has a directly beneficial effect on plants (fig-ure 4.5). Although the reasons for this stimula-tion are not yet understood, certain types ofhumus cause roots to grow longer and have morebranches, resulting in larger and healthier plants.In addition, soil microorganisms produce a va-riety of root-stimulating substances that behaveas plant hormones.


Darkening of the Soil

Organic matter tends to darken soils. You caneasily see this in coarse-textured sandy soilscontaining light-colored minerals. Under well-drained conditions, a darker soil surface allowsa soil to warm up a little faster in the spring.This provides a slight advantage for seed germi-nation and the early stages of seedling develop-ment, which is often beneficial in cold regions.

Protection Against Harmful Chemicals

Some naturally occurring chemicals in soils canharm plants. For example, aluminum is an im-portant part of many soil minerals and, as such,poses no threat to plants. As soils become moreacidic, especially at pH levels below 5.5, alumi-num becomes soluble. Some soluble forms ofaluminum, if present in the soil solution, aretoxic to plant roots. However, in the presenceof significant quantities of soil organic matter,the aluminum is bound tightly and will not doas much damage.

Organic matter is the single most importantsoil property that reduces pesticide leaching. Itholds tightly onto a number of pesticides. This

prevents or reduces leaching of these chemicalsinto groundwater and allows time for detoxifi-cation by microbes. Microorganisms can changethe chemical structure of some pesticides, in-dustrial oils, many petroleum products (gas andoils), and other potentially toxic chemicals, ren-dering them harmless.

ORGANIC MATTERAND NATURAL CYCLES

The Carbon Cycle

Soil organic matter plays a significant part in anumber of global cycles. People have becomemore interested in the carbon cycle because thebuildup of carbon dioxide (CO2) in the atmo-sphere is thought to cause global warming. Car-bon dioxide

is also released to the atmosphere

when fuels, such as gas, oil, and wood, areburned. A simple version of the natural carboncycle, showing the role of soil organic matter, isgiven in figure 4.6. Carbon dioxide

is removed

from the atmosphere by plants and used to makeall the organic molecules necessary for life.

Color and Organic Matter

In Illinois, a hand-held chart has been devel-oped to allow people to estimate percent soilorganic matter. Their darkest soils — almostblack — indicates from 3.5 to 7 percent or-ganic matter. A dark brown soil indicates 2 to3 percent and a yellowish brown soil indicates1.5 to 2.5 percent organic matter. (Color maynot be as clearly related to organic matter inall regions, because the differences in theamount of clay and the types of mineralspresent also influence soil color.)

Each percent organic matter in the

top 6 inches of soil contains about

the same quantity of carbon as all the

atmosphere directly over the field!

Sunlight provides plants with the energy theyneed to carry out this process. Plants, as well asthe animals feeding on plants, release carbondioxide

back into the atmosphere as they use

organic molecules for energy.


The largest amount of carbon present on theland is not in the living plants, but in soil or-ganic matter. That is rarely mentioned in dis-cussions of the carbon cycle. More carbon isstored in soils than in all plants, all animals andthe atmosphere. Soil organic matter contains anestimated four times as much carbon as livingplants. As soil organic matter is depleted, it be-comes a source of carbon dioxide

for the atmo-

sphere. When forests are cleared and burned, alarge amount of carbon dioxide is released tothe atmosphere. There is a potentially larger re-lease of carbon dioxide

following conversion of

forests to agricultural practices that rapidly de-plete the soil of its organic matter. There is as

much carbon in 6 inches of soil with 1 percentorganic matter as there is in the atmosphereabove a field. If organic matter decreases from 3percent to 2 percent, the amount of carbon di-oxide in the atmosphere could double. (Ofcourse, wind and diffusion move the carbon di-oxide to other parts of the globe.)

The Nitrogen Cycle

Another important global cycle in which organicmatter plays a major role is the nitrogen cycle.This cycle is of direct importance in agriculture,because available nitrogen for plants is com-monly deficient in soils. Figure 4.7 shows the

Figure 4.6 The role of soil organic matter in the carbon cycle. Losses of carbon from the field are indicated bythe dark border around the words describing the process.

carbon dioxide (CO2)(0.04% in the atmosphere)

root respirationand soil organic

matterdecomposition

respirationin stems

and leaves crop harvest

photosynthesis

crop andanimal

residues

carbonin soil

organic matter erosion


nitrogen cycle and how soil organic matter en-ters into the cycle. Some bacteria living in soilsare able to “fix” nitrogen, converting nitrogengas to forms that other organisms, including cropplants, can use. Inorganic forms of nitrogen, likeammonium and nitrate, exist in the atmospherenaturally, although air pollution causes higheramounts than normal. Rainfall and snow deposit

inorganic nitrogen forms on the soil. Inorganicnitrogen also may be added in the form of com-mercial nitrogen fertilizers. These fertilizers arederived from nitrogen gas in the atmospherethrough an industrial fixation process.

Almost all of the nitrogen in soils exists aspart of the organic matter, in forms that plantsare not able to use as their main nitrogen source.

Figure 4.7 The role of soil organic matter in the nitrogen cycle. Losses of nitrogen from the field are indicatedby the dark border around the words describing the process.

denitrification

plantuptake

crop andanimal

residues

crop harvest

nitrogen in soilorganic matter

volatilization

nitrogenfixation

NH4+ + OH - NH3 + H2O

ammoniumNH4

+

nitrateNO3

-

legumes

atmospheric fixationor

fertilizer production

NH4+ + NO3

-

free-livingbacteria

immobilization

erosion

NO3- N2 + N2O

leaching togroundwater

�

nitrogen gas (N2)(78% of atmosphere)


The Water Cycle

Organic matter plays an important part in thelocal, regional, and global water, or hydrologic,cycle due to its role in promoting water infiltra-tion into soils and storage within the soil. Wa-ter evaporates from the soil surface and fromliving plant leaves as well as from the ocean andlakes. Water then returns to the earth, usuallyfar from where it evaporated, as rain and snow.Soils high in organic matter, with excellent tilth,enhance the rapid infiltration of rainwater intothe soil. This water may be available for plantsto use or it may percolate deep into the subsoiland help to recharge the groundwater supply.Since groundwater is commonly used as a drink-ing water source for homes and for irrigation,recharging groundwater is important. When thesoil’s organic matter level is depleted, it is lessable to accept water, and high levels of runoffand erosion result. This means less water forplants and decreased groundwater recharge.

SOURCES

Allison, F.E. 1973. Soil Organic Matter and its Rolein Crop Production. Scientific Publishing Co.Amsterdam, The Netherlands.


Follett, R.F., J.W.B. Stewart, and C.V. Cole (eds).1987. Soil Fertility and Organic Matter as Criti-cal Components of Production Systems. SpecialPublication No.19. Soil Science Society ofAmerica. Madison, WI.

Lucas, R.E., J.B. Holtman, and J.L. Connor.1977. Soil carbon dynamics and croppingpractices. pp. 333–451. In Agriculture andEnergy (W. Lockeretz, ed.). Academic Press.

Almost all of the nitrogen in soils

exists as part of the organic matter,

in forms that plants are not able to

use as their main nitrogen source.

Bacteria and fungi convert the organic forms ofnitrogen into ammonium and different bacteriaconvert ammonium into nitrate. Both nitrate andammonium can be used by plants.

Nitrogen can be lost from a soil in a numberof ways. When crops are removed from fields,nitrogen and other nutrients also are removed.The nitrate (NO3

-) form of nitrogen leachesreadily from soils and may end up in ground-water at higher concentrations than may be safefor drinking. Organic forms of nitrate as well asnitrate and ammonium (NH4

+) may be lost byrunoff water and erosion. Once freed from soilorganic matter, nitrogen may be converted toforms that end up back in the atmosphere. Bac-teria convert nitrate to nitrogen (N2) and nitrousoxide (N2O) gases in a process called denitrifi-cation, which occurs in saturated soils. Nitrousoxide (it’s called a “greenhouse gas”) contrib-utes to global warming. In addition, when itreaches the upper atmosphere, it helps to de-crease the ozone levels that protect the earth’ssurface from the harmful effects of ultraviolet(UV) radiation. So if you needed another rea-son not to apply excessive rates of fertilizers ormanures — in addition to the economic costsand the pollution of ground and surface waters— the possible formation of nitrous oxideshould make you cautious.


New York, NY. See this source for the Michi-gan study on the relationship between soilorganic matter levels and crop-yield potential.

Oshins, C. 1999. An Introduction to Soil Health.A slide set available at the Northeast RegionSARE website: www.uvm.edu/~nesare/slide.html.

Powers, R.F., and K. Van Cleve. 1991. Long-termecological research in temperate and borealforest ecosystems. Agronomy Journal 83:11–24. This reference compares the relativeamounts of carbon in soils with that in plants.

Stevenson, F.J. 1986. Cycles of Soil: Carbon, Ni-

trogen, Phosphorus, Sulfur, Micronutrients. JohnWiley & Sons. New York, NY. This referencecompares the amount of carbon in soils withthat in plants.

Strickling, E. 1975. Crop sequences and tillagein efficient crop production. Abstracts of the1975 Northeast Branch American SocietyAgronomy Meetings. pp. 20–29. See this sourcefor the Maryland experiment relating soil or-ganic matter to corn yield.

Tate, R.L., III. 1987. Soil Organic Matter: Biologi-cal and Ecological Effects. John Wiley & Sons.New York, NY.

33

The amount of organic matter in any particu-lar soil is a result of a wide variety of envi-

ronmental, soil, and agronomic influences. Someof these, such as climate and soil texture, arenaturally occurring. Human activityalso influences soil organic matter lev-els. Tillage, crop rotation, and manur-ing practices all have profound effectson the amount of soil organic matter.Pioneering work on the effect of natu-ral influences on soil organic matter levels wascarried out in the U.S. more than 50 years agoby Hans Jenny.

The amount of organic matter in soil is a re-sult of all the additions and losses of organicmatter that have occurred over the years (figure5.1). In this chapter, we will look at why differ-ent soils have different organic matter levels. Any-thing that adds large amounts of organic residuesto a soil may increase organic matter. On the other

5�

Amount of OrganicMatter in Soils

The depletion of the soil humus supply is apt to be

a fundamental cause of lowered crop yields.

—J.H. HILLS, C.H. JONES, AND C. CUTLER, 1908

hand, anything that causes soil organic matterto decompose more rapidly or be lost througherosion may deplete organic matter.

If additions are greater than losses, organicmatter increases. When additions areless than losses, there is a depletion ofsoil organic matter. When the systemis in balance, and additions equallosses, the quantity of soil organic mat-ter doesn’t change over the years.

NATURAL FACTORS

Temperature

In the United States, it is easy to see how tem-perature affects soil organic matter levels. Trav-eling from north to south, average hotter tem-peratures lead to less soil organic matter. As theclimate gets warmer, two things tend to happen


(as long as rainfall is sufficient): more vegeta-tion is produced because the growing season islonger, and the rate of decomposition of organicmaterials in soils also increases, because soil or-ganisms work more efficiently in warm weatherand for longer periods of the year. This increas-ing decomposition with warmer temperaturesbecomes the dominant influence determiningsoil organic matter levels.

Rainfall

Soils in arid climates usually have low amountsof organic matter. In a very dry climate, such asa desert, there is little growth of vegetation.Decomposition may be very low when the soilis dry and microorganisms cannot function well.When it finally rains, a very rapid burst of de-composition of soil organic matter occurs. Soilorganic matter levels generally increase as aver-age annual precipitation increases. With morerainfall, more water is available to plants andmore plant growth results. As rainfall increases,more residues return to the soil from grasses ortrees. At the same time, soils in high rainfall ar-eas may have less soil organic matter decompo-sition than well-aerated soils — decompositionis slowed by restricted aeration.

Soil Texture

Fine textured soils, containing high percentagesof clay, tend to have naturally higher amountsof soil organic matter than coarse textured sandsor sandy loams. The organic matter content ofsands may be less than 1 percent; loams mayhave 2 to 3 percent; and clays from 4 to morethan 5 percent. The strong bonds that developbetween clay and organic matter seem to pro-tect organic molecules from attack and decom-position by microorganisms. In addition, finetextured soils tend to have smaller pores andhave less oxygen than coarser soils. This alsocauses reduced decomposition of organic mat-ter. The lower rate of decomposition in soils withhigh clay contents is probably the main reasonthat their organic matter levels are higher thanin sands and loams.

Soil Drainage and Positionin the Topography

Some soils have a compact subsoil layer thatdoesn’t allow water to drain well. Decomposi-tion of organic matter occurs more slowly inpoorly aerated soils, when oxygen is limited orabsent, than in well-aerated soils. For this rea-son, organic matter accumulates in wet soil en-

Figure 5.1 Additions and losses of organic matter from soils.

soil organic matter

additions

crop residuesmanures

composts

lossesCO2

(respiration of soilorganisms)

erosion

AMOUNT OF ORGANIC MATTER IN SOILS 35

vironments. In a totally flooded soil, one of themajor structural parts of plants, lignin, doesn’tdecompose at all. The ultimate consequence ofextremely wet or swampy conditions is the de-velopment of organic (peat or muck) soils, withorganic matter contents of over 20 percent. Iforganic soils are artificially drained for agricul-tural or other uses, the soil organic matter willdecompose very rapidly. When this happens, theelevation of the soil surface actually decreases.Some homeowners in Florida were fortunate tosink corner posts below the organic level. Origi-nally level with the ground, those homes nowperch on posts atop a soil surface that has de-creased so dramatically the owners park undertheir homes.

Soils in depressions at the bottom of hills areoften wet because they receive runoff, sediments(including organic matter), and seepage fromup slope. Organic matter is not decomposed asrapidly in these landscape positions as in driersoils farther up slope. However, soils on a steepslope will tend to have low amounts of organicmatter because the topsoil is continually eroded.

Type of Vegetation

The type of plants that grow on a soil over theyears affects the soil organic matter level. Themost dramatic differences are evident when soilsdeveloped under grassland are compared withthose developed under forests. On natural grass-lands, organic matter tends to accumulate inhigh amounts and to be well distributed withinthe soil. This is probably a result of the deepand extensive root systems of native grasses.Their roots have high “turnover” rates, for rootdeath and decomposition constantly occurs asnew roots are formed. The high levels of organic

matter in soils that were once in grassland ex-plains why these are some of the most produc-tive soils in the world. By contrast, in forests,litter accumulates on top of the soil, and sur-face organic layers commonly contain over 50percent organic matter. However, subsurface min-eral layers in forest soils typically contain fromless than 1 to about 2 percent organic matter.

Acidic Soil Conditions

In general, soil organic matter decomposition isslower under acidic soil conditions than at moreneutral pH. In addition, acidic conditions, byinhibiting earthworm activity, encourage organicmatter to accumulate at the soil surface, ratherthan distributed throughout the soil layers.

HUMAN INFLUENCES

Soil erosion removes topsoil rich in organic mat-ter so that, eventually, only subsoils remain.Crop production obviously suffers when partor all of the most fertile layer of the soil is re-moved. Erosion is a natural process and occurson almost all soils. Some soils naturally erodemore easily than others and the problem is alsogreater in some regions than others. However,agricultural practices accelerate erosion. Nation-wide, soil erosion causes huge economic losses.It is estimated that erosion in the United Statesis responsible for annual losses of $500 millionin available nutrients and $18 billion in totalsoil nutrients.

Unless erosion is very severe, a farmer maynot even realize that a problem exists, but thatdoesn’t mean that crop yields are unaffected. Infact, yields may decrease by 5 to 10 percentwhen only moderate erosion occurs. Yields may


suffer a decrease of 10 to 20 percent or morewith severe erosion. The results of a study ofthree midwestern soils, shown in table 5.1, in-dicate that erosion greatly influences both or-ganic matter levels and water-holding ability.Greater amounts of erosion decreased the or-ganic matter contents of these loamy and clayeysoils. In addition, eroded soils stored less avail-able water than soils experiencing little erosion.

Organic matter also is lost from soils whenorganisms decompose more organic materialsduring the year than are added. This occurs as aresult of practices such as intensive tillage andgrowing crops that produce low amounts of resi-dues (see below).

Tillage Practices

Tillage practices influence both the amount oftopsoil erosion and the rate of decompositionof soil organic matter. Conventional plowingand disking of a soil to prepare a smooth seed-

bed breaks down natural soil aggregates anddestroys large, water-conducting channels. Thesoil is left in a physical condition that allowsboth wind and water erosion.

The more a soil is disturbed by tillage prac-tices, the greater the potential breakdown oforganic matter by soil organisms. During theearly years of agriculture in the United States,when colonists cleared the forests and plantedcrops in the East and farmers later moved to theMidwest to plow the grasslands, soil organicmatter decreased rapidly. In fact, the soils wereliterally mined of a valuable resource — organicmatter. In the Northeast and Southeast, it wasquickly recognized that fertilizers and soil amend-ments were needed to maintain soil productiv-ity. In the Midwest, the deep, rich soils of thetall-grass prairies were able to maintain theirproductivity for a long time despite acceleratedsoil organic matter loss and significant amountsof erosion. The reason for this was their unusu-ally high original levels of soil organic matter.

TABLE 5.1Effects of Erosion on Soil Organic Matter and Water

AVAILABLE

ORGANIC MATTER WATER CAPACITY

SOIL EROSION (%) (%)

Corwin slight 3.03 12.9

moderate 2.51 9.8

severe 1.86 6.6

Miami slight 1.89 16.6

moderate 1.64 11.5

severe 1.51 4.8

Morley slight 1.91 7.4

moderate 1.76 6.2

severe 1.60 3.6

—SCHERTZ ET AL., 1985.


Rapid soil organic matter decomposition bysoil organisms usually occurs when a soil isworked with a moldboard plow. Incorporatingresidues, breaking aggregates open, and fluff-ing up the soil allows microorganisms to workmore rapidly. It’s something like opening up theair intake on a wood stove, which lets in moreoxygen and causes the fire to burn hotter. InVermont, we found a 20-percent decrease in or-ganic matter after five years of growing corn ona clay soil that had previously been in sod for along time. In the Midwest, 40 years of cultiva-tion caused a 50-percent decline in soil organicmatter. Rapid loss of soil organic matter occursin the early years, because of the high initialamount of active (“dead”) organic matter avail-able to micro-organisms. After much of the ac-tive portion is lost, the rate of organic matterloss slows considerably.

With the current interest in reduced (con-servation) tillage, growing row crops in the fu-ture may not have such a detrimental effect onsoil organic matter. Conservation tillage prac-tices leave more residues on the surface andcause less soil disturbance than conventionalmoldboard plow and disk tillage. In fact, soilorganic matter levels usually increase when no-till planters place seeds in a narrow band of dis-turbed soil, while leaving the soil between plant-ing rows undisturbed. The rate of decomposi-tion of soil organic matter is lower because thesoil is not drastically disturbed by plowing anddisking. Residues accumulate on the surfacebecause the soil is not inverted by plowing.Earthworm populations increase, taking someof the organic matter deeper into the soil andcreating channels that help water infiltrate intothe soil. Decreased erosion also results from us-ing conservation tillage practices.

Crop Rotations and Cover Crops

At different stages in a rotation, different thingsmay be happening. Soil organic matter may de-crease, then increase, then decrease, and soforth. While annual row crops under conven-tional moldboard plow cultivation usually re-sult in decreased soil organic matter, perenniallegumes, grasses, or legume-grass forage cropstend to increase soil organic matter. The turn-over of the roots of these hay and pasture crops,plus the lack of soil disturbance, allow organicmatter to accumulate in the soil. This effect isseen in the comparison of organic matter in-creases when growing alfalfa compared to cornsilage (figure 5.2) In addition, different types ofcrops result in different quantities of residuesreturned to the soil. When corn grain is har-vested, more residues are left in the field thanafter soybeans, wheat, potatoes, or lettuce har-vests. Harvesting the same crop in different waysleaves different amounts of residues. When corngrain is harvested, more residues remain in thefield than when the entire plant is harvested forsilage (figure 5.3).

Figure 5.2 Organic carbon changes when growingcorn silage or alfalfa. Redrawn from Angers, 1992.

3.2

3.0

2.8

2.6

2.4

0 1 2 3 4 5

corn

pe

rce

nt

carb

on

years

alfalfa


Soil erosion is greatly reduced and topsoil richin organic matter is conserved when rotationcrops, such as grass or legume hay, are grownyear-round. The extensive root systems of sodcrops account for much of the reduction in ero-sion. Having sod crops as part of a rotation re-duces loss of topsoil, decreases decompositionof residues, and builds up organic matter by theextensive residue addition of plant roots.

Use of Organic Amendments

An old practice that helps maintain or increasesoil organic matter is to apply manures or otherorganic residues generated off the field. A studyin Vermont during the 1960s and 1970s foundthat between 20 and 30 tons (wet weight, in-cluding straw or sawdust bedding) of dairymanure per acre were needed to maintain soilorganic matter levels when silage corn wasgrown each year. This is equivalent to 1 to 11/2times the amount produced by a large Holsteincow over the whole year. Different manures canhave very different effects on soil organic mat-ter and nutrient availability. They differ in theirinitial composition and also are affected by howthey are stored and handled in the field.

ORGANIC MATTERDISTRIBUTION IN SOIL

In general, more organic matter is present nearthe surface than deeper in the soil (see figure5.4). This is one of the main reasons that top-soils are so productive, compared with subsoilsexposed by erosion or mechanical removal ofsurface soil layers. Much of the plant residuesthat eventually become part of the soil organicmatter are from the above-ground portion ofplants. When the plant dies or sheds leaves orbranches, it deposits residues on the surface.Although earthworms and insects help incor-porate the residues on the surface deeper intothe soil and the roots of some plants penetratedeeply, the highest concentrations still remainwithin 1 foot of the surface.

Litter layers that commonly develop on thesurface of forest soils may have very high or-ganic matter contents (figure 5.4a). Plowing for-est soils after removal of the trees incorporatesthe litter layers into the mineral soil. The incor-porated litter decomposes rapidly, and an agri-cultural soil derived from a light, sandy textureforest soil in the north or a silt loam in the south-east coastal plain would likely have a distribu-

Figure 5.3 Soil surface after harvest of corn silage or corn grain. Photos by Win Way.

a) corn silage b) corn grain


tion of organic matter similar to that indicatedin figure 5.4b. Soils of the tall-grass prairies havehigh levels of organic matter deep into the soilprofile (see figure 5.4c). After cultivation of thesesoils for 50 years, far less organic matter exists(figure 5.4d).

ACTIVEORGANIC MATTER

The discussion for almost all of this chapter hasbeen about amounts of total organic matter insoils. However, we should constantly keep inmind that we are interested in each of the dif-ferent types of organic matter in soils — the liv-ing, the dead (active), and the very dead (hu-mus). We don’t just want a lot of humus in soil,we also want a lot of active organic matter toprovide nutrients and aggregating glues whenit is decomposed. We want the active organicmatter because it supplies food to keep a di-

verse population of organisms present. As men-tioned earlier, when forest or prairie soils werefirst cultivated, there was a drastic decrease inthe organic matter content. Almost all of thedecline was due to a loss of the active (“dead”)part of the organic matter. It is the active frac-tion that increases relatively quickly when prac-tices, such as reduced tillage, rotations, covercrops, and manures, are used to increase soilorganic matter.

LIVINGORGANIC MATTER

In chapter 3, we talked about the various typesof organisms that live in soils. The weight offungi present in forest soils is much greater thanthe weight of bacteria. In grasslands, however,there are about equal weights of both. In agri-cultural soils that are routinely tilled, the weightof fungi is less than the weight of bacteria. As

Figure 5.4 Examples of soil organic matter content with depth. Modified from Brady and Weil, 1999.

de

pth

organic matter (%)

1 2 3 4 1 2 3 4 3 6 9 12 3 6 9 12

a) forest soil (litter layer ontop of mineral soil may be

30% or more organic)

b) agricultural soil(originally forest)

c) prairie soil d) agricultural soil(originally prairie)

1 ft

2 ft

3 ft


soils become more compact, larger pores areeliminated first. These are the pores in whichsoil animals, such as earthworms and beetles,live and function, so the number of such organ-isms in compacted soils decreases.

Different total amounts (weights) of living or-ganisms exist in various cropping systems. Ingeneral, high populations of diverse and activesoil organisms are found in systems with morecomplex rotations that regularly leave highamounts of crop residues and when other or-ganic materials are added to the soil. Organicmaterials may include crop residues, covercrops, animal manures, and composts. Leavesand grass clippings may be an important sourceof organic residues for gardeners. When cropsare rotated regularly, fewer parasite, disease,weed, and insect problems occur than when thesame crop is grown year after year.

On the other hand, frequent cultivation re-duces the number of many soil organisms astheir food supplies are depleted by decomposi-tion of organic matter. Compaction from heavyequipment causes harmful biological effects insoils. It decreases the number of medium to largepores, which reduces the volume of soil avail-able for air, water, and populations of organ-isms — such as mites and springtails — thatneed the large spaces in which to live.

SOURCES

Angers, D.A. 1992. Changes in soil aggregationand organic carbon under corn and alfalfa.Soil Science Society of America Journal 56:1244–1249.


Carter, V.G., and T. Dale. 1974. Topsoil and Civi-lization. University of Oklahoma Press. Nor-man, OK.

Hass, H.J., G.E.A. Evans, and E.F. Miles. 1957.Nitrogen and Carbon Changes in Great PlainsSoils as Influenced by Cropping and Soil Treat-ments. U.S. Department of Agriculture Tech-nical Bulletin 1164. U.S. Government Print-ing Office. Washington, D.C. This is a refer-ence for the large decrease in organic mattercontent of Midwest soils.

Jenny, H. 1980. The Soil Resource. Springer-Verlag. New York, NY.

Jenny, H. 1941. Factors of Soil Formation.McGraw-Hill. New York, NY. Jenny’s earlywork on the natural factors influencing soilorganic matter levels.

Magdoff, F.R., and J.F. Amadon. 1980. Yieldtrends and soil chemical changes resultingfrom N and manure application to continu-ous corn. Agronomy Journal 72:161–164. Seethis reference for further information on thestudies in Vermont cited in this chapter.

National Research Council. 1989. AlternativeAgriculture. National Academy Press. Wash-ington, D.C.

Schertz, D.L., W.C. Moldenhauer, D.F. Franz-meier, and H.R. Sinclair, Jr. 1985. Field evalu-ation of the effect of soil erosion on crop pro-ductivity. pp. 9–17. In Erosion and Soil Pro-ductivity. Proceedings of the national sympo-sium on erosion and soil productivity. Dec.10–11, 1984. New Orleans, LA. American So-ciety of Agricultural Engineers Publication 8-85. St. Joseph, MI.

Tate, R.L., III. 1987. Soil Organic Matter: Biologi-cal and Ecological Effects. John Wiley & Sons.New York, NY.

41

6�

Let’s Get PhysicalSoil Tilth, Aeration, and Water

Moisture, warmth, and aeration; soil texture; soil fitness;

soil organisms; its tillage, drainage and irrigation;

all these are quite as important factors in the make up

and maintenance of the fertility of the soil as are manures,

fertilizers, and soil amendments.

—J.L. HILLS, C.H. JONES, AND C. CUTLER, 1908

A soil’s physical condition has a lot to do with its ability to produce crops. A de-

graded soil usually has reduced water infiltrationand percolation (drainage into the subsoil), aera-tion, and root growth. This reduces theability of the soil to supply nutrients,render harmless many hazardous com-pounds (such as pesticides), and tomaintain a wide diversity of soil or-ganisms. Small changes in a soil’sphysical conditions can have a largeimpact on these essential processes. Creating agood physical environment, which is a criticalpart of building and maintaining a healthy soil,requires a certain amount of attention and care.

Let’s first look at the physical nature of a typi-cal agricultural soil. It usually contains about 50percent solid particles and 50 percent pores ona volume basis (figure 6.1). We discussed earlierhow organic matter is only a small, but very im-

portant component of the soil. The rest of thesoil particles are a mixture of various size min-erals, ranging from fine-grained microscopic clayto easily visible large sand grains. The relative

amounts of the various particle sizesdefines the texture of a soil, such as aclay, clay loam, loam, sandy loam, orsand. Although management prac-tices don’t change this basic soil physi-cal property, they may modify the ef-fects of texture on other properties.

The sizes of the spaces (pores) between theparticles and between aggregates are much moreimportant than the sizes of the particles them-selves. The total amount of pore space and therelative quantity of various size pores (large, me-dium, small, very small) govern water move-ment and availability for sustaining soil organ-isms and plants. We are interested mostly in thepores, because that’s where all the important pro-


cesses, such as water and air movement, takeplace. Soil organisms live and function in thepores, which is also where plant roots grow. Mostpores in a clay loam are small (generally lessthan 0.0004 inch), whereas most pores in aloamy sand are large (generally still smaller than0.1 inch). Although soil texture doesn’t changeover time, the total amount of pore space andthe relative amount of various size pores (calledthe pore size distribution) are strongly affectedby management.

WATER AND AERATION

The soil pore space can be filled with either wateror air, and their relative amounts change as thesoil wets and dries (figure 6.1). When all poresare filled with water, the soil is saturated andsoil gases can’t exchange with the atmospheric

gases. This means that carbon dioxide from re-spiring roots and soil organisms can’t escapefrom the soil and oxygen can’t enter, leading toundesirable anaerobic (no oxygen) conditions.On the other extreme, a soil with little watermay have good gas exchange, but it can’t sup-ply sufficient water to plants and soil organisms.

The way in which a soil holds and releaseswater is pretty similar to the way it works with asponge (figure 6.2). When it’s fully saturated (youtake the sponge out of a bucket of water), a sponge

Soil water is a rich mixture

that contains nutrients and

microorganisms.

Figure 6.1 Distribution of solids and pores in soil.

organicmatter

mineralmatter

air

water

soil wets-upduring rain

soil dries down

poressolids

LET’S GET PHYSICAL 43

loses water by gravity, but will stop drippingwithin about 30 seconds. It’s only the largest poresthat lose water during that rapid drainage be-cause they are unable to hold the water againstgravity. The sponge still contains a lot of waterwhen it stops dripping. The remaining water isin the smaller pores, which hold it more tightly.The sponge’s condition following drainage is al-most the same as a soil reaching field capacitywater content, which occurs after about two daysof free drainage following saturation by a lot ofrain or irrigation. If a soil contains mainly largepores, like a coarse sand, it loses a lot of waterthrough gravitational drainage. This is good be-cause these pores are now open for aeration, butit’s also bad because little water remains for

plants to use, leading to frequent drought stress.Coarse sandy soils have very small amounts ofwater available to plants before they reach theirwilting point (figure 6.2a). On the other hand, adense, fine-textured soil, such as a compactedclay loam, has mainly small pores, which holdon tightly to water and don’t release it as freedrainage (it has little gravitational water, figure6.2b). In this case, the soil will have long peri-ods of poor aeration, but more plant-availablewater than a coarse sand. Leaching of pesticidesand nitrates to groundwater is also controlledby the relative amounts of different size pores.The rapidly draining sands lose these chemicalsalong with the percolating water, but this ismuch less of a problem with clays.

Figure 6.2 Water storage for three soils. (Shaded area represents water stored in soil that is available forplant use.)

unavailablewater

gravitationalwater

unavailablewater

plant-availablewater

gravitational water

plant-available

water

unavailablewater

wilting point field capacity

plant-availablewater

a) sand

b) dense clay

c) aggregated loam

gravitationalwater

soil water contentsaturation0


The ideal soil is somewhere between the twoextremes. This can be found in a well aggre-gated medium-textured loam soil (figure 6.2c,figure 6.3). Such a soil has enough large porespaces between the aggregates to provide ad-equate drainage and aeration during wet peri-ods, but also has adequate amounts of smallpores and water-holding capacity to providesufficient water to plants and soil organismsbetween rainfall or irrigation events. Besidesholding and releasing water well, such soils alsoallow for good water infiltration, thereby increas-ing plant water availability and reducing runoffand erosion. This ideal soil condition is indi-cated by crumb-like aggregates, which are com-mon in good topsoil.

GOOD SOIL TILTH ISGOOD AGGREGATION

Good aggregation, or structure, helps to make ahigh quality soil. Aggregation in the surface soilis favored by organic matter and surface resi-due. As pointed out earlier, a continuous sup-ply of organic materials, roots of living plants,and mycorrhizal fungi hyphae are needed tomaintain good soil aggregation. Surface residuesprotect the soil from wind and raindrops andmoderate the temperature and moisture ex-tremes at the soil surface. An unprotected soilmay reach very high soil temperatures at thesurface and become very dry. Worms and in-sects will move deeper into the bare soil, devel-oping a surface zone containing few active or-ganisms. Many small microorganisms, such asbacteria and fungi that live in thin films of wa-ter, will die or become inactive, slowing the natu-ral process of organic matter cycling. Large andsmall organisms function better in a soil that iswell protected by crop residue cover, a mulch,or a sod, which helps maintain good soil aggre-gation. An absence of both erosion and the forcesthat cause compaction helps maintain good sur-face aggregation.

WATER INFILTRATION, RUNOFF,AND EROSION

As rainfall reaches the ground, most water ei-ther infiltrates into the soil or runs off the sur-face (some may stand in ruts or depressionsbefore infiltrating or evaporating). The maxi-mum amount of rainwater that can enter a soilin a given time, called infiltration capacity, is in-fluenced by the soil type, soil structure, and thesoil moisture at the start of the rain. Early in a

Figure 6.3 A well aggregated soil has a range ofpore sizes. This medium size soil crumb is madeup of many smaller ones. Very large pores occurbetween the medium size aggregates.

small pore

large pore

intermediatepore

aggregate (crumb)


storm, water usually enters a soil readily. Whenthe soil becomes more moist, it can soak up lesswater. If rain continues, runoff is produced dueto the soil’s reduced infiltration rate. When anintense storm hits an already saturated soil, run-off occurs very rapidly, because the soil’s abilityto absorb water is low. Rainfall or snowmelt onfrozen ground generally poses even greater run-off concerns, as pores are blocked with ice. Run-off happens more readily with poorly structuredsoils, because they have fewer large pores toquickly conduct water downward. As soil tilthdegrades, water infiltration decreases, produc-ing more runoff.

Runoff water concentrates into tiny streams,which loosen soil particles and take them down-hill. As runoff water gains more energy, it scoursaway more soil. Runoff also carries agriculturalchemicals and nutrients, which end up instreams, lakes, and estuaries. Soil degradationin many of our agricultural and urban water-sheds has resulted in increased runoff duringintense rainfall and increased problems withflooding. Also, the lower infiltration capacity ofdegraded soils reduces the amount of water thatis available to plants, as well as the amount thatpercolates through the soil into undergroundaquifers. This underground water feeds streamsslowly. Watersheds with degraded soils experi-ence lower stream flow during dry seasons dueto low groundwater recharge and increasedflooding during times of high rainfall due to highrunoff.

Soil erosion is the result of exposing the soildirectly to the destructive energy of raindropsand wind. Soil erosion has long been known todecrease soil quality, and at the same time causesedimentation in downstream or downwind ar-eas. Soil is degraded when the best soil material

— the surface layer — is removed by erosion.Erosion also selectively removes the more eas-ily transported finer soil particles. Severelyeroded soils, therefore, become low in organicmatter and have less favorable physical proper-ties, leading to a reduced ability to sustain cropsand increased potential for harmful environmen-tal impacts.

Some ancient farming civilizations recognizedsoil erosion as a problem and developed effec-tive methods for runoff and erosion control.Evidence of ancient terracing methods are ap-parent in various parts of the world, notably inthe Andean region of South America and inSoutheast Asia. Other cultures effectively con-trolled erosion using mulching and intercrop-ping, thereby protecting the soil surface fromthe elements. (Some ancient desert civilizations,such as the Anasazi in the Southwestern U.S.and the Nabateans in the Middle East, took ad-vantage of surface runoff to harvest water togrow crops in downhill depressions. Their meth-ods, however, were specific to very dry condi-tions.) For most agricultural areas of the worldtoday, water and wind erosion still cause exten-sive damage (including the spread of deserts)

For most agricultural areas of the

world today, water and wind erosion

still cause extensive damage

(including the spread of deserts)

and remain the greatest threat to

agricultural sustainability.


and remain the greatest threat to agriculturalsustainability.

Tillage degrades land even beyond promot-ing water and wind erosion by exposing soil tothe elements. It can also cause erosion by di-rectly moving soil down the slope to lower ar-eas of the field. In complex topographies — suchas seen in figure 6.4 — this ultimately results inthe removal of surface soil from the knolls andits deposition in depressions (swales) at the bot-tom of the slopes. What causes this type of till-age erosion? Gravity causes more soil to bemoved by the plow or harrow down slope thanup slope. Soil is thrown farther down slope whentilling in the down slope direction than is thrownuphill when tilling in the up slope direction (fig-ure 6.5a). Down slope tillage typically occurs atgreater speed than when traveling uphill, mak-ing the situation even worse. Tillage along the

contour also results in down slope soil move-ment. Soil lifted by a tillage tool comes to rest ata slightly lower position on the slope (figure6.5b). A more serious situation occurs whenusing a moldboard plow along the contour. Thisis typically performed by throwing the soil downthe slope, as better inversion is obtained, thanby trying to turn the furrow up the slope (figure6.5c).

Tillage causes soil to move downhill.

Soil loss from slopes due to tillage erosioncan far exceed losses from water or wind ero-sion. On the other hand, tillage erosion doesnot generally result in off-site damage, becausethe soil is merely moved from higher to lower

Figures 6.4 Effects of tillage erosion on soils. Photo by NRCS.


positions within a field. However, it is anotherreason to reduce tillage on sloping fields!

SOIL COMPACTION

A soil becomes more compact, or dense, whenaggregates or individual particles of soil areforced closer together. Soil compaction has vari-ous causes and different visible effects. Threetypes of soil compaction may occur (figure 6.6):

� surface crusting� plow layer compaction� subsoil compaction

Surface crusting occurs when soil is unpro-tected by surface residue or a plant canopy andthe energy of raindrops disperses wet aggregates,pounding them together into a thin, but dense,surface layer. The sealing of the soil reduceswater infiltration and the surface forms a hardcrust when dried. If the crusting occurs soonafter planting, it may delay or, in some cases,prevent seedling emergence. Even when thecrust is not severe enough to limit germination,it can reduce water infiltration. Soils with sur-face crusts are prone to high rates of runoff anderosion (see figure 4.4 in chapter 4). You can

Figure 6.5 Three causes of erosion resulting from tilling soils on slopes.

region of soil loss region of soil accumulation

a. up-and-downhill tillage

b. tillage (chisel, disc, etc.) along contour

c. plowing along contour, throwing furrow downhill

� �

��


reduce surface crusting by leaving more resi-due on the surface and maintaining strong soilaggregation.

Plow layer compaction — compaction of theentire surface layer — has probably occurred tosome extent in all intensively worked agricul-tural soils. There are three primary causes forsuch compaction — erosion, reduced organicmatter levels, and forces exerted by field equip-ment. The first two result in a reduced supplyof sticky binding materials and a subsequent lossof aggregation.

Compaction of soils by heavy equipment andtillage tools is especially damaging when soilsare wet. To understand this, we need to know alittle about soil consistence, or how soil reacts toexternal forces. At very high water content, asoil may behave like a liquid (figure 6.7) andsimply flow as a result of the force of gravity —

as with mudslides during excessively wet peri-ods. At slightly lower water contents, soil canbe easily molded and is said to be plastic. Uponfurther drying, the soil will become friable — itwill break apart rather than mold under pres-sure.

The point between plastic and friable soil,the plastic limit, has important agricultural im-plications. When a soil is wetter than the plas-tic limit, it is seriously compacted if tilled ortraveled on, because soil aggregates are pushedtogether into a smeared, dense mass. This is whyyou often see smeared cloddy furrows or deeptire ruts in a field (figure 6.8). When the soil isfriable (the water content is below the plasticlimit) it breaks apart when tilled and aggregatesresist compaction by field traffic. This is whythe potential for compaction is so strongly in-fluenced by timing of field operations.

Figure 6.6 Plants growing in a) soil with good tilth and b) soil with all three types of compaction.

porous(loose-fitting)crumbs and

blocks)

surface crust

germinatingseed

tightly packedcrumbs

large blockswith few

cracks

subsoilcompaction

a) good soil structure b) compacted soil


A soil’s consistency is strongly affected by itstexture (figure 6.7). For example, as coarse-tex-tured sandy soils drain, they rapidly change frombeing plastic to friable. Fine-textured loams andclays need longer drying periods to lose enoughwater to become friable. This extra drying timedelays field operations.

Surface crusting and plow layer compactionare especially common with intensively tilledsoils. This is often part of a vicious cycle in whicha compacted soil tills up very cloddy (figure6.9a), and then requires extensive secondary till-age and packing trips to create a satisfactoryseedbed (figure 6.9b). Natural aggregates breakdown and organic matter decomposes in theprocess — contributing to more compaction inthe future. Although the final seedbed may beideal at the time of planting, rainfall shortly af-ter planting may cause surface sealing and fur-ther settling (figure 6.9c), because few sturdy

Figure 6.8 Deep tire ruts in a hay field after liquidmanure was applied when soil was wet and plastic.

aggregates are present to prevent the soil fromdispersing. The result is a dense plow layer witha crust at the surface. Some soils may hardsetlike cement, even after the slightest drying, slow-ing plant growth. Although the soil becomessofter when it re-wets, this provides only tem-porary relief to plants.

Figure 6.7 Soil consistency states for a sand and a clay soil (friable soil is best for tillage).

sand

loose friable

plastic limit

clay

hard friable plastic

liqui

d

0 soil water content saturation


Subsoil compaction — compacted soil belowthe normally tilled surface layer — is usuallycalled a plow pan, although it’s commonly causedby more than just plowing. Subsoil is easily com-pacted, because it is usually wetter and tends tobe naturally dense, higher in clay content, lowerin organic matter, and have naturally lower ag-gregation than topsoil. Also, subsoil is not loos-ened by regular tillage and cannot easily beamended with additions of organic materials, soits compaction is more difficult to manage.

Subsoil compaction is the result of either di-rect loading or the transfer of forces of compac-tion from the surface. Direct loading occurs bythe pressure of a tillage implement, especially aplow or disk, pressing on the subsoil. It alsooccurs when a field is moldboard plowed and aset of tractor wheels is placed in the open fur-row, thereby directly compacting the soil belowthe plow layer (figure 6.10). Subsoil compac-tion also occurs when farmers run heavy vehicleswith poor weight distribution. The load exertedon the surface is transferred into the soil alonga cone-shaped pattern (figure 6.11). With in-creasing depth, the force of compaction is dis-tributed over a larger area, thereby reducing thepressure in deeper layers. When the loadingforce at the surface is small, say through footFigure 6.9 Three tilth stages for a compacted soil.

Check Before Tilling

To be sure that a soil is ready for equipmentuse, you can do the simple “ball test” by tak-ing a handful of soil from the lower part ofthe plow layer and trying to make a ball out ofit. If it molds easily and sticks together, thesoil is too wet. If it crumbles readily, it is suf-ficiently dry for tillage or heavy traffic.

a) Stage 1: Cloddy soil after tillage makesfor a poor seedbed.

b) Stage 2: Soil is packed and pulverized tomake a fine seedbed.

c) Stage 3: Raindrops disperse soil aggregates,forming a surface crust.


Figure 6.10 Tractor wheels in open furrow during plowing compacts subsoil.

Figure 6.11 Forces of heavy loads are transferred deep into the soil, especially when wet.

depth of tillage

dry soil wet soil


traffic or a light tractor, the pressure exertedbelow the plow layer is minimal. But when theload is high, the pressures at depth are suffi-cient to cause considerable soil compaction.When the soil is wet, compaction forces nearthe surface are more easily transferred to thesubsoil. Clearly, the most severe compactiondamage to subsoils occurs by heavy vehicle traf-fic during wet conditions.

CONSEQUENCESOF COMPACTION

As compaction pushes soil particles closer to-gether, the soil becomes more dense and porespace is lost. When the bulk density increasesduring compaction, mainly larger pores areeliminated. Loss of aggregation from compac-tion is particularly harmful for fine and medium-textured soils that depend on these pores forgood infiltration and percolation of water, as wellas air exchange with the atmosphere. Althoughcompaction can damage coarse-textured soils,these soils depend less on aggregation, becausepores between many of their particles are suffi-ciently large to allow good water and air move-ment. Compacted soil becomes hard when driedand can restrict root growth and the activity ofsoil organisms. The resistance to penetration,called soil strength, for a moist, high-qualitysoil is well-below the critical level (300 poundsper square inch (psi)), when root growth ceasesfor most crops. As the soil dries, its strength in-creases, but may not exceed the critical level formost (or all) of the moisture range. A compactedsoil has a very narrow water content range forgood root growth. It’s harder in the wet range— where it may even be above the critical level,

depending on the severity of compaction. Whenit dries, a compacted soil hardens quicker thana well-structured soil, rapidly reaching a hard-ness well above the 300 psi level that restrictsroot growth.

Actively growing roots need pores with di-ameters greater than about 0.005 inch, the sizeof most root tips. Roots must enter the pore andanchor themselves before continuing growth.Compacted soils that have few or no large poresdon’t allow plants to be effectively rooted — lim-iting growth and water and nutrient uptake.

What happens when root growth is limited?The root system will probably have short thickroots and few fines ones or root hairs (see figure6.6). The few existing thick roots are able tofind some weak zones in the soil, often by fol-lowing crooked patterns. These roots have thick-ened tissue and are not efficient at taking upwater and nutrients. In many cases, roots indegraded soils do not grow below the tilled layerinto the subsoil (see figure 6.6) — it’s just toodense and hard for them to grow. Deeper root

Some Crops More SensitiveThan Others

Compaction doesn’t affect all crops to thesame extent. An experiment in New Yorkfound that direct-seeded cabbage and snapbeans were more harmed by compactionthan cucumbers, table beets, sweet corn, andtransplanted cabbage. Much of the compac-tion damage was caused by secondary ef-fects, such as prolonged soil saturation afterrain, reduced nutrient availability or uptake,and greater pest problems.


penetration is especially critical under rain-fedagriculture. The limitation on deep root growthby subsoil compaction increases the probabilityof yield losses from drought stress.

There is a more direct effect on plant growthbeyond reduced root growth that limits the vol-ume of the soil that is used for water and nutri-ent supply. A root system that’s up against me-chanical barriers sends a chemical signal to theplant shoot, which then slows down respirationand growth. This seems to be a natural survivalmechanism similar to when plants experiencewater stress. In fact, because some of the samehormones are involved — and mechanical re-sistance increases when the soil dries — youusually can’t separate the effects of compactionfrom those of drought.

THE WATER RANGEFOR BEST PLANT GROWTH

The limitations to plant growth caused by com-paction and water extremes can be combinedinto the concept of the optimum water range forplant growth — the range of water contents forwhich neither drought, mechanical stress, norlack of aeration reduces plant growth (figure6.12). This range, referred to by scientists as theleast-limiting water range, is bounded on twosides — when the soil is too wet and when it’stoo dry.

The optimum water range in a well-struc-tured soil has its field capacity on the wet end,as water above this water content readily drainsout by gravity. On the dry end is the wilting

Figure 6.12 The optimum water range for crop growth for two different soils.

well-structured soil poor aeration

droughtstress

optimum water range

compacted soil

root resistance optimumwater range

pooraeration

very dry

field capacity

soil water contentsaturation


point — beyond which the soil holds water tootightly to be used by plants. However, the soilwater range for best growth in a compacted soilis much narrower. A severely compacted soil atfield capacity is still too wet because it lacks largepores and is poorly aerated even after the soildrains. Good aeration requires about 20 percentof the pore space (about 10 percent of the vol-ume of the whole soil) to be air-filled. On thedry end, plant growth in a compacted soil iscommonly limited by soil hardness rather thanby lack of available water. Plants in compactedsoils experience more stress during both wet anddry periods than plants in soils with good tilth.The effects of compaction on crop yields usu-ally depend on the length and severity of exces-

sive wet or dry periods and when those periodsoccur relative to critical times for plant growth.

SOURCES

Hillel, D. 1991. Out of the Earth: Civilization andthe Life of the Soil. University of CaliforniaPress. Berkeley, CA.

Letey, J. 1985. Relationship between soil physi-cal properties and crop production. Advancesin Soil Science 1:277–294.

Ontario Ministry of Agriculture, Food, and Ru-ral Affairs (OMFARA). 1997. Soil Manage-ment. Best Management Practices Series.Available from the Ontario Federation ofAgriculture, Toronto, Ontario (Canada).

da Silva, A.P., B.D. Kay, and E. Perfect. 1994.Characterization of the least limiting waterrange of soils. Soil Science Society of AmericaJournal 58:1775–1781.

Unger, P.W., and T.C. Kaspar. 1994. Soil com-paction and root growth: a review. AgronomyJournal 86:759–766.

Soehue, W. 1958. Fundamentals of pressure dis-tribution and soil compaction under tractortires. Agricultural Engineering 39:276–290.

Plants in compacted soils

experience more stress during both

wet and dry periods than plants

in soils with good tilth.

55

We used the term cycle earlier when discuss-ing the flow of nutrients from soil to plant

to animal to soil, as well as global carbon andnitrogen cycles (chapter 4). Some farmers de-pend more on natural soil nutrientcycles — as contrasted with purchasedcommercial fertilizers — to providefertility to plants. Is it really possibleto depend forever on the natural cy-cling of all the nutrients the cropneeds? Let’s first consider what a cyclereally is and how it differs from the other waysthat nutrients move from one location to an-other.

When nutrients move from one place to an-other, that is a flow. There are many differenttypes of nutrient flows that can occur. Whenyou buy fertilizers or animal feeds, nutrients are“flowing” onto the farm. When you sell sweetcorn, apples, alfalfa hay, or milk, nutrients are

7�

Nutrient Cycles and Flows

Increasingly…emphasis is being laid on the

direction of natural forces, on the conservation

of inherent richness, on the acquirement of plant

food supplies from the air and subsoil.


“flowing” off the farm. Flows that involve prod-ucts entering or leaving the farm gate are man-aged intentionally, whether or not you are think-ing about nutrients. Other flows are unplanned:

when nitrate is lost from the soil byleaching to groundwater or when run-off waters take nutrients along witheroded topsoil to a nearby stream.

When crops are harvested andbrought to the barn to feed animals,that is a nutrient flow, as is the return

of animal manure to the land. Together thesetwo flows are a true cycle, because nutrientsreturn to the fields from which they came. Inforests and natural grassland, the cycling of nu-trients is very efficient. In the early stages ofagriculture, where almost all people lived neartheir fields, nutrient cycling was also efficient(figure 7.1a). However, in many types of agri-culture, especially modern “industrial-style”


farming, there is little real cycling of nutrients,because there is no easy way to return nutrientsshipped off the farm. In addition, nutrients incrop residues don’t cycle very efficiently whenthe soil is without living plants for long peri-ods, and nutrient runoff and leaching losses aremuch larger than from natural systems.

The first major break in the cycling of nutri-ents occurred as cities developed and nutrientsbegan to routinely travel with the farm prod-ucts to feed the growing urban populations. Fewnutrients now return to the soils that grew themmany miles away (figure 7.1b, 7.1c). The accu-mulated nutrients in urban sewage have pollutedwaterways around the world. Even with thebuilding of many new sewage treatment plantsin the 1970s and 1980s, effluent containingnutrients still flows into waterways, and sewagesludges are not always handled in an environ-mentally sound manner.

The trend to farm specialization has resultedin the second break in nutrient cycling by sepa-rating animals from the land that grows theirfeed. With specialized animal facilities (figure7.1c), nutrients accumulate in manure at thesame time that crop farmers purchase largequantities of fertilizers to keep their fields frombecoming nutrient deficient.

DIFFERING FLOW PATTERNS

Different types of farms may have distinctly dif-ferent nutrient flow patterns. Farms that areexclusively growing grain or vegetables have arelatively high annual nutrient export (figure7.2a). Nutrients usually enter the farm as eithercommercial fertilizers or various amendmentsand leave the farm as plant products. Some cy-cling of nutrients occurs as crop residues arereturned to the soil and decompose. A large

Figure 7.1 The patterns of nutrient flows change over time. From Magdoff et al., 1997.

a) early agriculture

humans

primaryconsumers(animals)


primary producers(plants)

b) urbanizing society(mid 19th to mid 20th century)

humans

c) industrial agriculture(mid- to late 20th century)

humans



nutrients nutrients

energy andnutrients


NUTRIENT CYCLES AND FLOWS 57

nutrient outflow is common, however, becausea large portion of the crop is usually exportedoff the farm. For example, an acre of a good cropof tomatoes or onions usually contains over 100lbs. of nitrogen, 20 lbs. of phosphorus, and 100lbs. of potassium. For agronomic crops, the an-nual exports of nutrients is about 100 lbs. of

nitrogen, 6 lbs. of phosphorus, and 50 lbs. ofpotassium per acre for corn grain and about 150lbs. of nitrogen, 20 lbs. of phosphorus and 130lbs. of potassium per acre for grass hay.

It should be fairly easy to balance inflows andoutflows on crop farms, at least theoretically. Inpractice, under good management, nutrients are

Figure 7.2 Nutrient flows and cycles on a) crop and b) dairy farms (larger flows indicated by thicker lines).

a) vegetable or agronomic crop farm

fertilizersand lime crop residues

farm growncrops

crops

leaching,runoff, and

volatilization

b) dairy farm

feeds andminerals,bedding,fertilizers,and lime

farm growncrops

crop residuesand manures

soil

milkandmeat

leaching,runoff, and

volatilization

mineralization release from CEC

desorption and dissolutionfrom minerals

soil

mineralizationrelease from CEC

desorption and dissolutionfrom minerals


depleted a bit by crop growth and removal un-til soil test levels fall too low, and then they’reraised again with fertilizers or manures (seechapter 19).

A contrasting situation occurs on dairy farms,if all of the forage is produced on the farm, butgrains and minerals are purchased (figure 7.2b).In this situation, there are more sources for thenutrients coming onto the farm — with feedsand minerals for animal consumption usually alarger source than fertilizers. Most of the nutri-ents consumed by animals end up in the ma-nure — 60 to over 90 percent of the nitrogen,phosphorus, and potassium. Compared withcrop farms, more nutrients flow onto many dairyfarms and fewer flow off per acre. Under thissituation, nutrients will accumulate on the farmand may eventually cause environmental harmfrom excess nitrogen or phosphorus.

Two different nutrient flows occur whenmanure on livestock farms is applied to the fieldsused for growing the feeds. The nutrients in themanure that came from farm-grown feed sourcesare completing a true cycle. The nutrients in themanure that entered the farm as purchased feedsand mineral supplements are not participatingin a true cycle. These nutrients are completinga flow that might have started in a far-away farmor mine and are now just being transported fromthe barn to the field.

Animal operations that import all feeds andthat have a limited land base to use the manurehave the greatest potential to accumulate highamounts of nutrients. Contract growers of chick-ens are an example of this practice.

If there is enough cropland to grow most ofthe grain and forage needs, low amounts of im-ported nutrients and export per acre will result.The relatively low amounts of nutrients exported

per acre from animal products makes it easierto rely on nutrient cycling on a mixed livestock-crop farm that produces most of its feed, thanon a farm growing only crops.

IMPLICATIONS OFNUTRIENT FLOW PATTERNS

Long distance transportation of nutrients is cen-tral to the way in which the modern food sys-tem functions. On average, the food we eat hastraveled about 1,300 miles from field to proces-sor to distributor to consumer. Exporting wheatfrom the U.S. Pacific Northwest to China in-volves an even longer distance, as does importof apples from New Zealand to New York. Thenutrients in concentrated commercial fertilizersalso travel large distances from the mine or fac-tory to distributors to the field. The specializa-tion of the corn and soybean farms of the Mid-west and the hog and chicken mega-farms cen-tralized in a few regions, such as Arkansas, theDelmarva Peninsula and North Carolina, hascreated a unique situation. The long distanceflows of nutrients from crop farms to animalfarms requires the purchase of fertilizers on thecrop farms; meanwhile, the animal farms areoverloaded with nutrients.

Of course, the very purpose of agriculture inthe modern world — the growing of food andfiber and the use of the products by people livingaway from the farm — results in a loss of nutri-ents from the soil, even under the best possiblemanagement. In addition, leaching losses of nu-trients, such as calcium, magnesium, and potas-sium, are accelerated by natural acidification, aswell as by acidification caused by the use of fer-tilizers. Soil minerals — especially in the “young”soils of glaciated regions and in arid regions not

NUTRIENT CYCLES AND FLOWS 59

subject to much leaching — may supply lots ofphosphorus, potassium, calcium, and magne-sium and many other nutrients. A soil with plen-tiful active organic matter also may supply nu-trients for a long time. Eventually, however, nu-trients will need to be applied to a continuallycropped soil. Nitrogen is the only nutrient youcan “produce” on the farm — legumes and theirbacteria working together can remove nitrogenfrom the atmosphere and change it into formsthat plants can use. However, sooner or lateryou will need to apply some phosphorus or po-tassium, even to the richest soils. If the farm isin a mixed crop-livestock system that exportsonly animal products, it may take a very longtime to deplete a rich soil, because so few nutri-ents per acre are exported with those products.For crop farms, especially in humid regions, thedepletion occurs more rapidly, because more nu-trients are exported per acre each year.

Three Different Flow Patterns

There are three main nutrient flow patterns, witheach one having implications for the long-termfunctioning of the farm. Imports of nutrientsmay be less than exports, imports may be greaterthan exports, and imports may equal exports.

Imports are less than exports. For farms “liv-ing off capital” and drawing down the suppliesof nutrients from minerals and organic matter,nutrient concentrations continually decline. Thiscan continue for awhile, just like a person cancontinue to live off savings in a bank accountuntil the money runs out. At some point, theavailability of one or more nutrients becomesso low that crop yields decrease. If this condi-tion is not remedied, the farm becomes less andless able to produce food and its economic con-dition will decline. This is clearly not a desir-able situation for either the farm or the country.Unfortunately, the low productivity of much ofAfrica’s agricultural lands is partially caused bythis type of nutrient flow pattern.

Imports are much larger than exports. Ani-mal farms with inadequate land bases pose adifferent type of problem. As animal numbersincrease, relative to the available cropland andpasture, larger purchases of feeds (containingnutrients) are necessary. As this occurs, there isless land available — relative to the nutrientloads — to spread manure. Ultimately, the op-eration exceeds the capacity of the land to as-similate all the nutrients and pollution of groundand surface waters occurs. This pattern of nu-trient flow is not environmentally acceptable.However, under current conditions, it may bemore economical than a more balanced pattern.

The issue eventually becomes not whethernutrients will be imported onto the farm, butrather, what source of nutrients you should use.Will the nutrients brought onto the farm be com-mercial fertilizers, traditional amendments(limestone), biologically fixed nitrogen, im-ported feeds or minerals for livestock, organicmaterials, such as manures, composts and slud-ges, or some combination of sources?

On average, the food we eat

has traveled about 1,300 miles

from field to processor to

distributor to consumer.


Imports and exports are close to balance.From the environmental perspective and for thesake of long-term soil health, fertility should beraised to — and then maintained at — optimallevels. The best way to keep desirable levels oncethey are reached is to roughly balance inflowsand outflows. Soil tests can be very helpful tofine-tune a fertility program and make sure thatlevels are not building up too high or beingdrawn down too low (see chapter 19). This canbe a challenge and may not be economically pos-sible for all farms. This is easier to do on a mixedcrop-livestock farm than on either a crop farm

or a livestock farm that depends significantlyon imported feeds.

SOURCES

Magdoff, F., L. Lanyon, and W. Liebhardt. 1997.Nutrient cycling, transformations, and flows:Implications for a more sustainable agricul-ture. Advances in Agronomy 60: 1–73.

Magdoff, F., L. Lanyon, and W. Liebhardt. 1998.Sustainable Nutrient Management: A Role forEveryone. Northeast Region Sustainable Ag-riculture Research and Education Program.Burlington, VT.

P A R T T W O

EcologicalSoil and CropManagement

63

Building high-quality soils takes a lot ofthought and action over many years. Of

course, there are things that can be done rightoff — plant a cover crop this fall or just make aNew Year’s resolution not to work soilsthat really aren’t ready in the spring(and then stick with it). Other changestake more time. You need to study care-fully before drastically changing croprotations, for example. How will thenew crops be marketed and are the nec-essary labor and machinery available?

There are three different general managementapproaches to enhancing soil health. First, vari-ous practices to build up and maintain high lev-els of soil organic matter are key. Second, devel-oping and maintaining the best possible soilphysical condition often requires other types ofpractices, in addition to those that directly im-pact soil organic matter. Paying better attention

8�

Managing for High Quality Soils:Organic Matter, Soil Physical Condition,

Nutrient Availability

Because organic matter is lost from the soil through decay,

washing, and leaching, and because large amounts are

required every year for crop production, the necessity of

maintaining the active organic-matter content of the soil, to

say nothing of the desirability of increasing it on many

depleted soils, is a difficult problem.

—A. F. GUSTAFSON, 1941

to soil tilth and compaction is more importantthan ever, because of the use of very heavy fieldmachinery. Lastly, although good organic mattermanagement goes a long way toward providing

good plant nutrition in an environmen-tally sound way, good nutrient manage-ment involves additional practices.

ORGANIC MATTERMANAGEMENT

It is difficult to be sure exactly why problemsdevelop when organic matter is depleted in aparticular soil. However, even in the early 20thcentury, agricultural scientists proclaimed,“Whatever the cause of soil unthriftiness, thereis no dispute as to the remedial measures. Doc-tors may disagree as to what causes the disease,but agree as to the medicine. Crop rotation! Theuse of barnyard and green manuring! Humus


maintenance! These are the fundamental needs”(Hills, Jones, and Cutler, 1908). Close to a cen-tury later, these are still some of the major rem-edies available to us.

There seems to be a contradiction in our viewof soil organic matter. On one hand, we wantcrop residues, dead microorganisms, and ma-nures to decompose. If soil organic matterdoesn’t decompose, then no nutrients are madeavailable to plants, no glue to bind particles ismanufactured, and no humus is produced tohold on to plant nutrients as water leachesthrough the soil. On the other hand, numerousproblems develop when soil organic matter issignificantly depleted through decomposition.This dilemma of wanting organic matter to de-compose, but not wanting to lose too much,means that organic materials must be continu-ally added to the soil. A supply of active organicmatter must be maintained so that humus cancontinually accumulate. This does not mean thatorganic materials must be added to each fieldevery year. However, it does mean that a fieldcannot go without additions of organic residuesfor many years without paying the consequences.

Do you remember that plowing a soil is simi-lar to opening up the air intake on a wood stove?What we really want in soil is a slow, steadyburn of the organic matter. You get that in awood stove by adding wood every so often and

making sure the air intake is on a medium set-ting. In soil, you get a steady burn by addingorganic residues regularly and by not disturb-ing the soil too often.

There are three general management strate-gies for organic matter management. First, usecrop residues more effectively and find newsources of residues to add to soils. New residuescan include those you grow on the farm, suchas cover crops, or those available from variouslocal sources. Second, be sure to use a numberof different types of materials — crop residues,manures, composts, cover crops, leaves, etc. Itis important to provide varied residue sourcesto help develop and maintain a diverse groupof soil organisms. Third, implement practicesthat decrease the loss of organic matter from soilsbecause of accelerated decomposition or erosion.

Soil Organic Matter Levels

Raising and maintaining soil organic mat-ter levels. It is not easy to dramatically increasethe organic matter content of soils or even tomaintain good levels once they are reached. Im-proving organic matter content requires a sus-tained effort that includes a number of ap-proaches to return organic materials to soils andminimize soil organic matter losses. It is espe-cially difficult to raise the organic matter con-tent of soils that are very well aerated, such ascoarse sands, because added materials are de-composed so rapidly. Soil organic matter levelscan be maintained with less organic residue inhigh clay-content soils with restricted aerationthan in coarse-textured soils.

All practices that help to build organic mat-ter levels do at least one of two things — addmore organic materials than was done in the pastor decrease the rate of organic matter loss from

Soil Organic MatterManagement Strategies

� Increase additions of organic residues tosoils.

� Use varied sources of organic materials.� Decrease losses of organic matter from soils.

MANAGING FOR HIGH QUALITY SOILS 65

soils (table 8.1). Those practices that do bothmay be especially useful. Practices that reducelosses of organic matter either slow down therate of decomposition or decrease the amountof erosion. Soil erosion must be controlled tokeep organic matter-enriched topsoil in place.In addition, organic matter added to a soil musteither match or exceed the rate of loss by de-composition. These additions can come frommanures and composts brought from off thefield, crop residues and mulches remaining fol-lowing harvest, or cover crops. Reduced tillagelessens the rate of organic matter decomposi-tion and also may result in less erosion. Whenreduced tillage increases crop growth and resi-dues returned to soil, it is usually a result of bet-ter water infiltration and storage and less sur-face evaporation. It is not possible in this bookto give specific soil organic matter managementrecommendations for all situations. In chapters9 through 15, we will evaluate management op-tions and issues associated with their use.

How much organic matter is enough? Un-like the case with plant nutrients or pH levels,there are no accepted guidelines for organicmatter content. We do know some generalguidelines. For example, 2 percent organic mat-ter in a sandy soil is very good, but in a claysoil, 2 percent indicates a greatly depleted situ-ation. The complexity of soil organic mattercomposition, including biological diversity oforganisms as well as the actual organic chemi-cals present, means that there is no simple in-terpretation for total soil organic matter tests.

Using Organic Materials

Crop residues. Crop residues are usually thelargest source of organic materials available tofarmers. The amount of crop residue left afterharvest varies depending on the crop. Soybeans,potatoes, lettuce and corn silage leave little resi-due. Small grains, on the other hand, leave moreresidue, while sorghum and corn harvested for

TABLE 8.1Effects of Different Management Practices

on Gains and Losses of Organic Matter

GAINS LOSSES

MANAGEMENT PRACTICE INCREASES DECREASES

� Add materials from off the field yes no(manures, composts, other organic materials)

� Better utilize crop residue yes no

� Include high residue producing crops in rotation yes no

� Include sod crops (grass/ legume forages) in rotation yes yes

� Grow cover crops yes yes

� Reduce tillage intensity yes/no* yes

� Use conservation practices to reduce erosion yes/no* yes

* practice may increase crop yields, resulting in more residue


grain leave the most. A ton or more of crop resi-dues per acre may sound like a lot of organicmaterial being returned to the soil. However,keep in mind that after residues are decomposedby soil organisms only about 10 to 20 percentof the original amount is converted into stablehumus.

The amount of roots remaining after harvestalso can range from very low to fairly high. Fora crop of corn, roots may account for over a tonof dry weight per acre (thus more than 41/2 tonsof surface residues plus roots — about 60 per-cent of the total plant — remain following aMidwest grain harvest of about 120 bu. per acre).

The estimated root residues (from Prince Ed-ward Island in Canada) give some idea of thedifferences that you might find (table 8.2).

Some farmers remove above ground residuesfrom the field for use as animal bedding or tomake compost. Later, these residues return tocontribute to soil fertility as manures or com-posts. Sometimes, residues are removed fromfields, to be used by other farmers or to makeanother product. There is renewed interest inusing crop residues as a wood substitute to makea variety of products, such as particleboard. Thisactivity could cause considerable harm becauseresidues are not returned to soils.

Crop Residues

The amount of residue left in the field after harvest depends on the type of crop and its yield. Thetable on the left contains the amounts of residues found in California’s highly productive, irrigated SanJoaquin Valley. These residue amounts are higher than would be found on most farms, but the relativeamounts for the various crops are interesting.

Residues of Common Crops in theMidwest and Great Plains

CROP TONS/ACRE

Corn (120 bu.) 31/2

Sorghum (80 bu.) 21/2

Wheat (35 bu.) 2

Soybeans (35 bu.) less than 1

—FROM VARIOUS SOURCES

Crop Residues in theSan Joaquin Valley (California)

CROP TONS/ACRE

Corn (grain) 5

Broccoli 3

Cotton 21/2

Wheat (grain) 21/2

Sugarbeets 2

Safflower 11/2

Tomatoes 11/2

Lettuce 1

Corn (silage) 1/2

Garlic 1/2

Wheat (after baling) 1/4

Onions 1/4

—MITCHELL ET AL., 1999


Burning of wheat, rice, and other crop resi-dues in the field is a common practice in parts ofthe United States as well as in other countries.Residue is usually burned to help control insectsor diseases or to make next year’s fieldwork easier.Residue burning may be so widespread in a givenarea that it causes a local air pollution problem.Burning also diminishes the amount of organicmatter returned to the soil and the amount ofprotection against raindrop impact.

Sometimes, important needs for crop residuesand manures may prevent their use in main-taining or building soil organic matter. For ex-ample, straw may be removed from a grain fieldto serve as mulch in a strawberry field. Thesetrade-offs of organic materials can sometimescause a severe soil-fertility problem if allowedto continue for a long time. This issue is of muchmore widespread importance in developingcountries where resources are scarce. There, cropresidues and manures frequently serve as fuelfor cooking or heating when gas, coal, oil, orwood are not available. In addition, straw maybe used in making bricks or used as thatch for

housing or to make fences. Although it is com-pletely understandable that people in resource-poor regions use residues for such purposes, thenegative effects of these uses on soil productiv-ity can be substantial. An important way to in-crease agricultural productivity in developingcountries is to find alternative sources for fueland building materials to replace the crop resi-dues and manures traditionally used.

Using residues as mulches. Crop residues orcomposts can be used as a mulch on the soilsurface. This occurs routinely in some reducedtillage systems when high residue-yielding cropsare grown or when killed cover crops remainon the surface. In some small-scale vegetableand berry farming, mulching is done by apply-ing straw from off-site. Strawberries grown inthe colder northern parts of the country are rou-tinely mulched with straw for protection fromwinter heaving. The straw is blown on in latefall and is then moved into the interrows in thespring, providing a surface mulch during thegrowing season.

Mulching has numerous benefits, including:

� enhanced water availability to crops (betterinfiltration into the soil and less evaporationfrom the soil);

� weed control;� less extreme changes in soil temperature;� reduced splashing of soil onto leaves and

fruits and vegetables (making them lookbetter as well as reducing diseases); and

� reduced infestations of certain pests (Colo-rado potato beetle on potatoes is less severewhen potatoes are grown in a mulch system).

On the other hand, residue mulches in coldclimates can delay soil warming in the spring,

TABLE 8.2Estimated Root Residue

Produced by Crops

ESTIMATED

ROOT RESIDUES

CROP (LBS./ACRE)

Italian ryegrass 2,600–4,500

Winter cereal 2,200–2,600

Red clover 2,200–2,600

Spring cereal 1,300–1,800

Soybeans 500–900

Potatoes 300–600

—TOPP ET AL., 1995


reduce early season growth, and increase prob-lems with slugs during wet periods. Of course,one of the reasons for the use of plastic mulches(clear and black) for crops like tomatoes andmelons is to help warm the soil.

Effects of ResidueCharacteristics on Soil

Decomposition rates and effects on aggre-gation. Residues of various crops and manureshave different properties and, therefore, havedifferent effects on soil organic matter. Materi-als with low amounts of hard-to-degrade hemi-cellulose and lignin, such as cover crops whenstill very green and soybean residue, decomposerapidly (figure 8.1) and have a shorter-term ef-fect on soil organic matter levels than residueswith high levels of these chemicals (for example,corn and wheat). Manures, especially those that

contain lots of bedding (high in hemicelluloseand lignin), are decomposed more slowly andtend to have more long-lasting effects on totalsoil organic matter than crop residues or ma-nures without bedding. Also, cows — becausethey eat a diet containing lots of forages, whichthey do not completely decompose — havemanure with longer lasting effects on soils thannon-ruminants, such as chickens and hogs, thatare fed exclusively a high-grain/low-fiber diet.Composts contribute little active organic mat-ter to soils, but add a lot of well decomposedmaterials (figure 8.1).

In general, residues containing a lot of cellu-lose and other easy-to-decompose materials willhave a greater effect on soil aggregation thancompost, which has already undergone decom-position. Because aggregates are formed fromby-products of decomposition by soil organisms,organic additions like manures, cover crops, and

Figure 8.1 Different types of residues have varying effects on soils (thicker lines indicate more material,dashed line indicates very small percent of that type). Modified from Oshins, 1999.

rapid decompositionand nutrient release

very slowdecomposition

sugars,cellulose

proteins

hemicelluloselignin

humus

cover crop(green)

compost


straw will enhance aggregation more than com-post. (However, adding compost does improvesoils in many ways, including increasing the wa-ter holding capacity.)

Although it’s important to have adequateamounts of organic matter in soil, that isn’tenough. A variety of residues is needed to pro-vide food to a diverse population of organisms,nutrients to plants, and to furnish materials thatpromote aggregation. Residues low in hemicel-lulose and lignin usually have very high levelsof plant nutrients. On the other hand, straw orsawdust (containing a lot of lignin) can be usedto build up organic matter, but a severe nitro-gen deficiency and an imbalance in soil micro-bial populations will occur unless a readily avail-able source of nitrogen is added at the same time(see discussion of C:N ratios below). In addi-tion, when insufficient N is present, less of theorganic material added to soils actually ends upas humus.

C:N Ratio of organic materials and nitro-gen availability. The ratio of the amount of aresidue’s carbon to the amount of nitrogen in-fluences nutrient availability and the rate ofdecomposition. The ratio, usually referred to asthe C:N ratio, may vary from around 15:1 foryoung plants, to between 50 to 80:1 for the oldstraw of crop plants, to over 100:1 for sawdust.For comparison, the C:N ratio of soil organic mat-ter is usually in the range of about 10 to 12:1 andthe C:N of soil microorganisms is around 7:1.

The C:N ratio of residues is really just an-other way of looking at the percentage of nitro-gen (figure 8.2). A high C:N residue has a lowpercentage of nitrogen. Low C:N residues haverelatively high percentages of nitrogen. Cropresidues are usually pretty close to 40 to 45 per-cent carbon, and this figure doesn’t change muchfrom plant to plant. On the other hand, nitro-gen content varies greatly depending on the typeof plant and its stage of growth.

Figure 8.2 Nitrogen release and immobilization with changing nitrogen content. Based on data of Vigiland Kissel, 1991.

C:N

rat

io

90

80

70

60

50

40

30

20

10

00 1 2 3 4 5 6 7

% N

short term reduction innitrogen availability

(immobilization)

nitrogen release


If you want crops growing immediately fol-lowing the application of organic materials, caremust be taken to make nitrogen available.

Nitrogen availability from residues variesconsiderably. Some residues, such as fresh,young, and very green plants, decompose rap-idly in the soil and, in the process, may readilyrelease plant nutrients. This could be comparedto the effect of sugar eaten by humans, whichresults in a quick burst of energy. Some of thesubstances in older plants and in the woody por-tion of trees, such as lignin, decompose veryslowly in soils. Materials, such as sawdust andstraw, mentioned above, contain little nitrogen.Well-composted organic residues also decom-pose slowly in the soil because they are fairlystable, having already undergone a significantamount of decomposition.

Mature plant stalks and sawdust that haveC:N over 40:1 (table 8.3) may cause temporaryproblems for plants. Microorganisms using ma-terials containing 1 percent nitrogen (or less)need extra nitrogen for their growth and repro-duction. They will take the needed nitrogen fromthe surrounding soil, diminishing the amountof nitrate and ammonium available for crop use.This reduction of soil nitrate and ammoniumby microorganisms decomposing high C:N resi-dues is called immobilization of nitrogen.

When microorganisms and plants competefor scarce nutrients, the microorganisms usu-ally win, because they are so well distributed inthe soil. Plant roots are in contact with only 1 to2 percent of the entire soil volume whereas mi-croorganisms populate almost the entire soil.The length of time during which the nitrogennutrition of plants is adversely affected by im-mobilization depends on the quantity of resi-dues applied, their C:N ratio, and other factors

influencing microorganisms, such as fertilizationpractices, temperature, and moisture conditions.If the C:N ratio of residues is in the teens or low20s, corresponding to greater than 2 percentnitrogen, then there is more nitrogen presentthan the microorganisms need for residue de-composition. When this happens, extra nitro-gen becomes available to plants fairly quickly.Green manure crops and animal manures are inthis group of residues. Residues with C:N in themid-20s to low 30s, corresponding to about 1to 2 percent nitrogen, will not have much effecton short-term nitrogen immobilization or release.

Sewage sludge on your fields? In theory, theuse of sewage sludges on agricultural landsmakes sense as a way to resolve problems re-lated to people living in cities, far removed fromthe land that grows their food. However, thereare some troublesome issues associated with ag-

TABLE 8.3C:N Ratios of Selected

Organic Materials

MATERIAL C:N

Soil 10–12

Poultry manure 10

Clover and alfalfa (early) 13

Compost 15

Dairy manure (low bedding) 17

Alfalfa hay 20

Green rye 36

Corn stover 60

Wheat, oat, or rye straw 80

Oak leaves 90

Fresh sawdust 400

Newspaper 600

—C:N VALUES FROM VARIOUS SOURCES


ricultural use of sludges. By far, the most im-portant problem is that they frequently containcontaminants from industry and from variousproducts used around the home. Although manyof these metal contaminants naturally occur atlow levels in soils and plants, their high con-centrations in some sludges create a potentialhazard. The U.S. standards for toxic materials

in sludges are much more lenient than those inother industrialized countries and they permithigher loading of potentially toxic metals. So,although you are allowed to use many sludges,you should carefully examine a sludge’s contentsbefore applying it to your land.

Another issue is that sludges are producedby varied processes and, therefore, have differ-

Figure 8.3 C:N ratio of different size fractions of organic matter.

—MAGDOFF, F., UNPUBLISHED DATA, AVERAGE FOR THREE SOILS.

C:N Ratio of Active Organic Matter

As residues are decomposed by soil organisms,carbon is lost as CO2

, while nitrogen is mostly

conserved. This causes the C:N ratio of decom-posing residues to decrease. Although the C:Nratio for most agricultural soils is in the range of10 to 12:1, the different types of organic matter

within a soil have different C:N ratios. The largerparticles of soil organic matter have higher C:Nratios, indicating that they are less decomposedthan smaller fractions. Microscopic evidence alsoindicates that the larger fractions are less decom-posed than the smaller particles.

C:N

26

24

22

20

18

16

14

12

10a) greaterthan sand

size

b) size ofcoarse sand

c) size offine sand

d) smaller than sandsize, probably not

active


ent properties. Most sludges are around neutralpH, but, when added to soils, cause some de-gree of acidification, as do most nitrogen fertil-izers. Because many of the problem metals aremore soluble under acidic conditions, the pHof soils receiving these materials should be moni-tored and maintained at around 6.8 or above.On the other hand, lime (calcium hydroxide andground limestone are used together) is addedto some sludges to raise the pH and kill diseasebacteria. The resulting “lime-stabilized” sludgehas extremely high levels of calcium, relative topotassium and magnesium. This type of sludgeshould be used primarily as a liming source andlevels of magnesium and potassium in the soilneed to be carefully monitored to be sure theyare present in reasonable amounts, comparedwith the high levels of added calcium.

The use of “clean” sludges — those contain-ing low levels of metal and organic contaminants— for agronomic crops is certainly an accept-able practice. Sludges should not be applied tosoils when growing crops for direct human con-sumption, unless it can be demonstrated that,in addition to low levels of potentially toxicmaterials, organisms dangerous to humans areabsent.

Application rates for organic materials. Theamount of residue added to a soil is often deter-mined by the cropping system. The crop resi-dues can be left on the surface or incorporatedby tillage. Different amounts of residue will re-main under different crops, rotations, or har-vest practices. For example, three or more tonsper acre of leaf, stalk, and cob residues remainin the field when corn is harvested for grain. Ifthe entire plant is harvested to make silage, thereis little left except the roots.

When “imported” organic materials arebrought to the field, you need to decide howmuch and when to apply them. In general, ap-plication rates of these residues will be basedon their probable contribution to the nitrogennutrition of plants. We don’t want to apply toomuch available nitrogen because it will bewasted. Nitrate from excessive applications oforganic sources of fertility may leach into ground-water just as easily as nitrate originating frompurchased synthetic fertilizers. In addition, ex-cess nitrate in plants may cause health prob-lems for humans and animals.

Sometimes the fertility contribution of phos-phorus may be the main factor governing ap-plication rates of organic material. Excess phos-phorus entering lakes can cause an increase inthe growth of algae and other aquatic weeds,decreasing water quality for drinking and rec-reation. In these locations, farmers must be care-ful to avoid loading the soil with too much phos-phorus, from either commercial fertilizers ororganic sources.

Effects of residue and manure accumula-tions. When any organic material is added tosoil, it decomposes relatively rapidly at first.Later, when only resistant parts (for example,straw stems high in lignin) are left, the rate ofdecomposition decreases greatly. This meansthat although nutrient availability diminisheseach year after adding a residue to the soil, thereare still long-term benefits from adding organicmaterials. This can be expressed by using a “de-cay series.” For example, 50, 15, 5, and 2 per-cent of the amount of nitrogen added in ma-nure may be released in the first, second, third,and fourth years following addition to soils. Inother words, crops in a regularly manured field


get some nitrogen from manure that was appliedin past years. So, if you are starting to manure afield, somewhat more manure will be needed inthe first year than will be needed in years 2, 3,and 4 to supply the same total amount of nitro-gen to a crop each year. After some years, youmay need only half of the amount used to sup-ply all the nitrogen needs in the first year.

Organic Matter Management onDifferent Types of Farms

Animal-based farms. It is certainly easier tomaintain soil organic matter in animal-based ag-ricultural systems. Manure is a valuable by-prod-uct of having animals. Animals also can use sod-type grasses and legumes as pasture, hay, andhaylage (hay stored under air-tight conditionsso that some fermentation occurs). It is easierto justify putting land into perennial forage cropsfor part of a rotation when there is an economicuse for the crops. Animals need not be on thefarm to have positive effects on soil fertility. Afarmer may grow hay to sell to a neighbor andtrade for some animal manure from theneighbor’s farm, for example. Occasionally, for-mal agreements between dairy farmers and veg-etable growers lead to cooperation on crop ro-tations and manure application.

Systems without animals. It is more challeng-ing, although not impossible, to maintain orincrease soil organic matter on non-livestockfarms. It can be done by using reduced tillage,cover crops, intercropping, living mulches, ro-tations that include crops with high amounts ofresidue left after harvest, and attention to othererosion-control practices. Organic residues, suchas leaves or clean sewage sludges, can sometimesbe obtained from nearby cities and towns. Straw

Maintaining Organic Matter in Small Gardens

There are a number of different ways thathome gardeners can maintain soil organic mat-ter. One of the easiest is using lawn grass clip-pings for mulch during the growing season.The mulch can then be worked into the soilor left on the surface to decompose until thenext spring. Also, leaves can be raked up inthe fall and applied to the garden. Cover cropscan also be used on small size gardens. Ofcourse, manures, composts, or mulch strawcan also be purchased.

There are a growing number of small-scalemarket gardeners, many with insufficient landto rotate into a sod type crop. They also mayhave crops in the ground late into the fall, mak-ing cover cropping a challenge. One possi-bility is to establish cover crops by over-seed-ing after the last crop of the year is well es-tablished. Another source of organic materi-als — grass clippings — are probably in shortsupply compared with the needs of croppedareas, but are still useful. It might also be pos-sible to obtain leaves from a nearby town.These can either be directly applied andworked into the soil or composted first. Aswith home gardeners, market gardeners canpurchase manures, composts, and strawmulch, but should get volume discounts onthe amounts needed for an acre or two.

or grass clippings used as mulch also add or-ganic matter when they later become incorpo-rated into the soil by plowing or by the activityof soil organisms. Some vegetable farmers use a“mow-and-blow” system where crops are grownon strips for the purpose of chopping them andspraying the residues onto an adjacent strip.


MAINTAINING SOILBIODIVERSITY

The role of diversity is critical to maintaining awell functioning and stable agriculture. Wheremany different types of organisms coexist, thereare fewer disease, insect, and nematode prob-lems. There is more competition for food andmore possibility that many types of predatorswill be found. This means that no single pestorganism will be able to reach a population high

enough to cause a major decrease in crop yield.We can promote a diversity of plant speciesgrowing on the land by using cover crops, inter-cropping, and crop rotations. However, don’tforget that diversity below the soil surface is asimportant as diversity above ground. Growingcover crops and using crop rotations help main-tain the diversity below ground, but adding ma-nures and composts and making sure that cropresidues are returned to the soil are also criticalfor promoting soil organism diversity.

Many of the practices discussed in this chapterand the other chapters in Part 2 help to reducethe severity of crop pests. It is now known thatplants have very sophisticated defense mecha-nisms against insects and diseases. When plantsare under environmental stresses caused by com-pact soils, droughty conditions, or excess nitro-gen, they are less able to combat pests and maybe even more attractive to them. On the otherhand, healthy plants growing on soils with goodbiological diversity are able to mount a strongdefense against many pests. For example, whenattacked by insects they may emit chemicals thatattract beneficial insects that are predators of thepest. In addition, good soil management de-creases levels of pests that live in the soil.

It is well established — and known by mostfarmers — that crop rotation can decrease dis-ease, insect, nematode, and weed pressures. Afew other examples are given below.

� Insect damage can be reduced by avoidingexcess nitrogen levels in soils through betternitrogen management.

Managing Soils and Crops to Minimize Pest Problems

� Root rots and severity of leaf diseases can bereduced with composts that contain lowlevels of available nitrogen, but still havesome active organic matter.

� Fungal diseases of roots and insect damageare decreased by lessening soil compaction.

� Many pests are kept under control by competi-tion for resources or direct antagonism (includ-ing the beneficials feeding on them). Goodquantities of a variety of organic materials helpmaintain a diverse group of soil organisms.

� Root surfaces are protected from fungal andnematode attack by high rates of beneficialmycorrhizal fungi. Most cover crops helpkeep mycorrhizal fungi spore counts highand promote higher rates of infection by thebeneficial fungi.

� Parasitic nematodes can be suppressed bycover crops.

� Residues of some cover crops, such as winterrye, reduce weed seed germination.

� Weed seed numbers are reduced in soils witha lot of biological activity, with both microor-ganisms and insects helping the process.


MANAGING SOIL PHYSICALCONDITIONS:

Developing and maintaining an optimumphysical environment. Plants thrive in a physi-cal environment that allows roots to actively ex-plore a large area, gets all the oxygen and waterneeded, and maintains a healthy mix of organ-isms. Although the soil’s physical environmentis strongly influenced by organic matter, thepractices and equipment used — from tillage toplanting to cultivation to harvest — have a ma-jor impact. If a soil is too wet — whether it haspoor internal drainage or it receives too muchwater — some remedies are needed to grow highyielding and healthy crops. Also, erosion —whether by wind or water — is an environmen-tal hazard that needs to be kept as low as pos-sible. Erosion is most likely when the surface ofa soil is bare and doesn’t contain sufficient me-dium- to large-size water-stable aggregates. Prac-tices for management of soil physical proper-ties are discussed in chapters 13 to 15.

NUTRIENT MANAGEMENT

Many of the practices that build up and main-tain soil organic matter also help enrich the soilwith nutrients or make it easier to manage nu-trients in ways that satisfy crop needs and arealso environmentally sound. For example, a le-gume cover crop increases a soil’s active organicmatter and reduces erosion, but it also adds ni-trogen that can be used by the next crop. Covercrops and deep-rooted rotation crops help tocycle nitrate, potassium, calcium, and magne-sium that might be lost to leaching below croproots. Importing mulches or manures onto the

farm also adds nutrients along with the organicmaterials. However, specific nutrient manage-ment practices are needed, such as testing ma-nure and checking its nutrient content beforeapplying additional nutrient sources. Other ex-amples of nutrient management practices notdirectly related to organic matter managementinclude applying nutrients timed to plant needs,liming acidic soils, and interpreting soil tests todecide on the appropriate amounts of nutrientsto apply (see chapters 16 to 19). Developmentof farm nutrient management plans and water-shed partnerships also improve soil while pro-tecting the local environment.

Many of the practices that build up

and maintain soil organic matter also

help enrich the soil with nutrients or

make it easier to manage nutrients in

ways that satisfy crop needs and are

also environmentally sound.

SOURCES

Barber, S.A. 1998. Chemistry of soil-nutrientinteractions and future agricultural sustain-ability. In Future Prospects for Soil Chemistry(P.M. Huang, D.L. Sparks, and S.A. Boyd,eds.). SSSA Special Publication No. 55. SoilScience Society of America. Madison, WI.


Cavigelli, M.A., S.R. Deming, L.K. Probyn, andR.R. Harwood (eds.). 1998. Michigan Field


Crop Ecology: Managing Biological Processes forProductivity and Environmental Quality. Michi-gan State University Extension Bulletin E-2646. East Lansing, MI.

Mitchell, J., T. Hartz, S. Pettygrove, D. Munk, D.May, F. Menezes, J. Diener, and T. O’Neill. 1999.Organic matter recycling varies with cropsgrown. California Agriculture 53(4):37–40.

Oshins. C. An Introduction to Soil Health. 1999.A slide set available at the Northeast RegionSARE website: www.uvm.edu/~nesare.slide.html

Topp, G.C., K.C. Wires, D.A. Angers, M.R. Car-ter, J.L.B. Culley, D.A. Holmstrom, B.D. Kay,G. P. Lafond, D.R. Langille, R.A. McBride, G.T.Patterson, E. Perfect, V. Rasiah, A.V. Rodd,and K.T. Webb. 1995. Changes in Soil Struc-ture. In The Health of Our Soils: Toward Sus-tainable Agriculture in Canada (D.F. Acton andL.J. Gregorich, eds.). Center for Land and Bio-logical Resources Research. Research Branch,Agriculture and Agri-Food Canada. Publica-tion 1906/E. http://res.agr.ca/CANSIS/PUB-LICATIONS/HEALTH/chapter06.html

77

Once cheap fertilizers became widely avail-able after World War II, many farmers, ex-

tension agents, and scientists looked down theirnoses at manure. People thought more abouthow to get rid of manure than how toput it to good use. In fact, some scien-tists tried to find out the absolute maxi-mum amount of manure that could beapplied to an acre without reducingcrop yields. Some farmers who didn’twant to spread manure actually piled itnext to a stream and hoped that next spring’s floodwaters would wash it away. We now know thatmanure, like money, is better spread around thanconcentrated in a few places. The economic con-tribution of farm manures can be considerable.The value of the nutrients in manure from a 70-cow dairy farm may exceed $7,000 per year; ma-nure from a 50-sow farrow-to-finish operation isworth about $4,000; and manure from a 20,000-

9�

Animal Manuresfor Increasing Organic Matter and

Supplying Nutrients

The quickest way to rebuild a poor soil is to practice

dairy farming, growing forage crops, buying . . .

grain rich in protein, handling the manure properly,

and returning it to the soil promptly.

— J. L. HILLS, C. H. JONES, AND C. CUTLER, 1908

bird broiler operation is worth about $3,000. Theother benefits to soil organic matter build-up,such as enhanced soil structure and better diver-sity and activity of soil organisms, may double

the value of the manure. If you’re notgetting the full fertility benefit from ma-nures on your farm, you may be wast-ing money.

Animal manures can have very dif-ferent properties, depending on theanimal species, feed, bedding, and ma-

nure-storage practices. The amounts of nutrientsin the manure that become available to crops alsodepend on what time of year the manure is ap-plied and how quickly it is worked into the soil.In addition, the influence of manure on soil or-ganic matter and plant growth is influenced bysoil type. In other words, it’s impossible to giveblanket manure application recommendations.They need to be tailored for every situation.


We’ll deal mainly with dairy cow manure,because there’s more information about its useon cropland. We’ll also offer general informa-tion about the characteristics and uses of someother animal manures.

MANURE HANDLING SYSTEMS

Solid versus Liquid

The type of barn on the farmstead frequentlydetermines how manure is handled on a dairyfarm. Dairy-cow manure containing a fairamount of bedding, usually around 13 to 20percent dry matter, is spread as a solid. This ismost common on farms where cows are kept inindividual stanchions. Liquid manure-handlingsystems are common where animals are kept ina “free stall” barn with little bedding. Liquidmanure is usually in the range of from 2 to 10percent dry matter (90 percent or more water).Manures with characteristics between solid andliquid are usually referred to as semi-solid orslurry, depending on the method of handling.

Composting manures is becoming an increas-ingly popular option for farmers. By compostingmanure you help stabilize nutrients, have asmaller amount of material to spread, and havea more pleasant material to spread (and if neigh-bors have complained about manure odors, thatmight be a big plus). Although it’s easier to com-post manure that has been handled as a solid,some farmers are separating the solids from liq-uid manure and then irrigating with the liquidand composting the solids. For a more detaileddiscussion of composting, see chapter 12.

Storage of Manure

Researchers have been investigating how bestto store manure to reduce the problems that

come with year-round manure spreading. Stor-age allows the farmer to apply manure when it’sbest for the crop and during appropriate weatherconditions. This reduces nutrient loss from themanure caused by water runoff from the field.However, significant losses of nutrients fromstored manure also may occur. One study foundthat, during the year, dairy manure stored inuncovered piles lost 3 percent of the solids, 10percent of the nitrogen, 3 percent of the phos-phorus, and 20 percent of the potassium. Cov-ered piles or well-contained liquid systems,which tend to form a crust on the surface, do abetter job of conserving the nutrients and sol-ids than unprotected piles. Poultry manure, withits high amount of ammonium, may lose 50 per-cent of its nitrogen during storage as ammoniagas volatilizes, unless precautions are taken toconserve nitrogen.

CHEMICAL CHARACTERISTICSOF MANURES

A high percentage of the nutrients in feeds passesright through animals and ends up in their ma-nure. Over 70 percent of the nitrogen, 60 per-cent of the phosphorus, and 80 percent of thepotassium in feeds may be available in manuresfor use on cropland. In addition to the nitro-gen, phosphorus, and potassium contributionsgiven in table 9.1, manures also contain signifi-cant amounts of other nutrients, such as cal-cium, magnesium, and sulfur. In regions wherethe micronutrient zinc tends to be deficient,there is rarely any deficiency on soils receivingregular manure applications.

The values given in table 9.1 must be viewedwith some caution, because the characteristicsof manures from even the same type of animal

ANIMAL MANURES 79

may vary considerably from one farm to another.Differences in feeds, mineral supplements, bed-ding materials, and storage systems make ma-nure analysis quite variable. Yet, as long as feed-ing, bedding, and storage practices remain un-changed on a given farm, manure characteris-tics will be similar from year to year.

The major difference among all the manuresis that poultry manure is significantly higher innitrogen and phosphorus than the other manuretypes. This is partially due to the difference in

feeds given poultry versus other farm animals.The relatively high percentage of dry matter inpoultry manure is also partially responsible forthe higher analyses of certain nutrients, whenexpressed on a wet ton basis.

It is possible to take the guesswork out ofestimating manure characteristics; most soil-test-ing laboratories will now analyze manure. Ma-nure analysis should become a routine part ofthe soil fertility management program on ani-mal-based farms.

TABLE 9.1Manure Characteristics

DAIRY COW BEEF COW CHICKEN HOG

DRY MATTER CONTENT (%)

Solid (fresh) 13 12 25 9

Liquid (fresh, diluted) 9 8 17 6

TOTAL NUTRIENT CONTENT (APPROXIMATE)

Nitrogen

lbs./ton 10 14 25 10

lbs./1,000 gal. 28 39 70 28

Phosphate

lbs./ton 5 9 25 6

lbs./1,000 gal. 14 25 70 9

Potash

lbs./ton 10 11 12 9

lbs./1,000 gal. 28 31 33 25

Manure Equivalents

Solid manure (tons, fresh) 20 11 5 16

Liquid manure (gal.) 7,200 4,000 1,500 5,700

—MODIFIED FROM MADISON ET AL., 1986.


EFFECTS OF MANURINGON SOILS

Effects on Organic Matter

When considering the influence of any residueor organic material on soil organic matter, thekey question is the amount of solids returnedto the soil. Equal amounts of different types ofmanures will have different effects on soil or-ganic matter levels. Dairy and beef manure con-tain undigested parts of forages, as well as bed-ding. They, therefore, have a high amount of com-plex substances, such as lignin, that do not de-compose readily in soils. Using this type of ma-nure results in a much greater long-term influ-ence on soil organic matter than does a poultrymanure without bedding. More solids are com-monly applied to soil with solid manure-han-dling systems than with liquid systems, becausegreater amounts of bedding are usually included.

When conventional tillage is used to grow acrop (such as corn silage), where the entireabove-ground portion is harvested, research in-dicates that an annual application of 20 to 30tons of the solid type of dairy manure per acreis needed to maintain soil organic matter (table9.2). As discussed above, a nitrogen-demand-ing crop, such as corn, may be able to use all ofthe nitrogen in 20 to 30 tons of manure. If moreresidues are returned to the soil by just harvest-ing grain, lower rates of manure application willbe sufficient to maintain or build up soil organicmatter.

An example of how manure addition mightbalance annual loss is given in figure 9.1. Onelarge Holstein “cow year” worth of manure isabout 20 tons. Although 20 tons of anything isa lot, when considering dairy manure, it trans-lates into a much smaller amount of solids. Ifthe approximately 5,200 pounds of solid mate-

Figure 9.1 Example of dairy manure addition just balancing soil organic matter losses.

20 tons fresh weight dairy manureat 13% dry matter = 5,200 lbs. of solids

x 0.25(75% decomposes

in first year)

gain from manure =1,300 lbs.

soil organic matter

2,000,000 lbs. in surface 6 inches x 0.0217 =43,400 lbs. of organic matter/acre

x 0.03(3% decomposes

per year)

lost by decomposition =

1,300 lbs.

ANIMAL MANURES 81

rial in the 20 tons is applied over the surface ofone acre and mixed with the 2 million poundsof soil present to a 6-inch depth, it would raisethe soil organic matter by about 0.3 percent.However, much of the manure will decomposeduring the year, so the net effect on soil organicmatter will be even less. Let’s assume that 75

percent of the solid matter decomposes duringthe first year and the carbon ends up as atmo-spheric CO2. At the beginning of the followingyear, only 25 percent of the original 5,200pounds, or 1,300 pounds of organic matter isadded to the soil. The net effect is an increase insoil organic matter of 0.065 percent (the calcu-

Application of manures causes many soil changes— biological, chemical, and physical. A few ofthese types of changes are indicated in table 9.2,which contains the results of a long-term experi-ment in Vermont with continuous corn silage on aclay soil. Manure counteracted many of the nega-tive effects of a monoculture cropping system inwhich few residues are returned to the soil. Soilreceiving 20 tons of dairy manure (wet weight, in-cluding bedding) maintained organic matter and

Manure Influences Many Soil Properties

CEC levels and close to the original pH (althoughacid-forming nitrogen fertilizers also were used).Manures, such as dairy and poultry, have limingeffects and actually counteract acidification.

High rates of manure addition caused a build-up of both phosphorus and potassium to highlevels. Soil in plots receiving manures were bet-ter aggregated and less dense and, therefore, hadgreater amounts of pore space than fields receiv-ing no manure.

TABLE 9.2Effects of 11 Years of Manure Additions on Soil Properties*

APPLICATION RATE (TONS/ACRE/YEAR)NONE 10 TONS 20 TONS 30 TONS

organic matter 4.3 4.8 5.2 5.5

CEC (me/100g) 15.8 17.0 17.8 18.9

pH 6.0 6.2 6.3 6.4

P (ppm) 6.0 7.0 14.0 17.0

K (ppm) 121.0 159.0 191.0 232.0

total pore space (%) 44.0 45.0 47.0 50.0

* Original levels: organic matter=5.2, pH=6.4, CEC=17.8 me/100gm, P=4 ppm, K=129 ppm.P and K levels with 20 and 30 tons of manure applied annually are much higher than cropneeds (see table 19.4a).

—MAGDOFF AND AMADON, 1980; MAGDOFF AND VILLAMIL, 1977.


lation is [1,300/2,000,000] x 100). Althoughthis does not seem like much added organic mat-ter, if a soil had 2.17 percent organic matter and3 percent of this was decomposed annually dur-ing cropping, then the loss would be 0.065 per-cent per year and the manure addition wouldjust balance this loss.

USING MANURES

Manures, like other organic residues that decom-pose easily and rapidly release nutrients, areusually applied to soils in quantities judged tosupply sufficient nitrogen for the crop beinggrown in the current year. It might be better forbuilding and maintaining soil organic matter toapply manure at higher rates, but doing so maycause undesirable nitrate accumulation in leafycrops and excess nitrate leaching to groundwa-ter. High nitrate levels in leafy-vegetable cropsare undesirable in terms of human health, andthe leaves of many plants seem more attractiveto insects. In addition, salt damage to crop plantscan occur from high manure application rates,especially when there is insufficient leaching byrainfall or irrigation. Very high amounts of addedmanures, over a period of years, also lead to highsoil phosphorus levels (table 9.2). It is a wasteof money and resources to add unneeded nutri-ents to the soil, nutrients which will only belost by leaching or runoff, instead of contribut-ing to crop nutrition.

Application Rates

A common per-acre rate of dairy-manure appli-cation is 10 to 30 tons fresh weight of solid, or4,000 to 11,000 gallons of liquid manure. Theserates will supply approximately 50 to 150

pounds of available nitrogen (not total) per acre.If you are growing crops that don’t need thatmuch nitrogen, such as small grains, 10 to 15tons of solid manure should supply sufficientnitrogen per acre. For a crop that needs a lot ofnitrogen, such as corn, 20 to 30 tons per acremay be necessary to supply its nitrogen needs.Low rates of about 10 tons per acre are also sug-gested for each of the multiple applications usedon a grass hay crop. In total, grass hay cropsneed at least as much total nitrogen applied asdoes a corn crop. There has been some discus-sion about applying manures to legumes. Thispractice has been discouraged because the le-gume uses the nitrogen from the manure, andmuch less nitrogen is fixed from the atmosphere.However, the practice makes sense on animalfarms where there is excess nitrogen.

For the most nitrogen benefit to crops, ma-nures should be incorporated into the soil im-mediately after spreading on the surface. Abouthalf of the total nitrogen in dairy manure comesfrom the ammonium (NH4

+) in urine. This am-monium represents almost all of the readilyavailable nitrogen present in dairy manure. Asmaterials containing urea or ammonium dry onthe soil surface, the ammonium is converted toammonia gas (NH3) and lost to the atmosphere.If dairy manure stays on the soil surface, about25 percent of the nitrogen is lost after one dayand 45 percent is lost after four days — but that45 percent of the total represents around 70 per-cent of the readily available nitrogen! This prob-lem is significantly lessened if about 1/2 inch ofrainfall occurs shortly after manure application,leaching ammonium from manure into the soil.Leaving manure on the soil surface is also a prob-lem because runoff waters may carry significantamounts of nutrients from the field. When this

ANIMAL MANURES 83

happens, crops don’t benefit as much from themanure application and surface waters becomepolluted. Some liquid manures — those withlow solids contents — penetrate the soil moredeeply. When applied at normal rates, thesemanures will not be as prone to lose ammoniaby surface drying.

Other nutrients contained in manures, in ad-dition to nitrogen, make important contribu-tions to soil fertility. The availability of phos-phorus and potassium in manures should besimilar to that in commercial fertilizers. (How-ever, some recommendation systems assumethat only around 50 percent of the phosphorusand 90 percent of the potassium is available.)The phosphorus and potassium contributionsof 20 tons of dairy manure is approximatelyequivalent to about 30 to 50 lbs. of phosphateand 180 to 200 lbs. of potash from fertilizers.The sulfur content as well as trace elements inmanure, such as the zinc previously mentioned,also add to the fertility value of this resource.

cant amounts of surface water penetration andthen drainage, such as a manure stack of well-bedded dairy or beef cow manure, may lose a lotof potassium. The 20 percent leaching loss ofpotassium from stacked dairy manure mentionedabove occurred because potassium was mostlyfound in the liquid portion of the manure.

Timing of Applications

Manures are best applied to annual crops, suchas corn, small grains, and vegetables, in one dosejust before soil tillage (unless a high amount ofbedding is used, which might tie-up nitrogenfor a while — see discussion of C:N in chapter8). This allows for rapid incorporation by plow,chisel, harrow, or disk. Even with reduced till-age systems, application close to planting timeis best, because the possibility of loss by runoffand erosion is reduced. It also is possible to in-ject liquid manures either just before the grow-ing season starts or as a sidedress to row crops.Fall manure applications on annual row crops,such as corn, may result in considerable nitro-gen loss, even if manure is incorporated. Lossesof nitrogen from fall-applied manure in humidclimates may be around 50 percent — resultingfrom leaching and denitrification before nitro-gen is available to next year’s crop.

Without any added nitrogen, perennial grasshay crops are constantly nitrogen deficient. Ap-plication of a moderate rate of manure — about50 lbs. worth of available nitrogen — in earlyspring and following each harvest is the bestway to apply manure. However, wet soils in earlyspring may not allow manure application with-out causing significant compaction.

Although the best use of manure is to applyit near the time when the crop needs the nutri-

N, P, and K in Hog, Dairy,and Beef Cattle Manures

FECES URINE

N 1/2 1/2

P most -

K - most

Because one-half of the nitrogen and almostall of the phosphorus is in the solids, much ofthese nutrients remain in sediments at the bot-tom when a liquid system is emptied withoutproperly agitating the manure. On the other hand,almost all of the potassium will be applied withthe liquid portion, even if it’s applied withoutthe solids. A manure system that allows signifi-


ents, sometimes insufficient storage capacitycauses farmers to apply it at other times. In thefall, manure can be applied to grasslands thatdon’t flood or to tilled fields that will either befall plowed or planted to a winter cover crop.Although legal in most states, it is not a goodpractice to apply manures when the ground isfrozen or covered with snow. The nutrient lossesthat can occur with runoff from winter-appliedmanure are both an economic loss to the farmas well as an environmental hazard. Winterspreading should be done only on an emergencybasis. However, new research on frost tillage hasshown that there are windows of opportunityfor incorporating winter-applied manure dur-ing periods when the soil has a shallow frozenlayer, 2 to 4 inches thick (see chapter 15). Farm-ers may use this time window to inject manureduring the winter.

POTENTIAL PROBLEMS

As we all know, too much of a good thing is notnecessarily good.

Excessive manure applications may causeplant-growth problems. It is especially impor-tant not to apply excess poultry manure, becausethe high soluble-salt content can harm plants.

Plant growth is sometimes retarded whenhigh rates of fresh manure are applied to soil im-mediately before planting. This problem usuallydoesn’t occur if the fresh manure decomposesfor a few weeks in the soil and can be avoidedby using a solid manure that has been storedfor a year or more. Injection of liquid manuresometimes causes problems when used onpoorly drained soils in wet years. The extra wa-ter applied and the extra use of oxygen by microor-ganisms may mean less aeration for plant roots.

When manures are applied regularly to a fieldto provide enough nitrogen for a crop like corn,phosphorus and potassium may build up to lev-els way in excess of crop needs (see table 9.2).Erosion of phosphorus-rich topsoils contributessediments and phosphorus to streams and lakes,polluting surface waters. When very high phos-

Ever hear of E. Coli 0157:H7?

A bacteria strain known as E. Coli (an abbre-viation that is pronounced e-COLE-eye) 0157:H7 has caused numerous outbreaks of severeillness in people who ate contaminated meat.It also has caused one known outbreak whenwater used to wash lettuce was contaminatedwith animal manure.

This particular bacteria is a resident ofcows’ digestive systems. It does no harm tothe cow, but — probably because of the cus-tomary practice of feeding low levels of anti-biotics when raising cattle — it is resistant toa number of commonly used antibiotics. Thisproblem only reinforces the common senseapproach to manure use. When using manurethat has not been thoroughly composted togrow crops for direct human consumption —especially leafy crops like lettuce that growlow to the ground and root crops such as car-rots and potatoes — special care should betaken. Before planting your crop, avoid prob-lems by planning a three-month period be-tween incorporation and harvest. For shortseason crops, this means that the manureshould be incorporated long before planting.Although there has never been a confirmedinstance of contamination of vegetables by E.Coli 0157: H7 or other disease organisms frommanure incorporated into the soil as a fertilityamendment, being cautious and erring on theside of safety is well justified.

ANIMAL MANURES 85

phorus build-up occurs from the continual ap-plication of manure applied at rates to satisfycrop nitrogen needs, it may be wise to switchthe application to other fields or to use strictsoil-conservation practices to trap sedimentsbefore they enter a stream. Including rotationcrops, such as alfalfa — that do not need ma-nure — allows a “draw-down” of phosphorusthat accumulates from manure application tograins. (However, this may mean finding anotherlocation to apply manure. For a more detaileddiscussion of nitrogen and phosphorus manage-ment, see chapter 17.)

Farms that purchase much of their animalfeed may have too much manure to safely useon their own land. Although they don’t usuallyrealize it, they are importing large quantities ofnutrients in the feed that remain on the farm asmanures. If they apply all these nutrients on asmall area of soil, nitrogen and phosphorus pol-lution of groundwater and surface water willoccur. It is a good idea to make arrangementswith neighbors for use of the excess manure.Another option, if local outlets are available, isto compost the manure (see chapter 14) and sellthe product to vegetable farmers, garden centers,landscapers, and directly to home gardeners.

SOURCES

Elliott, L. F., and F. J. Stevenson, eds. 1977. Soilsfor Management of Organic Wastes and Waste-waters. Soil Science Society of America. Madi-son, WI.

Madison, F., K. Kelling, J. Peterson, T. Daniel,G. Jackson, and L. Massie. 1986. Guidelinesfor Applying Manure to Pasture and Croplandin Wisconsin. Agricultural Bulletin A3392.Madison, WI.

Magdoff, F. R., and J. F. Amadon. 1980. Yield trendsand soil chemical changes resulting from N andmanure application to continuous corn. Agro-nomy Journal 72:161–164. See this referencefor dairy manure needed to maintain or increaseorganic matter and soil chemical changes un-der continuous cropping for silage corn.

Magdoff, F. R., J. F. Amadon, S. P. Goldberg, andG. D. Wells. 1977. Runoff from a Low-costManure Storage Facility. Transactions of theAmerican Society of Agricultural Engineers20:658–660, 665. This is the reference forthe nutrient loss that can occur from uncov-ered manure stacks.

Magdoff, F. R. and R. J. Villamil, Jr. 1977. ThePotential of Champlain Valley Clay Soils forWaste Disposal. Proceedings of the Lake Cham-plain Environmental Conference, Chazy, NY.July 15, 1976.

Maryland State Soil Conservation Committee.Undated. Manure Management Handbook—AProducer’s Guide. College Park, MD.

Ontario Ministry of Agriculture and Food. 1994.Livestock and Poultry Waste Management. BestManagement Practices Series. Available fromthe Ontario Federation of Agriculture, To-ronto, Ontario (Canada).

Ontario Ministry of Agriculture and Food. 1997.Nutrient Management. Best Management Prac-tices Series. Available from the Ontario Fed-eration of Agriculture, Toronto, Ontario(Canada).

Soil Conservation Society of America. 1976.Land Application of Waste Materials. Soil Con-servation Society of America. Ankeny, IA.

van Es, H.M., A.T. DeGaetano, and D.S. Wilks.1998. Space-time upscaling of plot-basedresearch information: frost tillage. NutrientCycling in Agroecosystems 50:85–90.

86

Darrell ParksManhattan, Kansas

F A R M P R O F I L E

Even if Darrell Parks didn’t like working with pigs,he would still raise hogs on his 400-acre farm inthe Flint Hills of Kansas, if only for the manure.Parks’ 30 sows provide manure that makes up akey part of his soil fertility program.

Parks raises corn, milo, wheat, soybeans andalfalfa. Recently, he received organic certification,so he no longer uses any purchased fertilizer. In-stead, he plants nitrogen-fixing legume cover cropssuch as red clover, Austrian winter peas and vetchto amend the soil and spot-treats with hog ma-nure to help areas in need of extra fertility.

“I’ve been working to better utilize farm-pro-duced manure and cover crops as well as a croprotation and management system that will allowme to eliminate purchased fertilizer, herbicides andinsecticides,” says Parks, who received a grant fromUSDA’s Sustainable Agriculture Research and Edu-cation (SARE) program to hone his use of manureon cropland.

Parks likes how manure corrects micronutri-ent deficiencies in his soil. He regularly tests hissoils, then targets problem areas with a thickerapplication of manure.

Cover crops supply his nitrogen. Parks grows alegume cover crop in the winter, followed by acash crop of milo or soybeans. On some fields,he’ll grow a wheat crop planted in the fall. Beforeplanting, he’ll treat the field with manure to en-sure the wheat will not lack nutrients. He followswheat with alfalfa or clover.

At the root of Parks’ program is increasing or-ganic matter in the soil, which will improve waterinfiltration and soil structure. The cover crops help

compensate for what Parks describes as “heavy”soils. He chooses cover crops such as sweet cloverthat break through compacted soil with their deeptaproots. He anticipates an improvement in soilstructure over the next five years as he continuesto perfect his rotation.

“Back in the ’20s and ’30s they did some ofthese things and had good systems in place, thenfertilizer became cheap and everyone forgot aboutcover crops as a possible solution,” he says. “I havesome fairly tight, heavy soils, and this is a way tomake those soils better over time.”

Parks has pushed his organic matter up above2 percent on his sandy soils and close to or morethan 3 percent on his heavy clay soils, but notesthat his tillage regime makes improving organicmatter content especially challenging. That’s whyhe remains committed to his dual nutrient regimeof both animal and “green” manures.

Moreover, his organic system, which shouldyield him more in the marketplace, demands it.

Conventional farmers in the area would ben-efit by emulating Parks’ heavy-on-cover-crops ro-tation, says Ed Reznicek of the Kansas Rural Cen-ter, who works with producers to develop crop-ping plans. Seeding clover under wheat, or frost-seeding it, makes for good forage, increased ni-trogen and much biomass, he says.

“From what I’ve seen, both Darrell’s weed con-trol and production seems to be increasing,”Reznicek says. “He’s motivated to net as much ashe can from his farming operation, using a strat-egy of lowering costs and finding alternativemarkets.”

87

U nderstanding the effect of cover crops on the soil and the productivity of subse-

quent crops comes down to us from antiquity.Chinese manuscripts indicate that the use ofgreen manures is probably 3,000 yearsold. Green manures were also com-monly used in ancient Greece andRome. There are three different termsused to describe crops grown specifi-cally to help maintain soil fertility andproductivity instead of for harvesting:green manures, cover crops, and catch crops.The terms are sometimes used interchangeablyand are best thought of from the grower’s per-spective. A green manure crop is usually grownto help maintain soil organic matter and increasenitrogen availability. A cover crop is grown mainlyto prevent soil erosion by covering the groundwith living vegetation and with living roots thathold onto the soil. This, of course, is related to

managing soil organic matter, because the top-soil lost during erosion contains the most organicmatter of any soil layer. A catch crop is grown toretrieve available nutrients still in the soil fol-

lowing an economic crop and preventsnutrients leaching over the winter.

Sometimes it’s confusing to decidewhich term to use — green manure,cover crop, or catch crop. We usuallyhave more than one goal when weplant these crops during or after our

main crop, and plants grown for one of thesepurposes may also accomplish the other twogoals. The question of which term to use is notreally important, so in our discussion below theterm cover crop will be used.

Cover crops are usually incorporated into thesoil or killed on the surface before they are ma-ture. (This is the origin of the term green ma-nure.) Since cover crop residues are usually low

10�

Cover Crops

Where no kind of manure is to be had, I think

the cultivation of lupines will be found the readiest

and best substitute. If they are sown about the middle

of September in a poor soil, and then plowed in,

they will answer as well as the best manure.

—COLUMELLA, FIRST CENTURY, ROME


in lignin content and high in nitrogen, they de-compose rapidly in the soil.

EFFECTS OF COVER CROPS

The benefits from cover crops depend on theproductivity of the one that you’re growing andhow long it’s left to grow before the soil is pre-pared for the next crop. The more residue youreturn to the soil, the better the effect on soilorganic matter. The amount of residue producedby the growth of a cover crop may be very small,as little as half a ton of dry matter per acre. Thisadds some active organic matter, but becausemost decomposes rapidly after it’s killed, thereis no measurable effect on the total amount oforganic matter present. On the other hand, goodproduction of hairy vetch or crimson clovercover crops may yield 11/2 to 21/2 tons to over 4tons per acre. If a crop like cereal rye is grownto maturity, it can produce 3 to 5 tons of resi-due.

A five-year experiment with clover in Cali-fornia showed that cover crops increased organicmatter in the top 2 inches from 1.3 to 2.6 per-cent and in the 2- to 6-inch layer from 1 to 1.2percent. Some researchers have found that covercrops do not seem to increase soil organic mat-ter. Low-growing cover crops that don’t producemuch organic matter may not be able to counterthe depleting effects of some management prac-tices, such as intensive tillage. Even if they don’tsignificantly increase organic matter levels, covercrops help prevent erosion and add at least someresidues that are readily used by soil organisms.

Cover crops also supply nutrients to the fol-lowing crop, suppress weeds, and break pestcycles. Cover crops help maintain high popula-

tions of mycorrhizal fungi spores, which helpsimprove inoculation of the next crop. Their pol-len and nectar are important food sources forpredatory mites and parasitic wasps, both im-portant for biological control of insect pests. Acover crop also provides a good habitat for spi-ders, and these general insect feeders help de-crease pest populations. Use of cover crops inthe Southeast has reduced the incidence ofthrips, bollworm, budworm, aphids, fall army-worm, beet armyworm, and white flies. Livingcover crop plants and their residues also increasewater infiltration into soil, thus compensatingfor the water that cover crops use.

SELECTION OF COVER CROPS

Before growing cover crops, you need to askyourself some questions.

� Which type should you plant?� When and how should you plant the crop?� When should the crop be killed or incorpo-

rated into the soil?

When you select a cover crop, you shouldconsider what you want to accomplish, the soilconditions, and the climate.

� Is the main purpose to add availablenitrogen to the soil or to provide largeamounts of organic residue?

� Is erosion control in the late fall and earlyspring your primary objective?

� Is the soil very acidic and infertile, with lowavailability of nutrients?

� Does the soil have a compaction problem?(Some species are especially good foralleviating compaction.)

COVER CROPS 89

� Is weed suppression your main goal?� Which species are best for your climate?

(Some species are more winter-hardy thanothers.)

� Will the climate and water-holding proper-ties of your soil cause a cover crop to use somuch water that it harms the followingcrop?

There are many types of plants that can beused as cover crops, with legumes and grasses(including cereals) the most extensively used.Leguminous crops are often very good covercrops. Summer annual legumes, usually grownonly during the summer, include soybeans, peas,and beans. Winter annual legumes that are nor-mally planted in the fall and counted on to over-winter include berseem clover, crimson clover,hairy vetch, and subterranean clover. Some, likecrimson clover, can only overwinter in regionswith mild frost. Hairy vetch, though, is able towithstand fairly severe winter weather. Bienni-als and perennials include red clover, white clo-ver, sweet clover, and alfalfa. It should be notedthat crops usually used as winter annuals aresometimes grown as summer annuals in cold,short-season regions. Also, summer annuals thatare easily damaged by frost, such as cowpeas,can be grown as a winter annual in the DeepSouth.

One of the main reasons for selecting legumesas cover crops is their ability to fix nitrogen fromthe atmosphere and add it to the soil. Legumessuch as hairy vetch or crimson clover that pro-duce a substantial amount of growth may sup-ply over 100 pounds of nitrogen per acre to thenext crop. However, other legumes, such as fieldpeas, bigflower vetch, and red clover, may sup-ply only 30 to 80 pounds of available nitrogen.

Nonleguminous crops used as cover cropsinclude the cereal grasses rye, wheat, oats, andbarley, as well as other grass family species, suchas ryegrass. Other cover crops, like buckwheat,rape, and turnips, are neither legumes norgrasses.

Some of the most important cover crops arediscussed below.

LEGUMES

If you grow a legume as a cover crop, don’t for-get to inoculate seeds with the bacteria that livein the roots and fix nitrogen. There are varioustypes of rhizobial bacteria that fix nitrogen. Someare specific to certain crops. There are differentstrains for alfalfa, clovers, soybeans, beans, peas,vetch and cowpeas. Unless you’ve recentlygrown a legume from the same general groupyou are currently planting, consider mixing theseeds with the appropriate commercial rhizobialinoculant before planting. The addition of sugarwater to the seed-inoculant mix helps the bac-teria stick to the seeds. Plant right away, so thebacteria don’t dry out. Inoculums are readilyavailable only if they are commonly used in yourregion. It’s best to check with your seed sup-plier a few months before you need the inocu-lant, so it can be special ordered, if necessary.

Inoculum Groups

red and white cloverscrimson and berseem clovers

alfalfa, sweet cloverpea, vetch, lentils

annual medicscowpea, lespedeza


Winter Annual Legumes

Berseem clover is an annual crop that is grownin the South during the winter. Some newer va-rieties have done very well in California, with“Multicut” outyielding “Bigbee.” It establishes eas-ily and rapidly and develops a dense cover, mak-ing it a good choice for weed suppression. It’salso drought tolerant and re-grows rapidly whenmowed or grazed. Berseem is also grown as asummer annual in the Northeast and Midwest.

Crimson clover is considered one of the bestcover crops for the southeastern United States.Where adapted, it grows in the fall and winter,and matures more rapidly than most other le-gumes. It also contributes a relatively largeamount of nitrogen to the following crop. Be-cause it is not very winter-hardy, crimson clo-ver is not usually a good choice for the north-ern portions of the South and further north. Innorthern regions, crimson clover can be grownas a summer annual, but that prevents an eco-nomic crop from growing during that field sea-son. Varieties like “Chief,” “Dixie,” and “Ken-tucky Select” are somewhat winter-hardy if es-tablished early enough before winter. Crimsonclover does not grow well on high pH (calcare-ous) or poorly drained soils.

Hairy vetch is grown in the Southeast, but iswinter-hardy enough to grow well in the mid-Atlantic states and even in most of the North-east and Midwest. Where adapted, hairy vetchproduces a large amount of vegetation and fixesa significant amount of nitrogen, contributingas much as 100 pounds of nitrogen per acre ormore to the next crop. Hairy vetch residues de-compose rapidly and release nitrogen morequickly than most other cover crops. This canbe an advantage when a rapidly growing, high-nitrogen-demand crop follows hairy vetch. Hairy

vetch will do better on sandy soils than manyother green manures, but needs good soil po-tassium levels to be most productive.

Subterranean clover is a warm climate win-ter annual that, in many situations, can com-plete its life cycle before a summer crop isplanted. When used this way, it doesn’t need tobe suppressed or killed and does not competewith the summer crop. If left undisturbed, it willnaturally re-seed itself from the pods that ma-ture below-ground. Because it grows low to theground and does not tolerate much shading, itis not a good choice to interplant with summerannual row crops.

Summer Annual Legumes

Cowpeas are native to central Africa and dowell in hot climates. The cowpea is, however,severely damaged by even a mild frost. It is deeprooted and is able to do well under droughtyconditions. It usually does better on low-fertil-ity soils than crimson clover.

Soybeans, usually grown as an economic cropfor their oil and protein-rich seeds, also can serveas a summer cover crop. They require a fertilesoil for best growth. As with cowpeas, soybeansare easily damaged by frost. Soybeans, if grownto maturity and harvested for seed, do not addmuch in the way of lasting residues.

Biennial and Perennial Legumes

Alfalfa is a good choice for well-drained soils,near neutral in pH, and high in fertility. The goodsoil conditions required for the best growth ofalfalfa make it a poor choice for problem situa-tions. Where adapted, it is usually grown in arotation for a number of years (see chapter 11).Alfalfa is commonly interseeded with smallgrains, such as oats, wheat, and barley, and it

COVER CROPS 91

grows after the grain is harvested. The alfalfavariety “Nitro” can be used as an annual covercrop because it is not very winter-hardy andusually winter kills under northern conditions.Nitro continues to fix nitrogen later into the fallthan winter-hardy varieties. However, it does notreliably winter kill every year, and the smallamounts of extra fall growth and nitrogen fixa-tion may not be worth the extra cost of the seedcompared with perennial varieties.

Crown vetch is only adapted to well-drainedsoils, but can be grown under lower fertilityconditions than alfalfa. It has been used suc-cessfully for roadbank stabilization and is ableto provide permanent groundcover. Crownvetch has been tried as an interseeded “livingmulch,” with only limited success at providingnitrogen to corn. However, it is relatively easyto suppress crownvetch with herbicides to re-duce its competition with corn.

Red clover is vigorous, shade tolerant, win-ter-hardy, and can be established relatively eas-ily. Red clover is commonly interseeded withsmall grains. Because red clover starts growingslowly, the competition between it and the smallgrain is not usually great. Red clover also suc-cessfully interseeds with corn in the Northeast.

Sweet clover (yellow blossom) is a reason-ably winter-hardy, vigorous-growing crop withan ability to get its roots into compacted sub-soils. It is able to withstand high temperaturesand droughty conditions better than many othercover crops. Sweet clover requires a soil pH nearneutrality and a high calcium level. As long asthe pH is high, sweet clover is able to grow wellon low-fertility soils. It is sometimes grown fora full year or more, since it flowers and com-pletes its life cycle in the second year. Whenused as a green manure crop, it is incorporatedinto the soil before full bloom.

White clover does not produce as muchgrowth as many of the other legumes and is alsoless tolerant of droughty situations. (New Zea-land types of white clover are more drought tol-erant than the more commonly used Dutchwhite clover.) However, because it does not growvery tall and is able to tolerate shading betterthan many other legumes, it may be useful inorchard-floor covers or as a living mulch. It isalso a common component of intensively man-aged pastures

GRASSES

A problem common to all the grasses is that ifyou grow the crop to maturity for the maximumamount of residue, you reduce the amount ofavailable nitrogen for the next crop. This iscaused by the high C:N ratio, or low percentageof nitrogen, in grasses near maturity. The prob-lem can be avoided by killing the grass early orby adding extra nitrogen in the form of fertil-izer or manure. Another way to help with thisproblem is to supply extra nitrogen by seedinga legume-grass mix.

Winter rye, also called cereal or grain rye, isvery winter-hardy and easy to establish. Its abil-ity to germinate quickly, together with its win-ter-hardiness, means that it can be planted laterin the fall than most other species. Winter ryehas been shown to have an allelopathic effect,which means that it can chemically suppressweeds. It grows quickly in the fall and also growsreadily in the spring.

Oats are not winter-hardy. Summer or fallseedings will winter-kill under most northernconditions. This provides a naturally killedmulch the following spring and may help withweed suppression. As a mixture with one of the


clovers, oats provide some quick cover in thefall. Oat stems help trap snow and conservemoisture, even after it has been killed by frost.

Annual ryegrass (not related to winter rye)grows well in the fall, if established early enough.It develops a very extensive root system and there-fore provides very effective erosion control, whileadding significant quantities of organic matter. Itmay winterkill in northern climates. Some cau-tion is needed with annual ryegrass, because itmay become a problem weed in some situations.

Sudangrass and sorghum-sudan hybrids arefast-growing summer annuals that produce a lotof growth in a short time. Because of their vig-orous nature, they are good at suppressingweeds. If they are interseeded with a low-grow-ing crop, such as strawberries or many veg-etables, you may need to delay seeding so themain crop will not be severely shaded. Sun-dangrass is especially helpful for loosening com-pacted soil.

OTHER CROPS

Buckwheat is a summer annual that is easilykilled by frost. It will grow better than many othercover crops on low-fertility soils. It also growsrapidly and completes its life cycle quickly. Buck-wheat can grow more than 2 feet tall in themonth following planting. It competes well withweeds, because it grows so fast and, therefore,is used to suppress weeds following an earlyspring vegetable crop. It is possible to grow morethan one crop of buckwheat per year in manyregions. Its seeds do not disperse widely, but itcan reseed itself and become a weed. Mow or tillit before seeds develop to prevent re-seeding.

Rape is a winter-hardy member of the cruci-fer (cabbage) family. It grows well under the

moist and cool conditions of late fall, when otherkinds of plants just sit there and get ready forwinter. Rape is killed by harsh winter condi-tions in the North, but is grown as a winter cropin the middle and southern sections of the coun-try. Members of the crucifer family do not de-velop mycorrhizal fungi associations, so rape willnot promote mycorrhizae in the following crop.

MIXTURES OF COVER CROPS

Mixtures of cover crops offer combined benefits.The most common mixture is a grass and le-gume, such as winter rye and hairy vetch or oatsand red clover. Mixed stands usually do a betterjob of suppressing weeds than a single species.Growing legumes with grasses helps compen-sate for the decreases in nitrogen availability forthe following crop when grasses are allowed tomature. In the mid-Atlantic region, the winterrye-hairy vetch mixture has been shown to pro-vide another advantage for managing nitrogen:When a lot of nitrate is left in the soil at the endof the season, the rye is stimulated (reducingleaching losses). When little nitrogen is avail-able, the vetch competes better with the rye, fix-ing more nitrogen for the next crop.

A crop that grows erect, such as winter rye,may provide support for hairy vetch and enableit to grow better. Mowing close to the groundkills vetch supported by rye easier than vetchalone. This may allow mowing instead of herbi-cide use, in no-till production systems.

TIMING COVER CROP GROWTH

If you want to accumulate a lot of organic mat-ter, it’s best to grow a cover crop for the wholegrowing season (see figure 10.1a). This means

COVER CROPS 93

there will be no income-generating crop grownthat year. This may be useful with very infertile,possibly eroded, soils. It also may help vegetableproduction systems when there is no manureavailable and where a market for hay crops jus-tifies a longer rotation.

Most farmers sow cover crops after the eco-nomic crop has been harvested (figure 10.lb.).In this case, as with the system shown in figure10.1a, there is no competition between the covercrop and the main crop. The seeds can be drilledinstead of broadcast, resulting in better covercrop stands. In the Deep South and in thecountry’s mid-section, you can usually plantcover crops after harvesting the main crop. Innorthern areas, there may not be enough time

to establish a cover crop following harvest. Evenif you are able to get it established, there will belittle growth in the fall to provide soil protec-tion or nutrient uptake. The choice of a covercrop to fit between main summer crops (figure10.1b) is severely limited in northern climatesby the short growing season and severe cold.Winter rye is probably the most reliable covercrop for these conditions. In most situations,there are a range of establishment options.

The third management strategy is to interseedcover crops during the growth of the main crop(figure 10.1c). This system is especially helpfulfor the establishment of cover crops in short-growing-season areas. Delay seeding the covercrop until the main crop is off to a good start

Figure 10.1 Three ways to time cover crop growth for use with a summer crop.

a

January July January July January

b

c

cover crop

main crop

maincrop

maincrop

cover crop

cover crop cover crop

maincrop

maincrop


and will be able to grow well despite the com-petition. Good establishment of cover crops re-quires moisture and, for small-seeded crops,some covering of the seed by soil or crop resi-dues. On the other hand, cereal rye is able toestablish well without seed covering, as long assufficient moisture is present. Farmers using thissystem usually broadcast seed during or just af-ter the last cultivation. Aerial seeding, “highboy”tractors, or detasseling machines are used tobroadcast green manure seed after a main cropis already fairly tall. When growing on smallerscale, seed is broadcast with the use of a hand-crank spin seeder.

When used in winter grain cropping systems,cover crops are established following grain har-

vest in late spring, interseeded with the grainduring fall planting, or frost-seeded in earlyspring (figure 10.2a). With some early-matur-ing vegetable crops, especially in warmer re-gions, it is also possible to establish cover cropsin late spring or early summer (figure 10.2b).Cover crops also fit into an early vegetable-win-ter grain rotation sequence (figure 10.2c).

No matter when you establish cover crops,they are usually killed before or during soilpreparation for the next economic crop. This isdone by mowing (most annuals are killed bymowing once they’ve flowered), plowing intothe soil, with herbicides, or naturally by winterinjury. Good suppression of vetch in a no-tillsystem has been obtained with the use of a modi-

Figure 10.2 Timing cover crop growth for winter grain, early vegetable, and vegetable-grain systems.

a

b

c

January July January July January

winter grain

cover crop

earlyvegetable

earlyvegetable

earlyvegetable

winter grain

cover crop

cover crop

cover cropcover crop

COVER CROPS 95

fied rolling stalk chopper. It is a good idea toleave a week or two between the time a covercrop is tilled in or killed and a main crop isplanted. This allows some decomposition tooccur and may lessen problems of nitrogen im-mobilization and allelopathic effects. It also mayallow for the establishment of a better seedbedfor small-seeded crops, such as some of the veg-etables. Establishing a good seedbed for cropswith small seeds may be difficult, because of thelumpiness caused by the fresh residues.

Cover crops are sometimes allowed to flowerto provide bees or other beneficial insects withpollen. However, if the plants actually set seed,the cover crop may re-seed unintentionally.Cover crops that may become a weed probleminclude buckwheat, ryegrass, crown vetch, andhairy vetch.

INTERCROPS

Growing a cover crop between the rows of amain crop has been practiced for a long time. Ithas been called a living mulch, an intercrop,polyculture (if more than one crop will be har-vested), and an orchard-floor cover. Intercrop-ping has many benefits. Compared with bare soil,a groundcover provides erosion control, betterconditions for using equipment during harvest-ing, higher water-infiltration capacity, and anincrease in soil organic matter. In addition, ifthe cover crop is a legume, a significant buildupof nitrogen may be available to crops in futureyears. Another benefit is the attraction of ben-eficial insects, such as predatory mites to flow-ering plants. Less insect damage has been notedunder polyculture than under monoculture.

Growing other plants near the main crop alsoposes potential dangers. The intercrop may har-bor insect pests, such as the tarnished plant bug.Most of the management decisions for using in-tercrops are connected with minimizing com-petition with the main crop. Intercrops, if theygrow too tall, can compete with the main cropfor light, or may physically interfere with themain crop’s growth or harvest. Intercrops maycompete for water and nutrients. Using inter-crops is a highly questionable practice if rainfallis barely adequate for the main crop and supple-mental irrigation isn’t available. One way to de-crease competition is to delay seeding the inter-

Cover crops that may become a

weed problem include buckwheat,

ryegrass, crown vetch, and

hairy vetch.

In drier areas and on droughty soils, such assands, late killing of a winter cover crop mayresult in moisture deficiency for the main sum-mer crop. In these situations, the cover cropshould be killed before too much water is re-moved from the soil. However, in warm climateswhere no-till methods are practiced, allowingthe cover crop to grow longer means more resi-due and better water conservation for the maincrop. Cover crop mulch may more than com-pensate for the extra water removed from thesoil during the later period of green manuregrowth.

In very humid regions or on wet soils, theability of an actively growing cover crop to“pump” water out of the soil by transpirationmay be an advantage (see figure 14.2). Lettingthe cover crop grow as long as possible resultsin more rapid soil drying and allows for earlierplanting of the main crop.


crop until the main crop is well established. Thisis sometimes done in commercial fruit orchards.Soil improving intercrops established by delayedplanting into annual main crops are usually re-ferred to as cover crops. Herbicides, mowing,and partial rototilling are used to suppress thecover crop and give an advantage to the maincrop. Another way to lessen competition fromthe cover is to plant the main crop in a rela-tively wide cover-free strip. This provides moredistance between the main crop and the inter-crop rows.

SOURCES

Allison, F.E. 1973. Soil Organic Matter and its Rolein Crop Production. Elsevier Scientific Publish-ing Co. Amsterdam, Netherlands. In his dis-cussion of organic matter replenishment andgreen manures (pp. 450–451), Allison citesa number of researchers who indicate thatthere is little or no effect of green manureson total organic matter, even though the sup-ply of active (rapidly-decomposing) organicmatter increases.

Hargrove, W.L. (ed.). 1991. Cover Cops for CleanWater. Soil and Water Conservation Society.Ankeny, IA.

MacRae, R.J., and G.R. Mehuys. 1985. The ef-fect of green manuring on the physical prop-erties of temperate-area soils. Advances in SoilScience 3:71–94.

Managing Cover Crops Profitably, 2nd Edition.1998. Sustainable Agriculture Network,Handbook Series, #3. USDA Sustainable Ag-riculture Research and Education Program.Beltsville, MD. An excellent source for prac-tical information about cover crops.

Miller, P.R., W.L. Graves, W.A. Williams, and B.A.Madson. 1989. Cover Crops for California Ag-riculture. Leaflet 21471. Division of Agricul-ture and Natural Resources, University ofCalifornia. Davis, CA. This is the referencefor the experiment with clover in California.

Pieters, A.J. 1927. Green Manuring Principles andPractices. John Wiley & Sons. New York, NY.

Power, J.F. (ed.). 1987. The Role of Legumes inConservation Tillage Systems. Soil Conserva-tion Society of America. Ankeny, IA.

Sarrantonio, M. 1997. Northeast Cover CropHandbook. Soil Health Series, Rodale Insti-tute, Kutztown, PA.

Smith, M.S., W.W. Frye, and J.J. Varco. 1987.Legume winter cover crops. Advances in SoilScience 7:95–139.

97

Peter Kenagy’s vegetable rotation offers an annualwindow of opportunity to grow cover crops, whichhe has used to good effect for 15 years.

Kenagy, who farms just more than 300 acres inOregon’s fertile Willamette Valley, grows sweetcorn, winter wheat, grass seed and green beans.In between those cash crops, he plants a variety ofcover crops to build the soil and control weeds.Late summer and fall provide a large window af-ter green beans, for example. In the ground just70 days, green beans come off in July or August.

It’s a perfect time, Kenagy says, to plant a sum-mer cover crop like sudangrass, which will growup to five feet tall before winter-killing with thefirst frost. The thick grass mulch continues to pro-vide a good ground cover when he plants corninto it in the spring.

“I have a huge gap between one crop and thenext,” says Kenagy, who experiments with striptillage to lessen his impact on the soil. “I have tocontrol weeds during that period, which is justone of a number of things a cover crop does.”

Kenagy also uses cover crops because they cap-ture excess nutrients, containing them from flow-ing into the adjacent Willamette River. In addi-tion, the crops catch silt from almost annual floodwaters. In November 1999, for example, he had100 acres under water.

“The more cover crop vegetation you have there,the more silt you catch,” he says. “I try to get a lotof growth before fall.”

Kenagy tries different covers to achieve differ-ent goals. He has planted common vetch and crim-son clover to fix nitrogen in the soil and triticale,ryegrass, rape and oats to cut winter erosion andtake up nutrients. The covers help aerate the soiland counter the effects of compaction.

Some years, he follows beans with oats anddwarf essex rape planted in August.

“Spring oats grow fast, and I’m shooting to getas much growth as possible in the fall to have asmuch coverage of the ground as possible,” he says.At times, he asks a neighbor to bring in a herd ofsheep to graze the oats.

Kenagy’s commitment to good soil goes beyondplanting cover crops. He participated in an experi-ment with Oregon State University researchers totest a strip-till machine that would disturb just 6inches of soil — just one-fifth of the soil surfacetypically plowed. (For information about strip till-age, also called zone tillage, see chapter 15.)

The machine cuts slots through vegetative resi-due, which Kenagy likes to think mimics a morenatural system. Grassland or forests, he points out,undergo perpetual cycles of accumulating newresidue and undergoing decomposition by soilfauna.

“One of the most abusive things farmers do tothe soil is till it, and most do it repeatedly,” hesays. “Strip till does less abuse to the soil, and keep-ing the residue on top is a much more natural wayfor it to be handled.”

Kenagy was written up in his local newspaperfor his commitment to forestry on the farm. Hegrows a mix of walnut, hazelnut, elderberry andcottonwood trees in a thick 200-foot-wide bufferalong the Willamette River.

Kenagy cuts some of the trees for timber, butretains a dense hedgerow and riparian area thatattracts wildlife and sops up nutrients to protectthe river.

“As a society, we’ve made much too big a foot-print on the land,” he told the Oregon StatesmanJournal. “I think it’s time to make it smaller.”

Peter KenagyAlbany, Oregon


99

11�

Crop Rotations

…with methods of farming in which grasses form an

important part of the rotation, especially those that

leave a large residue of roots and culms, the decline of

the productive power is much slower than when crops

like wheat, cotton, or potatoes, which leave little

residue on the soil, are grown continuously.

—HENRY SNYDER, 1896

There are very good reasons to rotate crops.Rotating crops usually means fewer problems

with insects, parasitic nematodes, weeds, anddiseases caused by bacteria, viruses, and fungi.Rotations are effective for controllinginsects like the corn rootworm, nema-todes like the soybean cyst nematode,and diseases like root rot of field peas.In addition, rotations that include le-gumes supply nitrogen to succeedingcrops. Growing sod-type forage grasses,legumes, and grass-legume mixes aspart of the rotation also increases soil organicmatter. When you alternate two crops, such ascorn and soybeans, you have a very simple ro-tation. More complex rotations require three ormore crops and a five- to 10-year (or more) cycleto complete.

Rotations are an important part of any sus-tainable agricultural system. Yields of crops

grown in rotations are frequently about 10 per-cent higher than when grown in monoculture.When you grow a grain or vegetable crop fol-lowing a legume, the extra supply of nitrogen

certainly helps. However, yields of cropsgrown in rotation are often higher thanin monoculture, even when both aresupplied with plentiful amounts of ni-trogen. In addition, following a non-legume crop with another nonlegumealso produces higher yields than a mo-noculture. For example, when you grow

corn following grass hay, or cotton followingcorn, you get higher yields than when corn orcotton are grown year after year. This yield ben-efit from rotations is sometimes called a rota-tion effect. Another important benefit of rota-tions is that growing a variety of crops in a givenyear spreads out labor needs and reduces riskcaused by climate or market conditions.


ROTATIONS INFLUENCE SOILORGANIC MATTER LEVELS

You might think you’re doing pretty well if soilorganic matter remains the same under a par-ticular cropping system. However, if you areworking soils with depleted organic matter, youneed to build up levels to counter the effects ofprevious practices. Maintaining an inadequatelylow level of organic matter won’t do!

The types of crops you grow, their yields, theamount of roots produced, the portion of thecrop that is harvested, and how you treat cropresidues will all affect soil organic matter. Soilfertility itself influences the amount of organicresidues returned, because more fertile soilsgrow higher-yielding crops, with more residues.

The decrease in organic matter levels whenrow crops are planted on a virgin forest or prai-rie soil is very rapid for the first five to 10 years,

but eventually, a plateau or equilibrium isreached. After that, soil organic matter levelsremain stable, as long as production practicesaren’t changed. An example of what can occurduring 25 years of continuously grown corn isgiven in figure 11.1. Soil organic matter levelsincrease when the cropping system is changedfrom a cultivated crop to a grass or mixed grass-legume sod. However, the increase is usuallymuch slower than the decrease that occurredunder continuous tillage.

A long-term cropping experiment in Missouricompared continuous corn to continuous sodand various rotations. More than 9 inches oftopsoil was lost during 60 years of continuouscorn. The amount of soil lost each year fromthe continuous corn plots was equivalent to 21tons per acre. After 60 years, soil under con-tinuous corn had only 44 percent as much top-soil as that under continuous timothy sod. A

Figure 11.1 Organic matter changes in the plow layer during long-term cultivation followed byhay-crop establishment.

org

anic

mat

ter

(%)

4

3

2

1

0

25 50 75 100years

corn(moldboard

plow)

grass sod

CROP ROTATIONS 101

six-year rotation consisting of corn, oats, wheat,clover, and two years of timothy resulted inabout 70 percent as much topsoil as found inthe timothy soil, a much better result than withcontinuous corn. Differences in erosion and or-ganic matter decomposition resulted in soil or-ganic matter levels of 2.2 percent for the unfer-tilized timothy and 1.2 percent for the continu-ous corn plots.

Two things happen when perennial forages(hay-type crops) are part of the rotation and re-main in place for some years during a rotation.First, the rate of decomposition of soil organicmatter decreases, because the soil is not con-tinually being disturbed. (This also happenswhen using no-till planting, even for non-sod-type crops, such as corn.) Second, grass and le-gume sods develop extensive root systems, partof which will naturally die each year, adding neworganic matter to the soil. Crops with extensiveroot systems stimulate high levels of soil bio-logical activity. The roots of a healthy grass orlegume-grass sod return more organic matter to

the soil than roots of most other crops. Olderroots of grasses die, even during the growingseason, and provide sources of fresh, active or-ganic matter. Roots of plants also continuallygive off, or exude, a variety of chemicals thatnourish nearby microorganisms.

We are not only interested in total soil or-ganic matter — we want a wide variety of dif-ferent types of organisms living in the soil. Wealso want to have a good amount of active or-ganic matter and high levels of well decomposedsoil organic matter, or humus, in the soil. Al-though most experiments have compared soilorganic matter changes under different croppingsystems, few experiments have looked at the ef-fects of rotations on soil ecology. The more resi-dues your crops leave in the field, the greaterthe populations of soil microorganisms. Experi-ments in a semiarid region in Oregon found thatthe total amount of microorganisms in a two-year wheat-fallow system was only about 25percent of the amount found under pasture.Conventional moldboard plow tillage systems

TABLE 11.1Comparison of Rotations:

Percent of Time Active Roots are Presentand Number of Species

ACTIVE NUMBER

ROOTING OF

ROTATION YEARS PERIOD (%) SPECIES

Corn-soybeans 2 32 2

Dry beans-winter wheat 2 57 2

Dry beans-winter wheat/cover 2 92 3

Dry beans-winter wheat-corn 3 72 3

Corn-dry beans-winter wheat/cover 3 76 4

Sugar beets-beans-wheat/cover-corn 4 65 5

—MICHIGAN FIELD CROP ECOLOGY, 1998.


are known to decrease the populations of earth-worms, as well as other soil organisms. Morecomplex rotations increase soil biological diver-sity. Including perennial forages in the rotationenhances this effect.

RESIDUE AVAILABILITY

As pointed out in chapters 3, 5, and 8, moreresidues are left in the field after some crops thanothers. High residue-producing crops should beincorporated into rotations whenever possible.

SPECIES RICHNESS ANDACTIVE ROOTING PERIODS

In addition to the quantity of residues remain-ing following harvest, a variety of types of resi-dues is also important. The goal should be aminimum of three different species in a rota-tion, with more if possible. The percent of thetime that living roots are present during a rota-tion is also important. The period that activeroots are present varies considerably, rangingfrom 32 percent of a corn-soybeans rotation to57 percent of the time for a beans-wheat rota-tion to 76 percent of the time for a 3-year beans-wheat-corn rotation (table 11.1).

FARM LABOR AND ECONOMICS

Before discussing appropriate rotations, let’s con-sider some of the possible effects on farm laborand finances. If you grow only one or two rowcrops, you must work incredibly long hours dur-ing planting and harvesting seasons. Includingforage hay crops and early harvested crops, alongwith those that are traditionally harvested in thefall, allows farmers to spread their labor overthe growing season, making the farm more eas-

ily managed by family labor alone. In addition,when you grow a more diversified group ofcrops, you are less affected by price fluctuationsof one or two crops. This may provide more year-to-year financial stability.

Although, as pointed out above, there aremany possible benefits of rotations, there are alsosome costs or complicating factors. It is criti-cally important to carefully consider the farmfamily’s labor and management capacity whenexploring diversification opportunities. You mayneed more equipment to grow a number of dif-ferent crops. There may be conflicts between la-bor needs for different crops; cultivation andsidedressing nitrogen fertilizer for corn in somelocations might occur at the same time as har-vesting hay. In addition, the more diversified thefarm, the less chance for time to relax.

GENERAL PRINCIPLES

Try to consider the following principles whenyou’re thinking about a new rotation:

1. Follow a legume forage crop, such as cloveror alfalfa, with a high nitrogen-demandingcrop, such as corn, to take advantage of thenitrogen supply.

2. Grow less nitrogen-demanding crops, suchas oats, barley, or wheat, in the second orthird year after a legume sod.

3. Grow the same annual crop for only one year,if possible, to decrease the likelihood of in-sects, diseases, and nematodes becoming aproblem.

4. Don’t follow one crop with another closelyrelated species, since insect, disease, andnematode problems are frequently shared bymembers of closely related crops.

CROP ROTATIONS 103

5. Use crop sequences that promote healthiercrops. Some crops seem to do well follow-ing a particular crop (for example, cabbagefamily crops following onions, or potatoesfollowing corn). Other crop sequences mayhave adverse effects, as when potatoes havemore scab following peas or oats.

6. Use crop sequences that aid in controllingweeds. Small grains compete strongly againstweeds and may inhibit germination of weedseeds, row crops permit mid-season culti-vation, and sod crops that are mowed regu-larly or intensively grazed help control an-nual weeds.

7. Use longer periods of perennial crops, suchas a forage legume, on sloping land and onhighly erosive soils. Using sound conserva-tion practices, such as no-till planting, ex-tensive cover cropping, or strip-cropping (apractice that combines the benefits of rota-tions and erosion control), may lessen theneed to follow this guideline.

8. Try to grow a deep-rooted crop, such as al-falfa, safflower, or sunflower, as part of therotation. These crops scavenge the subsoilfor nutrients and water, and channels leftfrom decayed roots can promote water infil-tration.

9. Grow some crops that will leave a significantamount of residue, like sorghum or cornharvested for grain, to help maintain organicmatter levels.

10.When growing a wide mix of crops — as isdone on many direct marketing vegetable farms— try grouping into blocks according to plantfamily, timing of crops (all early season cropstogether, for example), type of crop (root vs.fruit vs. leaf), or crops with similar culturalpractices (irrigated, using plastic mulch).

ROTATION EXAMPLES

It’s impossible to recommend specific rotationsfor a wide variety of situations. Every farm hasits own unique combination of soil and climateand of human, animal, and machine resources.The economic conditions and needs are alsodifferent on each farm. You may get useful ideasby considering a number of rotations with his-torical or current importance.

A five- to seven- year rotation was commonin the mixed livestock-crop farms of the north-ern Midwest and Northeast during the first halfof the 20th century. An example of this rotationis the following:

Year 1. CornYear 2. Oats (mixed legume/grass hay seeded)Years 3, 4, and 5. Mixed grass-legume hayYears 7 and 8. Pasture

The most nitrogen-demanding crop, corn,followed the pasture, and grain was harvestedonly two of every five to seven years. A less ni-trogen-demanding crop, oats, was planted in thesecond year as a “nurse crop” when the grass-legume hay was seeded. The grain was harvestedas animal feed and oat straw was harvested tobe used as cattle bedding; both eventually werereturned to the soil as animal manure. This ro-tation maintained soil organic matter in manysituations, or at least didn’t cause it to decreasetoo much. On prairie soils, with their very highoriginal contents of organic matter, levels stillprobably decreased with this rotation.

In the corn belt region of the Midwest, achange in rotations occurred as pesticides andfertilizers became readily available and animalswere fed in large feedlots, instead of on crop-producing farms. Once the mixed livestock


farms became grain-crop farms or crop-hogfarms, there was little reason to grow sod crops.In addition, government commodity price sup-port programs unintentionally encouraged farm-ers to narrow production to just two feed grains.The two-year corn-soybean rotation is betterthan monoculture, but it has a number of prob-lems, including erosion, groundwater pollutionwith nitrate and herbicides, depletion of soilorganic matter, and increased insect problems(see box). Research indicates that with highyields of corn grain there may be sufficient resi-dues to maintain organic matter. With soybeans,residues are minimal.

The Thompson mixed crop-livestock (hogsand beef) farm in Iowa practices an alternateseven-year corn belt rotation similar to the firstrotation we described. For fields that are con-venient for pasturing beef cows, the Thompsonrotation is as follows:

Year 1. CornYear 2. SoybeansYear 3. CornYear 4. Oats (mixed legume/grass hay seeded)Years 5, 6, and 7. Mixed grass-legume hay

Organic matter is maintained through a com-bination of practices that include the use ofmanures and municipal sewage sludge, greenmanure crops (oats and rye following soybeansand hairy vetch between corn and soybeans),

crop residues, and sod crops. These practiceshave resulted in a porous soil that has signifi-cantly lower erosion, higher organic matter con-tent, and more earthworms than neighbors’ fields

A four-year rotation under investigation inVirginia uses mainly no-till practices as follows:

Year 1. Corn, winter wheat no-till planted intocorn stubble.

Year 2. Winter wheat grazed by cattle, foxtailmillet no-till planted into wheat stubble andhayed or grazed, alfalfa no-till planted in fall.

Year 3. Alfalfa harvested and/or grazed.Year 4. Alfalfa is harvested and/or grazed as usual

until fall, then heavily stocked with animals toweaken it so that corn can be planted the nextyear.

This rotation follows many of the principlesdiscussed earlier in this chapter. It was designedby researchers, extension specialists, and farm-ers and is very similar to the older rotation de-scribed earlier. A few differences exist — thisrotation is shorter, alfalfa is used instead of clo-ver or clover-grass mixtures, and there is a spe-cial effort to minimize pesticide use under no-till practices. Weed-control problems occurredwhen going from alfalfa (fourth year) back tocorn. This caused the investigators to use falltillage followed by a cover crop mixture of win-ter rye and hairy vetch. Some success wasachieved suppressing the cover crop in thespring by just rolling over it with a disk harrowand planting corn through the surface residueswith a modified no-till planter. The heavy covercrop residues on the surface provided excellentweed control for the corn.

Traditional wheat-cropping patterns for thesemi-arid regions of the Great Plains and the North-west commonly include a fallow year to allow

For many years, the western corn rootwormwas effectively controlled by alternating be-tween corn and soybeans. Recently, popula-tions of the rootworm with a longer restingperiod have developed and they are able tosurvive the very simple rotation.

CROP ROTATIONS 105

storage of water and more mineralization of ni-trogen from organic matter for use by the nextwheat crop. However, the wheat-fallow systemhas several problems. Because no crop residuesare returned during the fallow year, soil organicmatter decreases unless manure or other organicmaterials are provided from off the field. Waterinfiltrating below the root zone during the fallowyear moves salts through the soil to the low partsof fields. Shallow groundwater can come to thesurface in these low spots and create “salineseeps,” where yields will be decreased. Increasedsoil erosion, caused by either wind or water, com-monly occurs during fallow years and organicmatter decreases (at about 2 percent per year, inone experiment). In this wheat monoculture sys-tem, the build-up of grassy weed populations,such as jointed goat grass and downy brome, alsoindicates that crop diversification is essential.

Farmers in this region who are trying to de-velop more sustainable cropping systems shouldconsider using a number of species, includingdeeper-rooted crops, in a more diversified rota-tion. This would increase the amount of resi-dues returned to the soil, reduce tillage, andlessen or eliminate the fallow period.

A four-year wheat-corn-millet-fallow rotationunder evaluation in Colorado was found to bebetter than the traditional wheat-fallow system.Wheat yields have been higher in this rotationthan wheat grown in monoculture. The extraresidues from the corn and millet also are help-ing to increase soil organic matter. Many pro-ducers are also including sunflower, a deep-root-ing crop, in a wheat-corn-sunflower-fallow ro-tation. Sunflower is also being evaluated in Or-egon as part of a wheat cropping sequence.

Vegetable farmers who grow a large selectionof crops find it best to rotate in large blocks —with each containing crops from the same fami-

lies or having similar production schedules orcultural practices. Many farmers are now usingcover crops to help “grow their own nitrogen,”utilize extra nitrogen that might be there at theend of the season, and add organic matter tothe soil. A four- to five-year vegetable rotationmight be as follows:

Year 1. Sweet corn followed by a hairy vetch/winter rye cover crop.

Year 2. Pumpkins, winter squash, summersquash followed by a rye or oats cover crop.

Year 3. Tomatoes, potatoes, peppers followedby a vetch/rye cover crop.

Year 4. Crucifers, greens, legumes, carrots,onions, and miscellaneous vegetablesfollowed by a rye cover crop.

Year 5. (If land is available) Oats and redclover or buckwheat followed by a vetch/ryecover crop.

Another rotation for vegetable growers usesa two- to three-year alfalfa sod as part of a six-to eight-year cycle. In this case, the crops fol-lowing the alfalfa are high-nitrogen-demandingcrops, such as corn or squash, followed by cab-bage or tomatoes, and, in the last two years,crops needing a fine seedbed, such as lettuce,onions, or carrots. Annual weeds in this rota-tion are controlled by the harvesting of alfalfa anumber of times each year. Perennial weedpopulations can be decreased by cultivationduring the row-crop phase of the rotation.

Most vegetable farmers do not have enoughland — or the markets — to have a multi-yearhay crop on a significant portion of their land.Aggressive use of cover crops will help to main-tain organic matter in this situation. Manures,composts, or other sources of organic materials,such as leaves, should also be applied every yearor two to help maintain soil organic matter.


SOURCES

Anderson, S.H., C.J. Gantzer, and J.R. Brown.1990. Soil physical properties after 100 yearsof continuous cultivation. Journal of Soil andWater Conservation 45:117–121.

Baldock, J.0., and R.B. Musgrave. 1980. Manureand mineral fertilizer effects in continuousand rotational crop sequences in central NewYork. Agronomy Journal 72:511–518.

Barber, S.A. 1979. Corn residue managementand soil organic matter. Agronomy Journal71:625–627.

Michigan Field Crop Ecology: Managing BiologicalProcesses for Productivity and EnvironmentalQuality. 1998. Cavigelli, M. A., S. R. Deming,L. K. Probyn, and R. R. Harwood (eds.).Michigan State University Extension Bulle-tin E-2646. East Lansing, MI.

Coleman, E. 1989. The New Organic Grower.Chelsea Green. Chelsea, VT. See this refer-ence for the vegetable rotation.

Francis, C.A., and M.D. Clegg. 1990. Crop ro-tations in sustainable production systems. InSustainable Agricultural Systems (C.A. Ed-wards, R. Lal, P. Madden, R.H. Miller, and G.House, eds.). Soil and Water ConservationSociety. Ankeny, IA.

Gantzer, C.J., S.H. Anderson, A.L. Thompson,and J.R. Brown. 1991. Evaluation of soil lossafter 100 years of soil and crop management.Agronomy Journal 83:74–77. This source de-scribes the long-term cropping experimentin Missouri.

Grubinger, V.P. 1999. Sustainable Vegetable Pro-duction: From Start-Up to Market. Natural Re-source and Agricultural Engineering Service,Ithaca, NY.

Havlin, J.L., D.E. Kissel, L.D. Maddux, M.M.Claassen, and J.H. Long. 1990. Crop rota-tion and tillage effects on soil organic carbon

and nitrogen. Soil Science Society of AmericaJournal 54:448–452.

Luna, J.M., V.G. Allen, W.L. Daniels, J.F. Fonte-not, P.G. Sullivan, C.A. Lamb, N.D. Stone,D.V. Vaughan, E.S. Hagood, and D.B. Taylor.1991. Low-input crop and livestock systemsin the southeastern United States. pp. 183-205. In Sustainable Agriculture Research andEducation in the Field. Proceedings of a con-ference, April 3–4, 1990. Board on Agricul-ture, National Research Council. Washing-ton, D.C.: National Academy Press. This isthe reference for the rotation experiment inVirginia.

National Research Council. 1989. AlternativeAgriculture. National Academy Press. Wash-ington, D.C. This is the reference for the ro-tation used on the Thompson farm.

Peterson, G.A., and D.G. Westfall. 1990. Sus-tainable dryland agroecosystems. In Conser-vation Tillage. Proceedings of the Great PlainsConservation Tillage System Symposium, Au-gust 21–23, 1990, Bismark, ND. Great PlainsAgricultural Council Bulletin No. 131. Seethis reference for the wheat-corn-millet-fal-low rotation under evaluation in Colorado.

Rasmussen, P.E., H.P. Collins, and R.W. Smiley.1989. Long-Term Management Effects on SoilProductivity and Crop Yield in Semi-Arid Re-gions of Eastern Oregon. USDA-AgriculturalResearch Service and Oregon State Univer-sity Agricultural Experiment Station, Colum-bia Basin Agricultural Research Center,Pendleton, OR. This describes the Oregonstudy of sunflowers as part of a wheat crop-ping sequence.

Werner, M.R., and D.L. Dindal. 1990. Effects ofconversion to organic agricultural practiceson soil biota. American Journal of AlternativeAgriculture 5(1): 24–32.

107

When the horse stable down the road went out ofbusiness, it forced Alex and Betsy Hitt to re-evalu-ate their farm fertility program. The Hitts, whoraise 75 varieties of vegetables and an equal num-ber of cut flowers just outside Chapel Hill, N.C.,were forced to search for an alternative to horsemanure to amend the soil on their five-acre farm.

The Hitts, who have made the most out of ev-ery acre, created an elaborate rotation that includesboth winter and summer cover crops to supplyorganic matter and nitrogen, lessen erosion andcrowd out weeds.

“We designed a rotation so that cover crops playa clear role,” Alex Hitt says. “Many times, whereother growers might say, ‘I need to grow a cashcrop,’ we’ll grow a cover crop anyway.”

The Hitts stay profitable, however, thanks to amarketing plan that takes full advantage of theirlocation near Chapel Hill, home to the Universityof North Carolina. Their more unusual producesuch as leafy greens, leeks and rapini find a homein restaurants, and — alongside their most profit-able lettuce, tomato, pepper, and flower crops —sell well at area farmers markets.

A typical year in one unit of the Hitts’ rotationincludes a cool-season crop, a summer cover cropsuch as soybeans and sudangrass, followed by afall season cash crop and then a winter cover.

“We have made a conscious decision in our rota-tion design to always have cover crops,” Alex Hittsays. “We have to — it’s the primary source for allof our fertility. If we can, we’ll have two covers onthe same piece of ground in the same year.”

While other farmers grow beans, corn or an-other profitable annual vegetable in the summer

after a spring crop, the Hitts don’t hesitate to takethe land out of production. Instead, Alex Hitt says,their commitment to building organic matter inthe soil yields important payoffs. The farm remainsessentially free of soil-borne diseases, which theyattribute to “so much competition and diversity”in the soil. And, despite farming on a five-percentslope, they see little or no erosion.

Alex and Betsy HittGraham, North Carolina


Alex and Betsy Hitt’s Rotation(cover crops in bold)

Year 1. Tomatoes (half no-till)Oats w/ Crimson Clover

Year 2. Cool Season FlowersSudangrass w/ SoybeansOats w/ Crimson Clover

Year 3. Spring LettuceSummer FlowersRye w/ Hairy Vetch

Year 4. No-till SquashYear 5. Over-wintered Flowers

Sudangrass w/ SoybeansRye w/ Hairy Vetch

Year 6. Peppers (half no-till)Wheat w/ Crimson Clover

Year 7. Summer FlowersOats w/ Crimson Clover

Year 8. Mixed Spring VegetablesCowpeas

Year 9. Over-wintered FlowersSudangrass w/ SoybeansOats w/ Crimson Clover

Year 10. Summer FlowersWheat w/ Hairy Vetch

108

“There are a billion benefits from cover crops,”Alex Hitt says. “We have really active soil — wecan see it by the good crops that we grow, and bythe problems that we don’t have.”

The Hitts’ rotation works well for growing flow-ers, a profitable direct-to-market crop that usu-ally requires less nitrogen than vegetables. Thechallenge, Alex Hitt says, is in choosing the rightcover prior to the next crop to get the maximumgrowth from the cover.

They continue to test different cover varieties— this year it’s Austrian winter peas and severaldifferent clovers — in a quest for covers that areeasy to establish and incorporate. The Hitts arebeginning to grow some crops in a no-till system,so an easy-to-kill cover crop is paramount.

Not only does the rotation help improve soilquality, but it also goes far toward controllingweeds. The covers smother weeds by crowdingand shading them out. A summer crop of cow-

peas, for example, covers all bare soil. Even moreeffective is the Hitts’ complex rotation, which con-founds the weeds by varying the timing and spac-ing of planting and cultivation season to season.

“We either have a different crop or we’re plant-ing it differently, so we don’t get the same weedsthe same time every year,” Alex Hitt says. “Whenwe went to a longer rotation and changed the tim-ing, we noticed it quickly.”

The Hitts keep in touch with their soil mineralbalance by testing all sections annually. They watchpH (calcium and magnesium), phosphorus andpotassium levels. Keeping the proper balance inthe soil, plus their complex 10-year rotation hashelped reduce agricultural pests, Alex Hitt says.

“The whole system works better,” he says. “Wedon’t have many diseases and we have a lot of bene-ficial insects. The whole thing is really in balance,and the rotation and cover crops have a lot to dowith that.”

109

Decomposition of organic materials takesplace naturally in forests and fields all

around us. Composting is the art and science ofcombining available organic wastes so that theydecompose to form a uniform andstable finished product. Composts areexcellent organic amendments forsoils. Composting reduces bulk, sta-bilizes soluble nutrients, and hastensthe formation of humus. Most organicmaterials, such as manures, crop resi-dues, grass clippings, leaves, sawdust, and manykitchen wastes, can be composted.

The microorganisms that do much of the workof rapid composting need high temperatures,plenty of oxygen, and moisture. These heat-lov-ing, or thermophilic, organisms work best be-tween about 110 and 130°F. Temperatures above140°F can develop in compost piles, helping kill

12�

Making and Using Composts

The reason of our thus treating composts of various

soils and substances, is not only to dulcify, sweeten,

and free them from the noxious qualities they otherwise

retain . . . [Before composting, they are] apter to ingender

vermin, weeds, and fungous . . . than to produce wholsome

[sic] plants, fruits and roots, fit for the table.

—J. EVELYN, SEVENTEENTH CENTURY

off weed seeds and disease organisms, but thisoverheating usually slows down the process. Attemperatures below 110°F, the less active meso-phylic organisms take over and the rate of com-

posting again slows down. The com-posting process is slowed by anythingthat inhibits good aeration or themaintenance of high enough tempera-tures and sufficient moisture.

Composting farm wastes and or-ganic residues from off the farm has

become a widespread practice. Accepting andcomposting lawn and garden wastes providessome income for farmers near cities and towns.They may charge for accepting the wastes andfor selling compost. Some farmers, especiallythose without animals or sod crops, may wantto utilize the compost as a source of organicmatter for their own soils.


MAKING COMPOSTS

Moisture

The amount of moisture in a compost pile isimportant. If the materials mat and rainwatercan’t drain easily through the pile, it may notstay aerobic in a humid climatic zone. On theother hand, if composting is done inside a barnor under dry climatic conditions, the pile maynot be moist enough to allow microorganismsto do their jobs. Moisture is lost during the ac-tive phase of composting, so it may be neces-sary to add water to a pile. In fact, even in ahumid region, it is a good idea to moisten thepile at first, if dry materials are used. However,if something like liquid manure is used to pro-vide a high-nitrogen material, sufficient mois-ture will most likely be present to start thecomposting process. The ideal moisture contentof composting material is about 40 to 60 per-cent, or about as damp as a wrung-out sponge.If the pile is too dry — 35 percent or less —ammonia is lost as a gas and beneficial organ-isms don’t repopulate the compost after the tem-perature moderates. Very dry, dusty compostsbecome populated by molds instead of the bene-ficial organisms we want.

Types of Starting Materials

The organic materials used should have lots ofcarbon and nitrogen available for the microor-ganisms to use. High-nitrogen materials, such aschicken manure, can be mixed with high-carbonmaterials like hay, straw, leaves, or sawdust.Compost piles are often built by alternating lay-ers of these materials. Turning the pile mixesthe materials together. Manure mixed with saw-dust or wood chips used for bedding can becomposted as is. Composting occurs most eas-ily if the average C:N ratio of the materials is about25 to 40 parts carbon for every part nitrogen(see chapter 8 for a discussion of C:N ratios).

There are too many different types of materi-als that you might work with to give blanketrecommendations about how much of each tomix to get the moisture content and the C:Ninto reasonable ranges so the process can get offto a good start. One example is given in the boxon the following page.

Types of “Composting”

Some people talk about “low temperature”composting — different types are called“sheet,” worm (vermicomposting), small pilecomposting — and “high temperature” com-posting. We like to use the term compostingonly when talking about the rapid decompo-sition that takes place at high temperatures.

Even Birds Do It?

The male brush turkey of Australia gathersleaves, small branches, moss, and other litterand builds a mound about 3 feet high and 5feet across. It then digs holes into the moundrepeatedly and refills them — helping to frag-ment and mix the debris. Finally, the pile iscovered with a layer of sticks and twigs.

The female lays her eggs in a hole dug intothe pile, which heats up to close to 100°Faround the eggs while the outside can bearound 65°F. The heat of the composting pro-cess frees the birds from having to sit on theeggs to incubate them.

—SEYMORE, 1991

MAKING AND USING COMPOSTS 111

Cornell University’s web site for compostingissues (http://www.cfe.cornell.edu/compost/Composting_homepage.html) features formulasto help you estimate the different proportionsof the specific materials you might want to usein the compost pile. Sometimes it will work outthat the pile may be too wet, too low in C:N(that means, too high in nitrogen), or too highin C:N (low in nitrogen). To balance your pile,you may need to add other materials, or changethe ratios used. The examples given above canbe remedied by adding dry sawdust or wood

chips in the first two cases and nitrogen fertil-izer in the third. If a pile is too dry, you can addwater with a hose or sprinkler system.

One thing to keep in mind is that not all car-bon is equally available for microorganisms. Lig-nin is not easily decomposed (we mentioned thiswhen discussing soil organisms in chapter 3 andagain in chapter 8, when we talked about thedifferent effects that various residues have whenapplied to soil). Although some lignin is decom-posed during composting — probably depend-ing on factors such as the type of lignin and the

A Sample Compost Recipe

Start with:

a) grass clippings (71% moisture, 45% C, and 2.4% N)b) leaves (35% moisture, 50% C, and 0.75% N)c) food scraps (80 % moisture, 42% C, and 5.0% N)

The ratio of the materials needed to get 60 percent moistureand 30:1 C:N is:

d) 100 lbs. of grass, 130 lbs. of leaves, and 80 lbs. of food scraps.

—RICHARD, TRAUTMANN & KRASNY, 1996

TABLE 12.1Total Versus Biodegradable Carbon and Estimated C:N Ratios

MATERIAL% CARBON C:N% CARBON C:N% LIGNIN % CELL WALL % NITROGEN

(TOTAL) (BIODEGRADABLE)

newsprint 39 116 18 54 21 97 0.34

wheat straw 51 89 34 58 23 95 0.58

poultry manure 43 10 42 9 2 38 4.51

maple wood chips 50 51 44 45 13 32 0.97

—RICHARD, TRAUTMANN & KRASNY, 1996


moisture content — high amounts of carbonpresent as lignin may indicate that not all ofcarbon will be available for rapid composting.When residues contain high amounts of lignin,it means that the effective C:N can be quite a bitlower than indicated by using total carbon inthe calculation (table 12.1). For some materi-als, there is little difference between the C:Ncalculated with total carbon versus using onlybiodegradable carbon.

It’s a good idea to avoid using certain materi-als, such as coal ash, wood chips from pressure-treated lumber, manure from pets, and largequantities of fats, oils, and waxes. These typesof materials are either difficult to compost ormay result in compost containing chemicals thatcan harm crops.

Wood chips or bark are sometimes used as abulking agent to provide a “skeleton” for goodaeration. These materials may be recycled byshaking the finished compost out of the bulk-ing material, which can then be used for a fewmore composting cycles.

Pile Size

A compost pile is a large natural convectivestructure — something like many chimneys allnext to each other — moving oxygen into thepile as carbon dioxide, moisture, and heat risefrom it. The materials need to fit together in away that allows oxygen from the air to flow infreely. On the other hand, it is also importantthat not too much heat escape from the centerof the pile. If small-sized particles are used, a“bulking agent” may be needed to make surethat enough air can enter the pile. Sawdust, dryleaves, hay, and wood shavings are frequentlyused as bulking agents. Tree branches need tobe “chipped” and hay chopped so that it doesn’t

mat and slow composting. Composting will takelonger when large particles are used, especiallythose resistant to decay.

The pile needs to be large enough to retainmuch of the heat that develops during compost-ing, but not so large and compacted that air can’teasily flow in from the outside. Compost pilesshould be 3 to 5 feet tall and about 6 to 10 feetacross the base after the ingredients have settled(see figure 12.1). (You might want it on the wideside in the winter, to help maintain the warmtemperatures, while gardeners can make com-post in a 3-feet tall by 3-feet wide pile in thesummer.) Easily condensed material should ini-tially be piled higher than 5 feet. It is possibleto have long windrows of composting materi-als, as long as they are not too tall or wide.

Turning the Pile

Turning the composting residues exposes all thematerials to the high-temperature conditions atthe center of the pile. Although the materials atthe top and on the sides of the pile are barelycomposting, they do provide insulation for the

Composting Animals

It is also possible to compost dead farm ani-mals, which are sometimes a nuisance to getrid of. Chickens and even dead cows havebeen successfully composted. Cam Tabb, aWest Virginia dairy and crop farmer, starts theprocess for large animals by laying the carcassthat’s been in the open for one day on a 3 to4 ft bed of sawdust. Then he covers it with 3to 4 ft of sawdust/horse manure and then turnsthe pile in 3 to 4 weeks. After all turning isdone, he uses new base material on top.


rest of the pile. Turning the pile rearranges allthe materials and creates a new center. If pilesare turned every time the interior reaches andstabilizes at about 140°F for a few days, it ispossible to complete the composting processwithin months. On the other hand, if you onlyturn the pile occasionally, it may take a year orlonger to complete. Equipment is now available

to quickly turn long compost windrows at large-scale composting facilities. Tractor-poweredcompost turners designed for composting onfarms are also available.

Although turning compost frequently speedsup the process, it may also dry out the pile andcause more nitrogen loss. If the pile is too dryyou might consider turning it when it’s rainingto help moisten it. If the pile is very wet, youmight want to turn it on a sunny day. Very fre-quent turning may not be advantageous, becauseit can cause physical breakdown of importantstructural materials that aid natural aeration. Theright amount of turning depends on a variety offactors, such as aeration, moisture, and tempera-ture. Turn your compost pile to avoid cold, wetcenters, break up clumps, and make the com-post more uniform later in the process beforeuse or marketing.

Figure 12.1 Compost pile dimensions and turning techniques.

c) After first turning(pile covered with composted material).

d) Composting finished(pile smaller than original size).

oxygen

water, heat, and carbondioxide

a) Early stage of composting(pile about 5 feet tall by 8 to 10 feet at base).

b) During first turning(covering now inside and partially

composted material used on top and sides).

oxygen

Minimum Turning Technique

Farm-quality composts can be produced byturning the pile only once or twice. To do thisyou need to carefully construct the pile —building it up to reasonable dimensions, us-ing and thoroughly mixing materials that givevery good porosity, and making sure the pilestays moist.


The Curing Stage

Following high-temperature composting, thepile should be left to cure for about one to threemonths. Usually this is done once pile tempera-tures reach 105°F and high temperatures don’treoccur following turning. Curing is especiallyneeded if the active (hot) process is short orpoorly managed. There is no need to turn thepile during curing because you are not trying tostimulate maximum decomposition and thereis less need for rapid oxygen entry into the pile’scenter when decomposition rate is slow. [How-ever, the pile may still need turning during thecuring stage if it is very large or it didn’t reallyfinish composting (determining when compostis finished is sometimes difficult), or if the pile

is soaked by rain.] Curing the pile furthers aero-bic decomposition of resistant chemicals andlarger particles. Common beneficial soil organ-isms populate the pile during curing, the pHbecomes closer to neutral, and ammonium isconverted to nitrate. Be sure to maintain watercontent around 50 percent during curing toensure that active populations of beneficial or-ganisms develop.

It is thought that the processes that occurduring the early curing process give compostsome of its disease-suppressing qualities. On theother hand, beneficial organisms require sourcesof food to sustain them. Thus, if composts areallowed to cure for too long — depleting all theavailable food sources — disease suppressionqualities may decrease and eventually be lost.

Research by Harry Hoitink and co-workers at OhioState University shows that composts can sup-press root and leaf diseases of plants. This sup-pression comes about because the plants aregenerally healthier (microorganisms produceplant hormones as well as chelates that makemicronutrients more available) and, therefore, arebetter able to resist infection. Beneficial organ-isms compete with disease organisms for nutri-ents as well as directly consume the disease-causing organisms or produce antibiotics that killbacteria. Some organisms, such as springtails andmites, “actually search out pathogen propagulesin soils and devour them,” according to Hoitink.In addition, he found that potting mixes contain-ing composts “rich in biodegradable organicmatter support microorganisms that induce sys-temic resistance in plants. These plants have el-

evated levels of biochemical activity relative todisease control and are better prepared to de-fend themselves against diseases.” This includesresistance to both root and leaf diseases.

Composts rich in available nitrogen may ac-tually stimulate certain diseases, as was foundfor Phytophthora root rot on soybeans, as wellas Fusarium wilts and fire blight on other crops.Applying these composts many months beforecropping, allowing the salts to leach away, orblending them with low nitrogen composts priorto application reduces the risk of stimulatingdiseases.

Composting can change certain organic ma-terials used as surface mulches — such as barkmulches — from stimulating disease to suppress-ing disease.

Disease Suppression by Composts


USING COMPOSTS

Finished composts generally provide only lowrelative amounts of readily available nutrients.During composting, much of the nitrogen is con-verted into more stable organic forms, althoughpotassium and phosphorus availability remainsunchanged. However, it should be kept in mindthat composts can vary significantly and somemay have high levels of nitrate. Even thoughmost composts don’t supply a large amount ofavailable nitrogen per ton, they still supply fairamounts of other nutrients in available formsand greatly help the fertility of your soil by in-creasing organic matter and by slowly releasingnutrients. Composts can be used on turf, inflower gardens, and for vegetable and agronomiccrops. Composts can be spread and left on thesurface or incorporated into the soil by plowingor rototilling. Composts also are used to growgreenhouse crops and are the basis of some pot-ting soil mixes.

ADVANTAGES OF COMPOSTING

Composted material is less bulky than the origi-nal material, and easier and more pleasant tohandle. During the composting process, carbondioxide and water are lost to the atmosphereand the size of the pile decreases by 30 to 60percent. In addition, many weed seeds and dis-ease-causing organisms may be killed by thehigh temperatures in the pile. Unpleasant odorsare eliminated. Flies, a common problem aroundmanures and other organic wastes, are much lessof a problem with composts. Composting re-duces or eliminates the decline in nitrogen avail-ability that commonly occurs when organic ma-terials, such as sawdust or straw, are added di-

rectly to soil. Composting is also very useful forrecycling kitchen wastes, leftover crop residues,weeds, and manures. Many types of local or-ganic waste, such as apple pumice, lake weeds,leaves, and grass clippings, can be composted.

There is evidence that compost applicationlowers the incidence of plant root and leaf dis-eases (see above). In addition, the chelates andthe direct hormone-like chemicals present incompost stimulate the growth of healthy plants.Then there are the positive effects on soil physi-cal properties that are derived from improvingsoil organic matter. All of these factors togethermay help explain some of the broad benefits toplant growth that are attributed to compost.

“I don’t make compost because

it makes me feel good. I do it

because composting is the only thing

I’ve seen in farming that costs less,

saves time, produces higher yields

and saves me money.”

—CAM TABB, DAIRY AND CROP FARMER,WEST VIRGINIA

If you have a large amount of organic wastebut not much land, composting may be veryhelpful. Also, since making compost decreasesthe solubility of nutrients, composting may helplessen pollution in streams, lakes, and ground-water. On many poultry farms and on beef feed-lots, where high animal populations on limitedland may make manure application a potentialenvironmental problem, composting may be thebest method for handling the wastes. Composted


material, with about half the bulk and weightand its higher commercial value than the ma-nure, can be economically transported signifi-cant distances to locations where nutrients areneeded.

Without denying these good reasons to com-post, there are frequently very good reasons tojust add organic materials directly to the soil,without composting. Compared with fresh resi-dues, composts may not stimulate as much pro-duction of the sticky gums that help hold ag-gregates together. Also, some uncompostedmaterials have more nutrients that are readilyavailable to feed plants than do composts. If yoursoil is very deficient in fertility, plants may needreadily available nutrients from residues. Rou-tine use of compost as an nitrogen source maycause high soil phosphorus levels to develop,because of the relatively low N:P ratio. Finally,

ProtectingDrinking Water Supplies

Composting of manure is of special interestin watersheds that supply drinking water tocities, such as those that serve New York. Theparasites Giardia Lamblia (beaver fever) andCryptosporidium parvum cause illness in hu-mans and are shed through animal manure,especially young stock. These organisms arevery resistant in the environment and are notkilled by chlorination. Composting of manure,however, is an economical option that killsthe pathogen and protects drinking water.

more labor and energy usually are needed tocompost residues before applying than to sim-ply apply the uncomposted residues directly.

SOURCES

Hoitink, H.A.J., D.Y. Han, A.G. Stone, M.S.Krause, W. Zhang, and W.A. Dick. 1997.Natural Suppression. American Nurseryman.October 1, 1997:90–97.

Epstein, E. 1997. The Science of Composting.Technomic Publishing Company.

Martin, D. L., and G. Gershuny (eds.). 1992.The Rodale Book of Composting: Easy Methodsfor Every Gardener. Rodale Press. Emmaus, PA.

Richard, T.L., N.M. Traubman, and M.E. Krasny.1996. Cornell composting website. http://www.cfe.cornell.edu/compost/

Rothenberger, R.R., and P.L. Sell. Undated. Mak-ing and Using Compost. University of MissouriExtension Leaflet (File: Hort 72/76/20M). Co-lumbia, MO.

Rynk, R. (ed.). 1992. On Farm Composting.NRAES-54. Northeast Regional AgriculturalEngineering Service. Ithaca, N.Y.

Seymour, R.S. 1991. The brush turkey. Scien-tific American. Dec. 1991.

Staff of Compost Science. 1981. Composting:Theory and practice for city, industry, and farm.The JG Press. Emmaus, PA.

Weil, R.R., D.B. Friedman, J.B. Gruver, K.R. Is-lam, and M.A. Stine. Soil Quality Research atMaryland: An Integrated Approach to Assess-ment and Management. Presented in Baltimore,MD at the 1998 ASA/CSSA/SSSA meetings.This is the source of the quote from Cam Tabb.

117

Despite one of the hottest, driest summers onrecord, West Virginia dairy farmer Cam Tabbyielded his typical harvest of 120 bushels of cornper acre, an enviable amount to those who watchedtheir crops wither in the fields. Neighbors won-dered whether Tabb enjoyed some kind of miracu-lous microclimate to soar through the drought of1999 — a severe dry spell lasting from April toSeptember in the mid-Atlantic region — withseemingly little impact.

“I get blamed for getting more water than theygot because the corn looks better,” laughs Tabb,who raises 170 dairy cows and grows small grainsand corn for grain and silage on 1,040 acres nearCharles Town, W.V. Instead, Tabb credits his strongyields to eight years of applying composted dairymanure to his fields.

“I get a healthier plant with a better root sys-tem because my soil structure is better,” he says.“So the rain that you do get really sinks in.”

Tabb’s compost treatments, combined with an-nual soil tests and rotations, have stood the test oftime. Not only have his practices improved hissoil and crop yields, but weather extremes like thedrought of 1999 have less of an impact on his farmthan those of other farmers who do not treat thesoil with such care.

Tabb has come a long way since he used to pilehis dairy manure on hard-packed ground andwatch it ice over in the winter. After hearing a WestVirginia University researcher talk about backyardcomposting at a town meeting, Tabb realized heneeded to add a carbon source and turn the pilesto encourage aeration. Once he began mixing insawdust from horse stalls and turning the piles,he was on his way to becoming a master composter.

Now, after years of fine-tuning his operation,Tabb can talk about compost for hours. He saysamending soil with compost in the spring leads tohealthy plants for the rest of the season. Moreover— here he gets especially animated — compostingreduces his volume of manure by half.

“The crop response and the reduction of ma-nure volume is what keeps me doing this,” he says.

Over the last few years, Tabb has worked withscientists to research the advantages of using com-post in his grain fields. One scientist compared anacre of corn grown in soil amended with Tabb’scompost against an acre of unamended soil on hisfarm. The difference in yields was extraordinary:up to 123 bushels of corn in the compost plotversus between 6 and 42 bushels in the conven-tional tract.

In another experiment, Tabb mixed 10,000pounds of fish into his piles for a scientist whoneeded to dispose of fish that turned up with abacterial disease during an aquaculture experi-ment. Contrary to what a visitor might expect, thecompost turned out to be rich — and odorless.Tabb says the Native Americans who taught earlyPilgrims to drop a dead fish into their soil beforeplanting were on to something.

“I built a windrow 60 feet long and 15 feethigh,” he recalls. “A month later, I would neverhave known the fish were there.”

Tabb grazes his herd on ryegrass pastures be-tween March and July. When the cows are in thebarn the remainder of the year, he collects themanure and composts it. He applies between 18and 24 tons of compost to his crop fields, depend-ing on soil test results, just once every three years.That way, he reduces the number of tractor trips

Cam TabbKearneysville, West Virginia


118

— and therefore, soil compaction — and still sup-plies all of the necessary nutrients, except extranitrogen. Measurements show his compost sup-plies about 9 pounds of nitrogen, 12 pounds ofphosphorus and 15 pounds of potash per ton.

“I use all I produce,” says Tabb, whose manypiles of compost stretch across a portion of hisfarm. The nutrient-supplying power of compostmore than compensates for the time Tabb spendsmaking his black gold. He uses a mix of dairymanure, leaves, grass, wood shavings, sawdust andhorse manure, then turns the pile three to fourtimes. He monitors pile temperatures and turnswhen it gets up to about 150 degrees, but doesn’tfollow any strict rules.

“I do it the lazy way,” he says. “I turn when Iget good and ready. It’s not an extra hard opera-tion.” Turning, which Tabb does with a front-endloader, pays for itself by reducing the pile.

Those extreme temperatures in the piles, whichreach as high as 150 degrees, kill both pathogensand weed seeds. In the spring, Tabb spreads a thinlayer of compost on part of his fields and plantsthrough it. When the soil is compacted, Tabb worksthe compost into the ground to help loosen it.

Tabb sees other benefits, too. He is especiallyproud of his up to 7-percent organic matter,pushed to such levels from eight years of usingcompost. The soil takes on a more spongy feel;Tabb says he sees little to no runoff from his com-post-treated fields. It also attracts worms. In a studycomparing two, one-acre blocks, researchers found23 worms per square foot in compost-treated soilversus less than one worm per square foot in un-treated soil on Tabb’s farm.

While Tabb reduces pesticide use thanks tocompost and a small grain-beans-corn-cover croprotation, he continues to spot-treat with a herbi-cide to control weeds. He plants using no-till strat-egies, often through cover crop residue and com-post.

Not only has Tabb changed his mind aboutcompost, but his neighbors are starting to takenotice. “People really thought I had lost my mindon this project,” he says. Recently, however, aneighbor asked him for compost for his wife’s gar-den.

“Before I handled the manure as a waste, not aresource,” he says. “Now, everything just growsbetter.”

119

The dust storms that hit the center of the U.S.during the 1930s were responsible for one

of the great migrations in our history. As WoodyGuthrie pointed out in his songs, soil erosion wasso bad that people saw little alterna-tive to abandoning their farms. Theymoved to other parts of the country insearch of work. Although changed cli-matic conditions and agricultural prac-tices improved the situation for a time,there was another period of acceleratedwind and water erosion during the 1970s and1980s.

Erosion by wind and water has occurred sincethe beginning of time. Although we should ex-pect some soil loss to occur on almost all soils,agriculture often increases erosion. Erosion isthe major hazard or limitation to the use of aboutone-half of all cropland in the United States! Onmuch of this land, erosion is occurring fast

13�

Reducing Soil Erosion

So long! It’s been good to know you.

This dusty old dust is a gettin’ my home.

And I’ve got to be drifting along.

—WOODY GUTHRIE, 1940

enough to reduce future productivity. As we dis-cussed earlier, erosion is also an organic matterissue, because it removes the soil layer highestin organic matter, the topsoil. The soil removed

from fields also has huge negative ef-fects off the farm, as sediment accu-mulates in streams, rivers, and reser-voirs or blowing dust reaches townsand cities.

A small amount of erosion is ac-ceptable, as long as new topsoil can

be created as rapidly as soil is lost. The maxi-mum amount of soil that can be lost to erosioneach year, while still maintaining reasonable pro-ductivity is called the soil loss tolerance or Tvalue. For a deep soil with a rooting depth ofgreater than 5 feet, the T value is 5 tons per acreeach year. Although this sounds like a hugeamount of soil loss, keep in mind that the weightof an acre of soil to 6 inches depth is about 2


million pounds, or 1,000 tons. So 5 tons isequivalent to about .03 inches ([5/1,000] x 6inches = 0.03 inch), enough to fill 200 bushelbaskets with soil. If soil loss continues at thisrate, at the end of 33 years about 1 inch will belost. On deep soils with good management oforganic matter, the rate of topsoil creation canbalance this loss. The soil loss tolerance amountis gradually reduced for soils with less rootingdepth. When the rooting depth is shallower than10 inches, the T value is about 1 ton per acreeach year. This is the same as 0.006 inch per yearand is equivalent to 1 inch of loss in 167 years.

When your soil loss is greater than the toler-ance value, productivity suffers in the long run.Yearly losses of 10 or 15 tons or more per acreoccur in many fields. Management practices areavailable to help reduce runoff and soil losses.For example, researchers in Washington statefound that erosion on winter wheat fields wasabout 4 tons each year when a sod crop wasincluded in the rotation, compared to about 15tons when sod was not included. An Ohio ex-periment where runoff from conventionallytilled and no-till continuous-corn fields wasmonitored showed that over a four-year period,runoff averaged about 7 inches of water eachyear for conventional tillage and less than one-tenth (0.1) inch for the no-till planting system.

SOLVING EROSIONPROBLEMS

Effective erosion control is possible without com-promising crop productivity. However, control-ling erosion is not always easy. It may requireconsiderable investment (as with terracing) ornew management strategies (as with no-till sys-tems). The numerous approaches to controllingerosion can be generally grouped into structural

solutions and agronomic management practices.Structures for reducing erosion generally involveengineering practices, where an initial investmentis made to build terraces, diversion ditches, dropstructures, etc. Agronomic practices to reduceerosion focus on changes in soil and crop man-agement. Appropriate conservation methods mayvary among fields and farms. Recently, there hasbeen a clear trend away from structural measuresin favor of agronomic management practices. Theprimary reasons for this change are:

� Management measures help control erosion,while also improving soil quality and cropproductivity.

� Significant advances have been made infarm machinery and methodologies foralternative soil and crop management.

� Structures generally focus on containingrunoff and sediment once erosion has beeninitiated, whereas management measures tryto prevent erosion from occurring in thefirst place.

Erosion: A Short-TermMemory Problem?

It’s difficult to fully appreciate erosion’s dam-age potential, because the most severe erosionoccurs during rare weather events and climateanomalies. Wind erosion during the dust-bowldays of the 1930s was especially damaging,resulting from several extremely dry years in arow. And about one-third of water erosiondamage that occurs in a particular field dur-ing a 30-year period commonly results from asingle extreme rainfall event! We must do ourbest to adequately protect our soils from thedamage that weather extremes can cause.

REDUCING SOIL EROSION 121

� Structures are often expensive to build andmaintain.

� Most structures do not reduce tillageerosion.

For long-term sustainability of crop produc-tion, use of agronomic management practices isusually preferred, although structural measurescan effectively complement them.

Erosion reduction works by either decreas-ing the shear forces of water and wind or bykeeping soil in a condition that can’t erode eas-ily. Many conservation practices actually pro-vide both. The soil organic matter managementpractices we discussed in the earlier chaptersall reduce erosion. We’ll also briefly cover otherimportant practices for keeping erosion to aminimum.

Reduced Tillage

Transition to tillage systems that increase sur-face cover (chapter 15) is probably the singlemost effective and economic approach to reduc-ing erosion. Restricted and no-till regimes suc-ceed in many cropping systems by providingsimilar or even better economic returns thanconventional tillage, while providing excellenterosion control. Maintenance of residues on thesoil surface and the lack of soil loosening bytillage greatly reduce dispersion of surface ag-gregates by raindrops and runoff waters. Theeffects of wind on surface soil are also greatlyreduced by leaving crop stubble on untilled soiland anchoring the soil with roots. These mea-sures facilitate infiltration of precipitation whereit falls, thereby reducing runoff and increasingplant water availability.

In cases where tillage is necessary, reducingits intensity and leaving some residue on the

surface slows down the loss of soil organic mat-ter and aggregation. Less tillage promotes higherinfiltration rates and reduces runoff and erosion.Leaving a rougher soil surface by eliminatingsecondary tillage passes and packers that crushnatural soil aggregates may significantly reducerunoff and erosion losses by preventing surfacesealing after intense rain.

Reducing or eliminating tillage also dimin-ishes tillage erosion and keeps soil from beingmoved downhill. The gradual losses of soil fromupslope areas exposes denser subsoil and mayin many cases further aggravate runoff and ero-sion. It is, however, possible to gradually reversethe effects of tillage erosion caused by using amoldboard plow. Because the plow moves soilforward and to the side, topsoil can be gradu-ally moved back up the slope, if plowing is per-formed diagonally to the slope in the uphill di-rection, with the soil being thrown 45 degreesto the front/right of its original location. Ofcourse, this approach may not give good soilinversion during plowing and does not addresswater and wind erosion concerns.

Significance of Soil Residues

Reduced-tillage and no-tillage practices resultin less soil disturbance and leave significantquantities of residue on the surface. Surfaceresidues are important because they interceptraindrops and can also slow down water run-ning over the surface. The amount of residueon the surface may be close to zero for themoldboard plow while continuous no-tillplanting may leave 90 percent or more of thesurface covered. Other reduced-tillage sys-tems, such as chiseling and disking (as a pri-mary tillage operation), typically leave morethan 30 percent of the surface covered.


Adding Organic Materials

Maintaining good soil organic matter levels helpskeep topsoil in place. A soil with more organicmatter usually has better tilth and less surfacecrusting. This means that more water is able toinfiltrate into the soil instead of running off thefield, taking soil with it. When you build uporganic matter, you help control erosion bymaking it easier for rainfall to enter the soil.

Adding organic materials regularly to soilsalso results in larger and more stable soil aggre-gates. Larger aggregates are not eroded by windor water as easily as smaller ones. Surface resi-due mulches provide both physical protectionof the soil surface from raindrop impact, as wellas food for large numbers of earthworms.

The adoption rate for no-till practices is lowerfor livestock-based farms than for grain and fi-ber farms. Manures may need to be incorpo-rated into the soil for best use of nitrogen, pro-tection from runoff, and odor control. Also, thesevere compaction sometimes resulting from useof heavy liquid manure spreaders on very moistsoils may need to be lessened by tillage. Directinjection of liquid organic materials in a zoneor no-till system is generally an option but re-quires additional equipment investments.

Cover Crops

Cover crops decrease erosion and increase waterinfiltration in a number of ways. Cover crops addorganic residues to the soil and help maintain tilthand organic matter levels. Cover crops frequentlygrow during seasons when the soil is especiallysusceptible to erosion, such as the early spring.Their roots help to bind soil and hold it in place.Because raindrops lose most of their energy whenthey hit leaves and drip to the ground, less soil

crusting occurs. Cover crops are especially effec-tive in reducing erosion if they are cut andmulched, rather than incorporated. See chapter10 for more information about cover crops.

Perennial Rotation Crops

Grass and legume forage crops can help lessenerosion because they maintain a cover on mostof the soil surface for the whole year. Their ex-tensive root systems hold soil in place. Ideally,such rotations are combined with reduced andno-tillage practices for the annual crops. Per-manent sod is a very good choice for steep soilsor other soils that erode easily.

Other Practices and StructuresFor Soil Conservation

Diversion ditches are frequently helpful forchanneling water away from the field withoutflowing over the entire area. Grassed waterwaysfor diversion ditches and other field water chan-nels do not reduce erosion from all of the field,but they do keep sediments on the field andreduce scouring of the channels. Grassed wa-terways help prevent surface water pollution byfiltering sediments out of runoff before theyreach a stream or pond.

Tilling and planting along the contour is asimple practice that helps control erosion. Whenyou work along the contour, instead of up anddown slope, wheel tracks and depressions causedby the plow, harrow, or planter will retain run-off water in small puddles and allow it to slowlyinfiltrate. This approach is not very effectivewhen dealing with steep erodible lands and alsodoes not reduce tillage erosion.

Alternating strips of row crops and peren-nial forages along the contour, referred to as stripcropping, is an effective way of reducing ero-

REDUCING SOIL EROSION 123

sion losses. In this system, erosion from the rowcrop is not allowed to worsen over long, unpro-tected slopes because the sediments are filteredout of runoff when the water reaches the sod ofthe forage crop. This conservation system is gen-erally effective in fields with moderate erosionpotential, and on farms with use for both rowand sod crops (for example, dairy farms). Re-search indicates that crop yields may be slightlyhigher when crops are grown in strips, ratherthan in the entire field. The increase in yield isprobably due to better use of light and soil wherethe different strips meet.

Terracing soil in hilly regions is an expensivepractice, but one which results in a more gradualslope and greatly reduced erosion. Well-con-structed and maintained structures can last along time, frequently making the high initialinvestment worthwhile.

Wind erosion is reduced by most of the samepractices that reduce water erosion — reducedtillage or no-till, cover cropping, and perennialrotation crops. In addition, practices that in-crease roughness of the soil surface diminish theeffects of wind erosion. The resulting increasein turbulent air movement near the land sur-face reduces the wind’s shear and its ability tosweep up soil material. Therefore, fields sub-jected to wind erosion may be rough-tilled. Also,tree shelterbelts planted at regular distancesperpendicular to the main wind direction act aswindbreaks and are very effective in reducingwind erosion losses.

SOURCES

American Society of Agricultural Engineers.1985. Erosion and Soil Productivity. Proceed-

ings of the national symposium on erosionand soil productivity, December 10–11, 1984,New Orleans, Louisiana. American Society ofAgricultural Engineers Publication 8-85. St.Joseph, MI.

Edwards, W.M. 1992. Soil structure: Processesand management. pp. 7–14. In Soil Manage-ment for Sustainability (R. Lal and F.J. Pierce,eds.). Soil and Water Conservation Society.Ankeny, IA. This is the reference for the Ohioexperiment on the monitoring of runoff.

Lal, R., and F.J. Pierce (eds.). 1991. Soil Man-agement for Sustainability. Soil and Water Con-servation Society. Ankeny, IA.

Ontario Ministry of Agriculture, Food, and Ru-ral Affairs. 1997. Soil Management. Best Man-agement Practices Series. Available from theOntario Federation of Agriculture, Toronto,Ontario (Canada).

Reganold, J.P., L.F. Elliott, and Y.L. Unger. 1987.Long-term effects of organic and conventionalfarming on soil erosion. Nature 330:370–372.This is the reference for the Washington Statestudy of erosion.

Smith, P.R. and M.A. Smith. 1998. Strip inter-cropping corn and alfalfa. Journal of Produc-tion Agriculture 10:345–353.

Soil and Water Conservation Society. 1991. CropResidue Management for Conservation. Pro-ceedings of a national conference, August 8–9, 1991, Lexington, KY. Soil and Water Con-servation Society. Ankeny, IA.

United States Department of Agriculture. 1989.The Second RCA Appraisal: Soil Water, and Re-lated Resources on Nonfederal Land in the UnitedStates, Analysis of Conditions and Trends. Gov-ernment Printing Office. Washington, DC.

125

We’ve discussed the benefits of cover crops,rotations, reduced tillage, and organic

matter additions for improving soil structure.However, these practices still may not preventcompacted soils unless specific stepsare taken to reduce the impact ofheavy loads from field equipment andinappropriately timed field operations.The causes of compaction were dis-cussed in chapter 6, and in this chap-ter we’ll discuss strategies to preventand lessen soil compaction. The first step is todecide whether compaction is a problem andwhich type is affecting your soils. The symp-toms, as well as remedies and preventive mea-sures, are summarized in table 14.1.

CRUSTING AND SURFACE SEALING

Crusting and surface sealing are easy to see atthe soil surface after heavy rains in the early

14�

Preventing and LesseningCompaction

A lasting injury is done by ploughing

land too wet.

—S. L. DANA, 1842

growing season, especially with clean-tilled soil.Keep in mind that it may not happen every year,if heavy rains do not occur before the plantcanopy protects the soil from direct raindrop

impact. Certain soil types, such assandy loams, are particularly suscep-tible to crusting. Their aggregates usu-ally aren’t very stable and, once bro-ken down, the small particles fill ingaps between the larger particles.

The impact of surface crusting ismost damaging when heavy rains occur betweenplanting and seedling emergence. The hard sur-face that forms may delay seedling emergence andgrowth until the crust mellows with the next rains.If such showers do not occur, the crop may beset back considerably. Crusting and sealing ofthe soil surface also reduce water infiltration ca-pacity. This increases runoff and erosion, andlessens the amount of available water for crops.


Reducing Surface Crusting

Crusting is a symptom of poor soil structure thatdevelops especially with intensively and clean-tilled soils. As a short-term solution, farmerssometimes use tools, such as rotary hoes, tobreak up the crust. The best long-term approachis to reduce tillage intensity, use tillage systemsthat leave residue or mulch on the surface, andimprove aggregate stability with organic matteradditions. Even residue covers as low as 30 per-cent will greatly reduce crusting and provide im-portant pathways for water entry. A good heavy-duty conservation planter — with rugged coul-ter blades for in-row soil loosening, tine wheelsto remove surface residue from the row, and ac-curate seed placement — is a key implementbecause it can successfully establish crops with-out intensive tillage (see chapter 15). Reducingtillage and maintaining significant amounts ofsurface residues not only prevent crusting, butalso rebuild the soil by reducing decompositionof organic matter. Practices that improve soilstructure, such as cover cropping, rotations withperennial crops, and adding organic materials,also help reduce crusting problems. Soils withvery low aggregate stability may sometimes ben-efit from surface applications of gypsum (cal-cium sulfate). The added calcium and the effectof the greater salt concentration in the soil wa-ter both promote aggregation.

PLOW LAYER AND SUBSOILCOMPACTION

Deep wheel tracks, extended periods of satura-tion, or even standing water following a rain orirrigation may indicate plow layer compaction.Compacted plow layers also tend to be extremelycloddy when tilled. A shovel can be used to vi-

sually evaluate soil structure and rooting. Thisis best done when the crop is in an early stageof development, but after the rooting system hada chance to get established. Digging can be veryeducational and provide good clues to the qual-ity of the soil. If you find a dense rooting sys-tem with many fine roots that protrude well intothe subsoil, you probably do not have a com-paction problem. Compacted soil shows littleaggregation, is more difficult to dig, and will digup in large clumps rather than granules. Com-pare the difference between soil and roots inwheel tracks and nearby areas.

Roots in a compacted plow layer are usuallystubby and have few root hairs. They also oftenfollow crooked paths as they try to find zonesof weakness. Rooting density below the plowlayer is an indicator for subsoil compaction.Roots are almost completely absent from thesubsoil below severe plow pans and often movehorizontally above the pan (figure 6.6). Keep inmind, however, that shallow rooted crops, suchas spinach and some grasses, may not necessar-ily experience subsoil compaction problemsunder those conditions.

Compaction also may be recognized by ob-serving crop growth. A poorly structured plowlayer will settle into a dense mass after heavyrains, leaving few large pores for air exchange.If soil wetness persists, anaerobic conditions mayoccur, causing reduced growth and denitrifica-tion (exhibited by leaf yellowing), especially inareas that are imperfectly drained. In addition,these soils may “hardset” if heavy rains are fol-lowed by a drying period. Crops in their earlygrowth are very susceptible to these problems(because roots are still shallow) and commonlygo through a noticeable period of stunted growthon compacted soils.

PREVENTING AND LESSENING COMPACTION 127

Reduced growth because of compaction af-fects the crop’s ability to fight or compete withpathogens, insects or weeds. These pest prob-lems may, therefore, become more apparent sim-ply because the crop is weakened. For example,compacted soils that are put into a no-till sys-

tem may initially show greater weed pressurebecause the crop is unable to effectively com-pete. Also, dense soils that are poorly aeratedare more susceptible to infestations of certainsoil-borne pathogens, such as Phytophthora dur-ing wet periods.

COMPACTION TYPE INDICATIONS

Surface crusting Breakdown of surface aggregatesand sealing of surface.

Poor seedling emergence.

Accelerated runoff and erosion.

TABLE 14.1Types of Compaction and Their Remedies

REMEDIES/PREVENTION

Reduce tillage intensity.

Leave residues on surface.

Add other sources of organic matter (manures,composts).

Grow cover crops.

Plow layer Deep wheel tracks.

Prolonged saturation orstanding water.

Poor root growth.

Hard to dig and resistantto penetrometer.

Cloddy after tillage.

Plow with moldboard or chisel plow, but reducesecondary tillage.

Do primary tillage before winter (if no erosiondanger exists).

Use zone builders.

Increase organic matter additions.

Use cover crops or rotation crops that can breakup compact soils.

Use better load distribution.

Use controlled traffic.

Don’t travel on soils that are wet.

Improve soil drainage.

Subsoil Roots can’t penetratesubsoil.

Resistant to penetrometer.

Don’t travel on soils that are wet.

Improve soil drainage.

Deep tillage with a subsoiler.

Use cover crops or rotation crops that penetratecompact subsoils.

Use better load distibution.

Use controlled traffic.

No wheels in open furrows.


Preventing or LesseningSoil Compaction

Preventing or lessening soil compaction gener-ally requires a comprehensive, long-term ap-proach to addressing soil health issues and rarelygives immediate results. Compaction on any par-ticular field may have multiple causes and thesolutions are often dependent on the soil type, cli-mate and cropping system. Let’s go over some gen-eral principles of how to solve these problems.

Tillage is a problem, but sometimes can bea solution. Tillage can either cause or lessenproblems with soil compaction. Repeated inten-sive tillage reduces soil aggregation and com-pacts the soil over the long term, causes erosionand loss of topsoil, and may bring about theformation of plow pans. On the other hand, till-age can relieve compaction by loosening the soiland creating pathways for air and water move-ment and root growth. This relief, as effective asit may be, is temporary. Tillage may need to berepeated in the next growing seasons if soilmanagement and traffic patterns stay the same.

Over time, farmers frequently use more in-tense tillage to offset the problems of cloddinessassociated with compaction of the plow layer.The solution to this problem is not necessarilyto stop tillage altogether. Compacted soils fre-quently become “addicted” to tillage and going“cold turkey” to a no-till system with a seriouslydegraded soil may result in failure. Practices thatperform some soil loosening with minor distur-bance at the soil surface help in the transitionfrom a tilled to an untilled system. This mayinclude a zone-building tool (figure 14.1 a) withnarrow shanks that disturb soil only where fu-ture plant rows will go. Also, paraplows (figure14.1b) that loosen the soil by lifting it from un-derneath can relieve some compaction. Another

Lessening and preventing soil compaction isimportant to improving soil health. The spe-cific approaches:

� should be selected based on where thecompaction problem occurs (subsoil,plow layer, or surface);

� must fit the soil and cropping system andtheir physical and economic realities; and

� are influenced by other managementchoices, such as tillage system and use oforganic matter amendments.

approach may be to gradually reduce tillage in-tensity through the use of tillage tools that leaveresidue on the surface (for example, chisels withstraight points, or specifically designed for high-residue conditions) and a good planter that en-sures good seed placement even with minimalsecondary tillage. Such a system reduces organicmatter losses and erosion over the long term and— through better germination rates — may pro-duce more crop residues.

Deep tillage (subsoiling) is a method to alle-viate compaction below the depth of normal till-age, although it is often erroneously seen as acure for all types of soil compaction. It is a rathercostly and energy-consuming practice thatshould not be done regularly. (Practices such as“zone building” and paraplowing also mayloosen the soil below the plow layer, but are lessrigorous and leave residue on the surface.) Deeptillage may be beneficial on soils that have de-veloped a plow pan. Simply shattering this panallows for deeper root exploration. To be effec-tive, deep tillage needs to be performed whenthe entire depth of tillage is sufficiently dry andin the friable state. The practice tends to be moreeffective on coarse-textured soils (sands, gravels),


Figure 14.1 Tillage implements that loosen soil with minimum surface disturbance.

as crops on those soils respond better to deeperrooting. The entire subsoil of fine-textured soilsis often hard, so the effects of deep tillage arethen less beneficial and in some cases even harm-ful. After performing deep tillage, it is impor-tant to prevent future recompaction of the soilby heavy loads and plows.

Better attention to working and travelingon the soil. Compaction of the plow layer orsubsoil is often the result of working or travel-ing on a field when it is too wet. The first stepwhen addressing compaction is to evaluate alltraffic and practices that occur on a field duringthe year and determine which field operationsare likely to be most damaging. The main crite-ria should be:

� The soil moisture condition under whichthe traffic occurs; and

� The relative compaction effects of varioustypes of field traffic (mainly defined byequipment weight and load distribution).

For example, with a late-planted crop, soil con-ditions during tillage and planting may be gener-

ally dry, and minimal compaction damage occurs.Likewise, mid-season cultivations usually do littledamage, because conditions are usually dry andthe equipment tends to be light. However, if thecrop is harvested under wet conditions, heavyharvesting equipment and uncontrolled traffic bytrucks that transport the crop off the field will doconsiderable compaction damage. In this sce-nario, emphasis should be placed on improvingthe harvesting operation. In another scenario, ahigh-plasticity clay loam soil is often spring-plowed when still too wet. Much of the compac-tion damage may occur at that time and alterna-tive approaches to tillage should be a priority.

Better load distribution. Improving the de-sign of field equipment may help reduce com-paction problems by better distributing vehicleloads. The ultimate example of this is the use oftracks, like those on a bulldozer, which espe-cially reduce the potential for subsoil compac-tion. (Beware! Tracked vehicles may tempt afarmer to traffic the land when it’s still too wet.Tracked vehicles have better flotation and trac-tion, but still cause compaction damage, espe-cially through smearing under the tracks.) Plow

a) zone-builder b) paraplow


layer compaction also can be reduced by lower-ing the inflation pressure of tires. A rule ofthumb: cutting tire inflation pressure in halfdoubles the size of the tire footprint and cutsthe contact pressure on the soil in half.

Use of multiple axles reduces the load ontires. Even though the soil receives more tirepasses, the resulting compaction is significantlyreduced. Using large, wide tires with low infla-tion pressure also helps decrease the compac-tion effect of loads on soil. Use of dual wheelssimilarly reduces compaction by increasing thefootprint, although this is less effective for re-ducing subsoil compaction, because the pres-sure cones from neighboring tires (figure 6.11)merge at shallower depths. Dual wheels are veryeffective at increasing traction, but again, posea danger because of the temptation (and abil-ity) to do field work under relatively wet condi-tions. Duals are not recommended on tractorsperforming seeding/planting operations becauseof the larger footprint.

drained soils. Subsurface (tile) drainage im-proves timeliness of field operations, helps drythe subsoil and, thereby, reduces compaction indeeper layers.

Clay soils often pose the greatest challengewith respect to compaction, because they remainin the plastic state for extended periods in thespring. After the top inch near the soil surfacedries out, it becomes a barrier that greatly re-duces further evaporation losses (figure 14.2).This keeps the soil below in a plastic state, pre-venting it from being worked or trafficked with-out causing excessive smearing and compactiondamage. For this reason, farmers often fall-tillclay soils. A better approach might be to usecover crops to dry the soil in the spring. Whena crop like winter rye grows rapidly in the spring,its roots effectively pump water from layers be-low the soil surface and allow the soil to transi-tion from the plastic to the friable state (figure14.2). Because these soils have high moisture-holding capacity, there is normally little concernabout cover crops depleting water for the fol-lowing crop.

Cover and rotation crops. Cover and rota-tion crops can significantly reduce soil compac-tion. The choice of cover/rotation crop shouldbe defined by the climate, cropping system,nutrient needs, and the type of soil compaction.Perennial crops commonly have active rootgrowth early in the growing season and canreach into the compacted layers when they arestill wet and relatively soft. Grasses generallyhave shallow, dense fibrous root systems thathave a very beneficial effect on the surface layer,but don’t help much with subsoil compaction.Crops with deep taproots, such as alfalfa, havefewer roots at the surface, but can penetrate intoa compacted subsoil. In many cases, a combi-

In colder climates, the tilth of com-

pacted plow layers can be improved

by rough-tilling the soil in the fall. The

freeze-thaw action on the loosened

soil helps create new aggregates.

Improved soil drainage. Fields that are im-perfectly drained often have more severe com-paction problems, because wet conditions per-sist and it is almost impossible to prevent trafficor tillage under those conditions. Improvingdrainage may go a long way toward preventingand reducing compaction problems on poorly


low evaporation rate

dry surfacelayer

preventsmoisture

losses fromdeeperlayers

depth oftillage

high transpiration rate

soil dries atgreater depths

because ofwater uptake by

roots

��

�

Figure 14.2 Cover crops enhance the drying of a clay soil. Without cover crops (left), evaporation lossesare low after the surface dries. With cover crops (right), water is removed from deeper in the soil, becauseof root uptake and transpiration from plant leaves, resulting in better tillage and traffic conditions.

nation of cover crops with shallow and deeprooting systems are preferred. Ideally, such cropsare part of the rotational cropping system, whichis typically done on ruminant livestock-basedfarms.

The relative benefits of incorporating ormulching a cover/rotation crop are site-specific.Incorporation through tillage loosens the soil,which may be beneficial if the soil has beenheavily trafficked. This would be the case witha sod crop that was actively managed for forageproduction, sometimes with traffic under rela-

tively wet conditions. Incorporation through till-age also encourages rapid nitrogen mineraliza-tion. Compared to plowing down a sod crop,cutting and mulching in a no-till or zone-till sys-tem reduces nutrient availability and does notloosen the soil. On the other hand, a heavy pro-tective mat at the soil surface provides someweed control and better water infiltration andretention. Some farmers have been successfulwith cut-and-mulch systems involving aggres-sive, tall cover/rotation crops, such as rye andsorghum-sudangrass.


Addition of other organic materials. Addinganimal manure, compost or sewage sludge ben-efits the surface soil layer in which they are in-corporated by providing a source of organicmatter. The long-term benefits of applying thesematerials, relative to soil compaction, may bevery favorable, but in many cases, the spreadersthemselves are a major cause of compaction.Livestock-based farms in humid regions usuallyapply manure using heavy spreaders (often withpoor load distribution) on wet or marginally drysoils, resulting in severe compaction of both thesurface layer and the subsoil. The need to in-corporate animal manure for efficient nitrogenuse and odor control is also often a barrier tothe adoption of no-till or zone-till systems. Thisproblem can be overcome only through an ad-ditional investment in manure injection tools.In general, the addition of organic materialsshould be done with care to obtain the biologi-cal and chemical benefits, while not aggravat-ing compaction problems.

Controlled traffic and permanent beds. Oneof the most promising, but rarely adopted, prac-tices for reducing soil compaction is the use ofcontrolled traffic lanes. In this system, traffic as-sociated with all field operations is limited tothe same lanes. A controlled traffic system is easi-est adopted with row crops in zone, ridge orno-till systems (not requiring full-field tillage,see chapter 15), because crop rows and trafficlanes remain recognizable year after year. Ridgetillage, in fact, dictates controlled traffic, aswheels cannot cross the ridges. Zone and no-till do not necessarily require controlled traffic,but greatly benefit from it, because the soil isnot regularly loosened by aggressive tillage.Adoption of controlled traffic lanes typically re-quires some adjustment of field equipment to

insure that all wheel traffic occurs in the samelanes and also requires considerable disciplinefrom equipment operators.

The primary benefit of controlled traffic isthe lack of compaction for most of the field atthe expense of limited areas that receive all thecompaction. Because the degree of soil compac-tion doesn’t necessarily worsen with each load(most of the compaction occurs with the heavi-est loading and does not greatly increase withother passes), damage in the traffic lanes is notmuch more severe than that occurring on thewhole field in a system with uncontrolled traf-fic. Controlled traffic lanes may actually havean advantage in that the consolidated soil is ableto bear greater loads, thereby better facilitating

Crops Particularly Hard on Soils

� Potatoes require intensive tillage andreturn low rates of residue to soil.

� Silage corn and soybeans return low ratesof residue.

� Many vegetable crops require timelyharvest, so field traffic occurs even whenthe soils are too wet.

Special care is needed to counter the nega-tive effects of such crops. These may includeselecting soil-improving crops to fill out therotation, extensive use of cover crops, usingcontrolled traffic, and adding extra organicmaterials, such as manures and composts. Inan 11-year experiment in Vermont with con-tinuous corn silage on a clay soil, we foundthat applications of dairy manure were criti-cal to maintaining good soil structure. Appli-cations of 0, 10, 20, and 30 tons (wet weight)of dairy manure per acre each year of the ex-periment resulted in pore space of 44, 45, 47,and 50 percent of the soil volume.


field traffic. Compaction also can be reducedsignificantly by maximizing traffic of farm trucksalong the field boundaries and using plannedaccess roads, rather than allowing them to ran-domly travel over the field.

…most of the compaction occurs

with the heaviest loading and

does not greatly increase

with other passes…

A permanent (raised) bed system is anotherway of controlling traffic. In this case, controlledtraffic is combined with soil shaping to improvethe physical conditions in the beds. Beds do notreceive traffic after they’ve been formed. This isespecially attractive where traffic on wet soil isunavoidable for economic reasons (for example,with certain fresh-market vegetable crops) andwhere it is useful to install equipment, such asirrigation lines, for multiple years.

SOURCES

Ontario Ministry of Agriculture, Food, and Ru-ral Affairs. 1997. Soil Management. Best Man-agement Practices Series. Available from theOntario Federation of Agriculture, Toronto,Ontario (Canada).

Kok, H., R.K. Taylor, R.E. Lamond, and S. Kes-sen. 1996. Soil Compaction: Problems and So-lutions. Cooperative Extension Service pub-lication AF 115. Kansas State University. Man-hattan, KS.

Using a Penetrometer forAssessing Compaction

A penetrometer is a tool that costs about $200and measures the resistance to soil penetra-tion. A penetrometer has a rod with a cone-shaped tip that is pushed into the soil. Whenpenetration resistance is greater than about300 psi, the soil is usually too hard for rootsto grow (see chapter 6). Remember that thestrength of the soil depends on the water con-tent as well as bulk density (also chapter 6),so penetrometer measurements need to berepeated several times during the growingseason to make a good assessment. However,you can sometimes get important informationfrom a single set of penetrometer measure-ments made when the soil is very moist (forexample at the beginning of the growing sea-son in humid regions). If penetrometer read-ings at that time are near or above 300 psi,they will surely be higher when the soil driesout later in the season. When making pen-etrometer measurements, try to notice soilstrength in both the plow layer and the sub-soil — corrective action may be different foreach case.

When using a penetrometer, also keep inmind that soil strength is extremely variableand multiple penetrations throughout the fieldshould be made and averaged before draw-ing conclusions. Penetrometers do not workwell in rocky soils, as the measurement is notvalid when the tip hits a rock. The devices arenot very good for predicting rooting behav-ior in clayey soils. Although clays may get veryhard upon drying, they may still have enoughlarge pores to allow roots to proliferate.

135

Although tillage is an ancient practice, the question of which tillage system is most

appropriate for any particular field or farm isstill difficult to answer. Before we discuss differ-ent tillage systems, let’s consider whypeople started tilling ground. Intensive,full-field tillage was first practiced byfarmers who grew small-grain crops,such as wheat, rye and barley, prima-rily in Western Asia, Europe, and North-ern Africa. Tillage was needed to con-trol weeds and give the crop a head-start beforea new flush of weeds germinated. It also stimu-lated mineralization of organic forms of nitro-gen to forms that plants could use. Mostly, how-ever, intensive tillage created a fine seedbed,thereby greatly improving germination. The soilwas typically loosened by plowing and thendragged to pulverize the clods and create a finelyaggregated and smooth seedbed. The loosened

15�

Reducing Tillage

…the crying need is for a soil surface similar to

that which we find in nature. …[and] the way to attain

it is to use an implement that is incapable of burying

the trash it encounters; in other words,

any implement except the plow.

—E.H.FAULKNER, 1943

soil also tended to provide a more favorable root-ing environment, facilitating seedling survivaland plant growth. From early on, animal trac-tion was employed to accomplish this arduous

task. At the end of the growing season,the entire crop was harvested, becausethe straw also had considerable eco-nomic value for animal bedding, roof-ing thatch, and brick making. Some-times, fields were burned after cropharvest to remove remaining crop resi-

dues and to control pests. Although this tillage-cropping system lasted for a long time, it re-sulted in excessive erosion, especially in theMediterranean region, where it caused exten-sive soil degradation. Eventually deserts spreadas the climate became drier.

Other ancient agricultural systems, notablythose in the Americas, did not use intensive full-field tillage for grain production. Instead, they


used a hoe for manual tillage that created smallmounds (hilling). This was well adapted to theregional staples of corn and beans, which havelarger seeds and require lower planting densi-ties than wheat, rye, and barley. Several seedswere placed in a small hill, often with the helpof a planting stick, and hills were spaced sev-eral feet apart. In many, but not all cases, thehills were elevated to provide a temperature andmoisture advantage to the crop. Compared withthe cereal-based systems growing only one cropin a monoculture, these fields often includedtwo or three plant species growing at the sametime. This hilling system was generally less proneto erosion than whole-field tillage, but climateand soil conditions on steep slopes still fre-quently caused considerable soil degradation.

A third tillage system was practiced as partof the rice-growing cultures in southern andeastern Asia. Here, paddies were tilled to con-trol weeds, puddle the soil, and create a denselayer that limited the downward losses of waterthrough the soil. The puddling process occurredwhen the soil was worked while wet — in theplastic or liquid consistency state — and wasspecifically aimed at destroying soil structure.This system was designed because rice plantsthrive under flooded conditions. There is littlesoil erosion, because paddy rice must be growneither on flat or terraced lands and runoff is con-trolled as part of the process of growing the crop.

Full-field tillage systems became more wide-spread as the influence of European culture ex-panded into other regions of the world. It’s bet-ter adapted to mechanized agriculture so the tra-ditional “hill crops” eventually became rowcrops. The invention of the moldboard plowprovided a more effective tool for weed controlby fully turning under crop residues, growing

weeds, and weed seeds. The development ofincreasingly powerful and comfortable tractorsmade tillage an easier task. In fact, it has be-come almost a recreational activity for somefarmers.

New technologies have lessened the need fortillage. The development of herbicides reducedthe need for tillage as a weed control method.New planters achieved better seed placement,even without preparing a seedbed beforehand.Amendments, such as fertilizers and liquid ma-nures, can be directly injected or band-applied.Now, there are even vegetable transplanters thatprovide good soil-root contact in reduced or no-till systems. Although herbicides often are usedto kill cover crops before planting the main crop,farmers and researchers have found that theycan obtain fairly good cover crop controlthrough well-timed mowing, rolling, or rolling/chopping — greatly reducing the amount ofherbicide needed.

Increased mechanization, intensive tillage,and erosion have degraded many agriculturalsoils to such an extent that people think theyrequire tillage to provide temporary relief fromcompaction. As aggregates are destroyed, crust-ing and compaction create a soil “addicted” to

Technologies have lessened

the need for tillage…

� herbicides

� new planters and transplanters

� new physical methods for cover

crop suppression

REDUCING TILLAGE 137

tillage. Except perhaps for organic crop produc-tion systems, where tillage is needed becauseherbicides aren’t used, a crop can be producedwith limited or no tillage with the same eco-nomic return as conventional tillage systems.Managing soil in the right way to make reducedtillage systems successful, however, remains aconsiderable challenge.

TILLAGE SYSTEMS

Tillage systems are often classified by the amountof surface residue left on the soil surface. Con-servation tillage systems are those that leavemore than 30 percent of the soil surface cov-ered with crop residue. This is considered to bea level at which erosion is significantly reduced(see figure 15.1). Of course, this partially de-pends on the amount of residue left after har-vest, which may vary greatly among crops andharvest method (for example, corn harvested forgrain or silage). Although surface residue covergreatly influences erosion potential, the sole fo-cus on it is somewhat misleading. Erosion po-tential also is affected by factors such as surfaceroughness and soil loosening. Another distinc-tion of tillage systems is whether they are full-field systems or restricted tillage systems (figure15.2). The benefits and limitations of varioustillage systems are compared in table 15.1.

Conventional Tillage

A full-field system manages the soil uniformlyacross the entire field surface. It typically involvesa primary pass to loosen the soil and incorporatematerials at the surface (fertilizers, amendments,weeds, etc.), followed by one or more secondarytillage passes to create a suitable seedbed. Pri-mary tillage tools are generally moldboard plows,

0 10 20 30 40 50 60 70 80

chisels, and disks, while secondary tillage is ac-complished with finishing disks, tine or toothharrows, rollers, packers, drags, etc. These till-age systems create a uniform and often finelyaggregated seedbed over the entire surface of thefield. Such systems appear to perform well be-cause they create near-ideal conditions for seedgermination and crop establishment.

Moldboard plowing is generally the least de-sirable practice because it is energy intensive,leaves very little residue on the surface, and of-ten requires multiple secondary tillage passes.It also tends to create plow pans. However, it isgenerally the most reliable practice and almostalways results in reasonable crop growth. Chisel

Figure 15.1 Soil erosion dramatically decreaseswith increasing surface cover. (Fall plow (FP), fallchisel (FC), no-till (NT), corn = circles, soybeans= no circles). Modified from Manuring, 1979.

This figure shows that:

• Surface residue reduces erosion.• Reduced tillage (chisel and no-till) leaves more

residue and results in less erosion than plowing.• Corn (circled) returns more residue than soybeans.

45

40

35

30

25

20

15

10

5

0

soil

loss

(to

ns/a

cre)

residue cover of soil surface (%)

FP

C

P

C

T

T


Figure 15.2 Four tillage systems.a) Chisel tillage: Shanks provide full-field soil loosening.b) No-till: Corn was directly planted into untilled soil. Photo by NRCS.c) Zone tillage: Planter loosens soil in the row and moves residues to the side.d) Ridge tillage: Crop is planted into small ridges without tillage.

a b

c d

implements generally provide results similar tothe moldboard plow, but require less energy andleave significantly more residue on the surface.Chisels also allow for more flexibility in depthof tillage, generally from 5 to 12 inches, withsome tools specifically designed to go deeper.Disks usually perform shallow tillage, depend-ing on their size, and still leave residue on thesurface. They can be used as both primary andsecondary tillage tools.

Although full-field tillage systems have theirdisadvantages, they often can help overcome cer-tain problems, such as compaction and highweed pressures. Organic farmers often use mold-board plowing as a necessity to provide adequateweed control and facilitate nitrogen release fromincorporated legumes. Livestock-based farmsoften use a plow to incorporate manure and tohelp make rotation transitions from sod cropsto row crops.

139

TILLAGE SYSTEM BENEFITS

TABLE 15.1Tillage System Benefits and Limitations

LIMITATIONS

Moldboard plow Easy incorporation of fertilizers andamendments.

Buries surface weed seeds.

Soil dries out fast.

Temporarily reduces compaction.

Chisel plow Same as above, but with moresurface residues.

Disc harrow Same as above.

No-till Little soil disturbance.

Few trips over field.

Low energy use.

Most surface residue cover anderosion protection.

Zone-till Same as above.

Ridge-till Easy incorporation of fertilizers andamendments.

Some weed control as ridges arebuilt.

Seed zone on ridge dries andwarms more quickly.

Leaves soil bare.

Destroys natural aggregation andenhances organic matter loss.

Surface crusting and acceleratederosion common.

Causes plow pans.

High energy requirements.

Same as above, but less aggressivedestruction of soil structure, lesserosion, less crusting, no plow pans,and less energy use.

Same as above

Hard to incorporate fertilizers andamendments.

Wet soils slow to dry and warm up inspring.

Can’t alleviate compaction by usingtillage.

Same as above, but fewer problemswith compaction.

Hard to use together with sod-type ornarrow-row crop in rotation.

Equipment needs to be adjusted totravel without disturbing ridges.

FULL-FIELD TILLAGE

RESTRICTED TILLAGE


that rougher soil has much higher water infil-tration rates and reduces problems with settlingand hardsetting after rains. Weed seed germi-nation is also generally reduced, but pre-emer-gence herbicides tend to be less effective thanwith smooth seedbeds. Reducing secondary till-age may, therefore, require emphasis on post-emergence weed control.

Restricted Tillage Systems

These systems are based on the idea that tillagecan be limited to the area around the plant rowand does not have to disturb the entire field.Three tillage systems fit this concept — no-till,zone-till, and ridge-till.

The no-till system (figure 15.2b) does notinvolve any soil loosening, except for a verynarrow and shallow area immediately aroundthe seed zone. This localized disturbance is typi-cally accomplished with a fluted, or ripple,coulter on a planter. This is the most extremechange from conventional tillage.

Besides incorporating surface residue, full-field tillage systems with intensive secondarytillage crush the natural soil aggregates. Thepulverized soil does not take heavy rainfall well.The lack of surface residue causes sealing at thesurface, which generates runoff and erosion andcreates hard crusts after drying. Also, intensivelytilled soil will settle after some rainfall and may“hardset” upon drying, thereby restricting rootgrowth.

Full-field tillage systems can be improved byusing tools, such as chisels (figure 15.2a), thatleave some residue on the surface. Reducingsecondary tillage also helps decrease negativeaspects of full-field tillage. Compacted soils tendto till up cloddy and intensive harrowing andpacking is then seen as necessary to create a goodseedbed. This creates a vicious cycle of furthersoil degradation with intensive tillage. Second-ary tillage often can be reduced through the useof state-of-the-art planters, which create a finelyaggregated zone around the seed without requir-ing the entire soil width to be pulverized. In-deed, a good planter is perhaps the most im-portant secondary tillage tool, because it helpsovercome poor soil-seed contact without de-stroying surface aggregates over the entire field.A fringe benefit of reduced secondary tillage is

A good planter is perhaps the

most important secondary tillage

tool, because it helps overcome

poor soil-seed contact without

destroying surface aggregates

over the entire field.

Restricted tillage systems are

based on the idea that tillage can

be limited to the area around the

plant row and does not have

to disturb the entire field.

No-till systems have been used successfullyon many soils in different climates. They areespecially well adapted to coarse-textured soils(sands and gravels), as they tend to be softerand less susceptible to compaction. It typicallytakes a few years for no-tilled soils to improve,


after which no-till crops often out-yield cropsgrown with conventional tillage. The quality ofno-tilled soils, by almost any measure, improvesover time. The maintenance of surface residueprotects against erosion and increases biologi-cal activity by protecting the soil from tempera-ture and heat extremes. Surface residues alsoreduce water evaporation, which — combinedwith deeper rooting — reduces the susceptibil-ity to drought.

Another system, usually called zone tillage(figure 15.2c), gets some of the benefits of soildisturbance in the soil around the plant rowwithout disturbing the entire field. It uses mul-tiple fluted coulters mounted on the front of aplanter (figure 15.3) to develop a fine seedbedof approximately 6 inches wide by 4 inchesdeep, and typically uses trash wheels to moveresidue away from the row. The system may alsoinclude a separate pass of a “zone building”implement during the off season (see figure

Figure 15.3 Zone-till planter. a. Coulters (cut upresidues and break up soil in seed zone); b. Ferti-lizer disc openers (place granular starter fertilizerin a band next to the seed); c. Spider (trash) wheels(move residue away from the row); d. Seed place-ment unit; e. Press wheels (create firm seedbed);and f. Wheel used for transporting the planter.

14.1). This typically involves a narrow shankor knife, sometimes used to inject fertilizers,combined with a trash remover or hilling disk(to perform in-row tillage and overcome com-paction problems). The term “strip tillage” of-ten is used to describe the latter system.

Ridge tillage (figure 15.2d) combines sometillage with a ridging operation. This system isparticularly attractive for cold and wet soils,because the ridges offer developing plants awarmer and better-drained environment. Theridging operation can be combined with me-chanical weed control and allows for band ap-plication of herbicides. This decreases the costof chemical weed control, allowing for about atwo-thirds reduction in herbicide use.

For fine and medium-textured soils, zone andridge tillage systems generally work better thanstrict no-till, especially in regions with cold andwet springs. Because these soils are more sus-ceptible to compaction, some soil disturbanceis probably beneficial. No-till is used success-fully for narrow-row crops, including smallgrains, perennial legumes and grasses. Zone andridge tillage are only adapted to wide-row cropswith 30-inch spacing or more.

WHICH TILLAGE SYSTEMFOR YOUR FARM?

This is difficult to answer. It depends on yourclimate, soils, cropping systems, and your ob-jectives. Here are some general guidelines.

Grain and vegetable farms have great flexi-bility with adopting reduced tillage systems. Inthe long run, limited disturbance and residuecover improve soil quality, reduce erosion, andboost yields. A negative aspect of these systemsis that, at first, they may require more herbi-

e

ab

c

fd


cides. However, combining reduced tillage withuse of cover crops frequently helps reduce weedproblems. Weed pressures typically decrease sig-nificantly after a few years. Mulched cover crops,as well as newly designed mechanical cultiva-tors, help provide effective weed control in high-residue systems. Some innovative farmers useno-till combined with a cover crop, which ismowed or otherwise killed to create a thickmulch. Steve Groff, a vegetable and field cropfarmer in Pennsylvania, modified a rolling stalkchopper to roll down and crimp his vetch/ryecover crop, providing weed control with mini-mal use of herbicides (see profile, p. 145).

Farmers need to be aware of potential soilcompaction problems with reduced tillage. If astrict no-till system is used on a compacted soil,especially on medium or fine-textured soils, se-rious yield reductions may occur. As discussedin chapter 6, dense soils have a narrow waterrange for which plant growth is not restricted.Crops growing on compacted soils are more sus-ceptible to inadequate aeration during wet pe-riods and restricted root growth and inadequatemoisture during drier periods. Compaction,

Figure 15.4 No-till transplanting of vegetablesinto a killed cover crop on the Groff farm inPennsylvania. Photo by Ray Weil.

therefore, reduces plant growth and makes cropsmore susceptible to pest pressures.

In poorly structured soils, tools like zone-builders and zone-till planters may provide com-paction relief in the row, while maintaining anundisturbed soil surface. Over time, soil struc-ture improves, unless recompaction occurs fromother field operations. Crops grown on imper-fectly drained soils tend to benefit greatly fromridging or bedding, because part of the root zoneremains aerobic during wet periods. These sys-tems also use controlled traffic lanes, whichgreatly reduce compaction problems. Unfortu-nately, matching wheel spacing and tire widthfor planting and harvesting equipment is notalways an easy task.

Before Converting to No-Till

An Ohio farmer asked one of the authors whatcould be done about a compacted, low or-ganic matter, and low fertility field that hadbeen converted to no-till a few years before.

Clearly, the soil’s organic matter and nu-trient levels should have been increased andthe compaction alleviated before the change.Once you’re committed to no-till, you’ve lostthe opportunity to easily and rapidly changethe soil’s fertility or physical properties. Therecommendation is really the same as forsomeone establishing a perennial crop likean apple orchard. Build up the soil and rem-edy compaction problems before convert-ing to no-till. It’s going to be much harder todo later on.

Once in no-till, there are some things thatcan be tried to break up compacted soils,such as a sorghum-sudangrass cover crop.However, a severe compaction problem mayrequire tilling up the soil and starting over.


For organic farms, as with traditional farmsbefore agrichemicals were available, full-fieldtillage may be necessary for mechanical weedcontrol and incorporation of manures and com-posts. Organic farming on lands prone to ero-sion may, therefore, involve tradeoffs. Erosioncan be reduced by using rotations with peren-nial crops and a modern planter to establish goodcrop stands without excessive secondary tillage.In addition, soil structure may be easier to main-tain, because organic farms generally use moreorganic inputs, such as manures and composts.

Livestock-based farms face special challengesrelated to applying manures or composts to thesoil. Although these materials may sometimesbe injected directly, some type of tillage usuallyis needed to avoid large losses of nitrogen byvolatilization and phosphorus and pathogens byrunoff. Transitions from sod to row crops areusually easier with some tillage. Farmers rais-ing livestock should try to reduce tillage as muchas possible and use methods that leave residueon the surface.

ROTATE TILLAGE SYSTEMS?

A tillage program does not need to be rigid. Whenchanging to reduced tillage, consider incorporat-ing nutrients and organic matter with the mold-board plow (see box on p. 142). Fields that arezone or no-tilled may occasionally need a full-field tillage pass to provide compaction relief or toincorporate amendments. Tilling a no-till or zone-till field should be done only if clearly needed.Although a flexible tillage program offers a num-ber of benefits, aggressive tillage with a mold-board plow and harrows on soils for which no-till is best adapted will destroy the favorable soilstructure built up by years of no-till management.

TIMING OFFIELD OPERATIONS

The success of a tillage system depends on manyother factors. For example, reduced tillage sys-tems, especially in the early transition years, mayrequire more attention to nitrogen management,as well as weed, insect, and disease control. Also,the performance of tillage systems may be af-fected by the timing of field operations. If till-age or planting is done when the soil is too wet(its water content is above the plastic limit) thencloddiness and poor seed placement may resultin poor stands. Tillage also is not recommendedwhen soil is too dry, because of excess dust cre-ation, especially on compacted soils. A “ball test”(Chapter 6) helps ensure that field conditionsare right.

Frost Tillage?

You may have heard of frost seeding legumesinto a pasture, hayfield, or winter wheat cropin very early spring, but probably never heardof tilling a frozen soil. It seems a strange con-cept, but some farmers are using frost tillageas a way to be timely and reduce unintendedtillage damage. It can be done after frost hasfirst entered the soil, but before it has pen-etrated more than 4 inches. Water moves up-ward to the freezing front and the soil un-derneath dries. This makes it tillable as longas the frost layer is not too thick. Compac-tion is reduced because equipment is sup-ported by the frozen layer. The resulting roughsurface is favorable for water infiltration andrunoff prevention. Some livestock farmers likefrost tillage as a way to incorporate or injectmanure in the winter.


Because soil compaction may affect the suc-cess of a reduced tillage system, a whole-systemapproach to soil management is needed. Forexample, no-till systems that also involve har-vesting operations with heavy equipment willsucceed only if traffic can be restricted to dryconditions or fixed lanes within the field. Evenzone-tillage methods may fail if heavy harvestequipment is used without fixed lanes. Soils thatare severely eroded and low in organic mattermay need careful management when making the

Optimum Tillage System

New agricultural technologies provide oppor-tunities to reduce tillage and improve soilquality. The optimum system for any farm de-pends mainly on soils and climate, as well asthe need for mechanical weed control, incor-poration of cover crops and animal manures,and lessening compaction. Tillage systemsshould change in the direction of those thatleave residue and mulches on the surface andthat limit the pulverization of soil aggregates.

transition to reduced tillage systems. In suchcases, methods that increase the soil organicmatter content and improve soil structure (forexample, cover cropping and organic amend-ments) before reducing tillage will improve theprobability for success of these systems. As sur-face residue levels increase with the start of re-duced tillage, some soil loosening may beneeded to relieve compaction.

SOURCES

Cornell Recommendations for Integrated Field CropProduction. 2000. Cornell Cooperative Exten-sion, Ithaca, NY.

Manuring. 1979. Cooperative Extension ServicePublication AY-222, Purdue University. WestLafayette, IN.

Ontario Ministry of Agriculture, Food, and Ru-ral Affairs. 1997. No-till: Making it Work. Avail-able from the Ontario Federation of Agricul-ture, Toronto, Ontario (Canada).

van Es, H.M., A.T. DeGaetano, and D.S. Wilks.1998. Upscaling plot-based research informa-tion: Frost tillage. Nutrient Cycling in Agro-ecosystems. 50: 85–90.

145

Steve Groff raises grains and vegetables every yearon his 175-acre farm in Lancaster County, Pa., buthis soil shows none of the degradation that canoccur with intensive cropping. Mixing cash cropssuch as corn, alfalfa, soybeans, broccoli, tomatoesand peppers with cover crops and a unique no-tillsystem, Groff has kept portions of his farm un-touched by a plow for close to two decades.

“No-till is a practical answer to concerns abouterosion, soil quality, and soil health,” says Groff,who won a national no-till award in 1999. “I wantto leave the soil in better condition than I found it.”

Groff confronted a rolling landscape pocked bygullies when he began farming with his father af-ter graduating from high school. They regularlyused herbicides and insecticides, tilled annuallyor semi-annually and rarely used cover crops. Likeother farmers in Lancaster County, they ignoredthe effects of tillage on a sloped landscape thatcauses an average of 9 tons of soil per acre to washinto the Chesapeake Bay every year.

Tired of watching two-feet-deep crevices formon the hillsides after every heavy rain, Groff be-gan experimenting with no-till to protect and im-prove the soil.

“We used to have to fill in ditches to get ma-chinery in to harvest,” Groff says. “I didn’t thinkthat was right.”

However, Groff stresses that switching to no-till alone isn’t enough. He has created a new sys-tem, reliant on cover crops, rotations, and no-till,to improve the soil. He’s convinced such methodscontribute to better yields of healthy crops, espe-cially during weather extremes.

He pioneered what he likes to call the “Perma-nent Cover” cropping system when the Pennsyl-

vania chapter of the Soil and Water ConservationSociety bought a no-till transplanter for vegetablecrops. Groff was one of the first farmers to try it.The machine allows him to transplant seedlingsinto slots cut into cover crop residue. The slotsare just big enough for the young plants and donot disturb the soil on either side. The result: Groffcan prolong the erosion-slowing benefits of covercrops.

Groff’s no-till system relies on winter covercrops and residues that blanket the soil virtuallyall year. In the fall, he uses a no-till seeder to planta combination of rye and hairy vetch. Groff likesthe pairing because of their complementary ben-efits. Their root structures grow in different pat-terns, and the vegetation left behind after killingleaves different residues on the soil surface.

Groff uses a rolling stalk-chopper — modifiedfrom Midwest machines that chop corn stalks af-ter harvest — to kill the covers in the spring. Thechopper flattens and crimps the cover crop, pro-viding a thick mulch. Once it’s flat, he makes apass with the no-till planter or transplanter.

The system creates a very real, side benefit inreduced insect pest pressure. Once an annual prob-lem, Colorado potato beetle damage has all butdisappeared from Groff’s tomatoes. Since he be-gan planting into the mulch, he has greatly re-duced spraying pesticides. The thick mat also pre-vents splashing of soil during rain, a primary causeof early blight on tomatoes.

“We have slashed our pesticide and fertilizerbill nearly in half, compared to a conventional till-age system,” Groff says. “At the same time, we’rebuilding valuable topsoil and not sacrificingyields.”


Steve GroffLancaster County, Pennsylvania

146

“No-till is not a miracle, but it works for me,”he says. “It’s good for my bottom line, I’m savingsoil, and I’m reducing pesticides and increasingprofits.”

Groff is convinced his crops are better thanthose produced in soils managed conventionally,especially during weather extremes. He has notedhigh earthworm populations and other biologicalactivity deep in the soil.

Ray Weil, a soils professor at the University ofMaryland who has spent time on Groff’s farm, con-curs.

“Steve’s subsoil is like other farmers’ topsoil,”he says.

Groff promotes his system at annual summerfield days that draw huge crowds of farmers and athis innovative web site, <www.cedarmeadowfarm.com>.

147

16�

Nutrient Management:An Introduction

The purchase of plant food is an important matter,

but the use of a [fertilizer] is not a cure-all, nor will it

prove an adequate substitute for proper soil handling.


Of the 18 elements needed by plants, onlythree — nitrogen (N), phosphorus (P),

and potassium (K) — are commonly deficientin soils. Deficiencies of other nutrients, such asmagnesium, sulfur, zinc, boron, andmanganese, certainly occur, but theyare not as widespread. However, in lo-cations with lots of young minerals thathaven’t been weathered much by na-ture, such as the Dakotas, potassiumdeficiencies are less common. Deficien-cies of sulfur, magnesium, and some micronu-trients may be more widespread in regions withhighly weathered minerals, such as the south-eastern states, or those with high rainfall, such asportions of the Pacific Northwest.

Environmental concerns have placed moreemphasis on better management of nitrogen and

phosphorus over the last few decades. Thesenutrients are critical to soil fertility management,but they are also responsible for widespread en-vironmental problems. Poor soil and crop man-

agement, the overuse of fertilizers, mis-use of manures, sludges and composts,and high animal numbers on limitedland area have contributed to surfaceand groundwater pollution in many re-gions of the U.S. Because both nitro-gen and phosphorus are used in large

quantities and their overuse has potential envi-ronmental implications, we’ll discuss them to-gether in chapter 17. Other nutrients, cationexchange, soil acidity (low pH) and liming, andarid and semi-arid region problems with so-dium, alkalinity (high pH), and excess salts arecovered in chapter 18.


THE BOTTOM LINE:NUTRIENTS AND PLANT HEALTH,

PESTS, PROFITS, ANDTHE ENVIRONMENT

Management practices are all related. The keyis to visualize them all as whole-farm manage-ment, leading you to the goals of better cropgrowth and better environmental quality. If a soilhas good tilth, no subsurface compaction, gooddrainage, adequate water, and a good supply oforganic matter, then plants should be healthyand have large root systems. A large and healthyroot system enables plants to efficiently take upnutrients and water from the soil and to usethose nutrients to produce higher yields.

Doing a good job of managing nutrients onthe farm and in individual fields is critical togeneral plant health and management of plantpests. Too much available nitrogen in the early

a. Build up and maintain high soil organicmatter levels.

b.Test manures and credit their nutrient contentbefore applying fertilizers or other amend-ments.

c. Incorporate manures into the soil quickly, ifpossible, to reduce nitrogen volatilizationand potential loss of nutrients in runoff.

d.Test soils regularly to determine the nutrientstatus and whether or not manures, fertiliz-ers, or lime are needed.

e. Balance nutrient inflows and outflows tomaintain optimal levels and allow a little“draw-down” if nutrient levels get too high.

f. Enhance soil structure and reduce field runoffby minimizing soil compaction damage.

g. Use forage legumes or legume cover crops toprovide nitrogen to following crops anddevelop good soil tilth.

h. Use cover crops to tie up nutrients in off-season, enhance soil structure, and reducerunoff and erosion.

i. Maintain soil pH in the optimal range for thecrops in your rotation.

j. When phosphorus and potassium are verydeficient, broadcast some of the fertilizer toincrease the general soil fertility level, andband apply some as well.

k. To get the most efficient use of the fertilizerwhen phosphorus and potassium levels are inthe medium range, consider band applica-tion at planting, especially in cool climates.

The ABCs of Nutrient Management

part of the growing season allows small-seededweeds, with few nutrient reserves, to get wellestablished. This early jump-start may then en-able them to out-compete crop plants later on.Crops do not grow properly if nutrients aren’tpresent in sufficient quantities and in reason-able balance to one another. Plants may bestunted if nutrients are low, or they may growtoo much foliage and not enough fruit if nitro-gen is too plentiful relative to other nutrients.Plants under nutrient stress, such as too low ortoo high nitrogen levels, are not able to emit asmuch of the natural chemicals that signal bene-ficial insects when insect pests feed on leaves orfruit. Stalk rot of corn is aggravated by low po-tassium levels. On the other hand, pod rot ofpeanuts is associated with excess potassiumwithin the fruiting zone of peanuts (the top 2 to3 inches of soil). Blossom-end rot of tomatoesis related to low calcium levels, often brought

NUTRIENT MANAGEMENT 149

Essential Nutrients for Plants

COMMON

ELEMENT AVAILABLE FORM SOURCE

needed in large amounts

Carbon CO2 atmosphere

Oxygen O2, H2O atmosphereand soilpores

Hydrogen H2O water in soilpores

Nitrogen NO3-, NH4

+ soil

Phosphorus H2PO4-, HPO4

-2 soil

Potassium K+ soil

Calcium Ca+2 soil

Magnesium Mg+2 soil

Sulfur SO4-2 soil

needed in small amounts

Iron Fe+2, Fe+3 soil

Manganese Mn+2 soil

Copper Cu+, Cu+2 soil

Zinc Zn+2 soil

Boron H3BO3 soil

Molybdenum MoO4-2 soil

Chlorine Cl- soil

Cobalt Co+2 soil

Nickel Ni+2 soil

Nutrient Management Goals

� Satisfy crop nutrient requirements foryield and quality.

� Minimize pest pressure caused by fertilityimbalances.

� Minimize the risk of damage to theenvironment.

� Minimize the cost of supplying nutrients.� Use local sources of nutrients whenever

possible.� Get full nutrient value from fertility

sources.—MODIFIED FROM OMAFRA, 1997

known sources of available nitrogen for plants.Mineralization of phosphorus and sulfur fromorganic matter is also an important source ofthese nutrients. As discussed earlier, organicmatter helps hold on to potassium (K+), calcium(Ca++), and magnesium (Mg++) ions. It also pro-vides natural chelates that maintain micronu-trients such as zinc, copper, and manganese, informs that plants can use.

on by droughty, or irregular rainfall/irrigation,conditions.

When plants either don’t grow well or are moresusceptible to pests, this affects the economic re-turn. Yield and crop quality usually are reduced,lowering the amount of money received. Therealso may be added costs to control pests that takeadvantage of poor nutrient management. In ad-dition, when nutrients are applied beyond plantneeds, it’s like throwing money away. And whennitrogen and phosphorus are lost from the soilby leaching to groundwater or running into sur-face water, entire communities may suffer frompoor water quality.

ORGANIC MATTER ANDNUTRIENT AVAILABILITY

The best single overall strategy for nutrient man-agement is to work to enhance soil organic mat-ter levels in soils. This is especially true for ni-trogen and phosphorus. Soil organic matter, to-gether with any freshly applied residues, are well


IMPROVING NUTRIENT CYCLINGON THE FARM

For economic and environmental reasons, itmakes sense to utilize nutrient cycles efficiently.Goals should include the reduction in long-dis-tance nutrient flows, as well as promoting “true”on-farm cycling. There are a number of strate-gies to help farmers reach the goal of better nu-trient cycling:

� Reduce unintended losses by promotingwater infiltration and better root healththrough enhanced management of soilorganic matter and physical properties. Theways in which organic matter can be builtup and maintained include increasedadditions of a variety of sources of organicmatter plus methods for reducing losses viatillage and conservation practices.

� Enhance nutrient uptake efficiency bycarefully using fertilizers and amendments.Better placement and synchronizing appli-cation with plant growth both improveefficiency. Sometimes, changing plantingdates or switching to a new crop creates abetter match between the timing of nutrientavailability and crop needs.

� Tap local nutrient sources by seeking localsources of organic materials, such as leavesor grass clippings from towns, aquaticweeds harvested from lakes, produce wastefrom markets and restaurants, food process-ing wastes, and clean sewage sludges (seediscussion on sewage sludge in chapter 8).Although some of these do not contribute totrue nutrient cycles, the removal of agricul-turally usable nutrients from the “wastestream” makes sense and helps developmore environmentally sound nutrient flows.

� Promote consumption of locally producedfoods by supporting local markets as well asreturning local food wastes to farmland.When people purchase locally producedfoods there are more possibilities for truenutrient cycling to occur. Some CommunitySupported Agriculture farms, where sub-scriptions are paid before the start of thegrowing season, encourage their members toreturn produce waste to the farm forcomposting, completing a true cycle.

� Reduce exports of nutrients in farmproducts by adding animal enterprises tocrop farms (fewer exports, and more reasonto include forage legumes and grasses inrotation). The best way to both reducenutrient exports per acre, as well as to makemore use of forage legumes in rotations, isto add an animal (especially a ruminant)enterprise to a crop farm. Compared withselling crops, feeding crops to animals andexporting animal products results in farfewer nutrients leaving the farm. (Keep in

Strategies for ImprovingNutrient Cycles

� Reduce unintended losses.� Enhance nutrient uptake efficiency.� Tap local nutrient sources.� Promote consumption of locally produced

foods.� Reduce exports of nutrients in farm

products.� Bring animal densities in line with the land

base of the farm.� Develop local partnerships to balance

flows among different types of farms.


mind that, on the other hand, raisinganimals with mainly purchased feed is thebest way to overload a farm with nutrients.)

� Bringing animal densities in line with theland base of the farm can be accomplishedby renting or purchasing more land — togrow a higher percentage of animal feedsand for manure application — or byreducing animal numbers.

� Develop local partnerships to balance flowsamong different types of farms. As pointedout in chapter 8 when we discussed organicmatter management, sometimes neighboringfarmers cooperate with both nutrientmanagement and crop rotations. This isespecially beneficial when a livestock farmerhas too many animals and imports a highpercentage of feed and a neighboringvegetable farm has a need for nutrients andhas an inadequate land base to allow arotation that includes a forage legume. Bycooperating with nutrient management androtations, both farms win, sometimes inways that were not anticipated (see “win-win” box). Encouragement and coordina-tion from an extension agent may helpneighboring farmers work out cooperativeagreements. It is more of a challenge as thedistances become greater.

Some livestock farms that are overloaded withnutrients are finding that composting is an at-tractive alternative way to handle manure. Dur-ing the composting process, volume and weightare greatly reduced (see chapter 12), resulting inless material to transport. Organic farmers arealways on the lookout for reasonably priced ani-mal manures and composts. The landscape in-dustry also uses a fair amount of compost. Local

or regional compost exchanges can help removenutrients from overburdened animal operationsand place them on nutrient-deficient soils.

USING FERTILIZERSAND AMENDMENTS

There are four main issues when applying nu-trients:

� How much is needed?� What source(s) should be used?� When should the fertilizer or amendment

be applied?� How should the fertilizer or amendment be

applied?

Chapter 19 details use of soil tests to helpyou decide how much fertilizer or organic nu-trient sources to apply. Here we will go over howto approach the other three issues.

Win-Win Cooperation

Cooperation between Maine potato farmersand their dairy farm neighbors has led to bet-ter soil and crop quality for both types offarms. As potato farmer John Dorman ex-plains, after some years of cooperating witha dairy farm on rotations and manure man-agement, soil health “…has really changedmore in a few years than I’d have thoughtpossible.” Dairy farmer Bob Fogler feels thatthe cooperation with the potato farmer al-lowed his family to expand the dairy herd.He notes that “we see fewer pests and bet-ter quality corn. Our forage quality has im-proved. It’s hard to put a value on it, but for-age quality means more milk.”

FROM HOARD’S DAIRYMAN, 4/10/99


Nutrient Sources: CommercialFertilizers vs. Organic Materials

There are numerous fertilizers and amendmentsnormally used in agriculture (some are listed intable 16.1). Fertilizers such as urea, triple su-perphosphate, and muriate of potash (potassiumchloride) are convenient to store and use. Theyare also easy to blend to meet nutrient needs inspecific fields and provide predictable effects.Their behavior in soils and the ready availabil-ity of the nutrients is well established. The tim-ing, rate, and uniformity of nutrient applicationis easy to control when using commercial fertil-izers. However, there also are drawbacks to us-ing commercial fertilizers. All of the commonlyused nitrogen materials (those containing urea,ammonia, and ammonium) are acid forming,and their use in humid regions, where nativelime has been weathered out, requires more fre-quent lime additions. Also, the high nutrientsolubility can result in salt damage to seedlingswhen excess fertilizer is applied close to seedsor plants. Because nutrients in commercial fer-tilizers are readily available, under some circum-stances more may leach to groundwater thanwhen using organic nutrient sources. For ex-ample, high rainfall events on a sandy soil soonafter ammonium nitrate fertilizer application willprobably cause more nitrate loss than if a com-post had been applied or a legume cover croprecently incorporated. Likewise, sediments lostby erosion from fields fertilized with commer-cial fertilizers probably will contain more avail-able nutrients than those from fields fertilizedwith organic sources, resulting in more severewater pollution.

Organic sources of nutrients have many othergood qualities too. They usually provide a moreslow release source of fertility and the nitrogen

availability is more evenly matched to the needsof growing plants. Sources like manures or cropresidues commonly contain all the needed nu-trients, including the micronutrients — but theymay not be present in the proper proportion forsoil and crop needs. These materials are alsosources of soil organic matter, providing foodfor soil organisms and forming aggregates andhumus.

One of the drawbacks to organic materials isthe variable amounts and uncertain timing ofnutrient release for plants to use. The value ofmanure as a nutrient source depends upon thetype of animal, its diet, and how the manure ishandled. For cover crops, the nitrogen contri-bution depends upon the species, amount ofgrowth in the spring, and weather. Also, ma-nures typically are bulky and may contain a highpercentage of water — so considerable work is

Organic Farming vs.Organic Nutrient Sources

We’ve used the term “organic sources” of nu-trients to refer to nutrients contained in cropresidues, manures, and composts. These typesof materials are used by all farmers—”con-ventional” or “organic.” Both also use lime-stone and a few other materials. However,most of the commercial fertilizers listed intable 16.1 are not allowed in organic produc-tion. In place of sources such as urea, anhy-drous ammonia, diammonium phosphate(DAP), concentrated superphosphate, andmuriate of potash, organic farmers use prod-ucts that come directly from minerals such asgreensand, granite dust, and rock phosphate.Other organic products come from parts oforganisms such as bone meal, fish meal, soy-bean meal, and bloodmeal (see table 16.2).

153

TABLE 16.1Composition of Various Common Amendments

and Commercial Fertilizers (%)

N P2O5 K2O Ca Mg S Cl

N Materials

Anhydrous ammonia 82

Aqua ammonia 20

Ammonium nitrate 34

Ammonium sulfate 21 24

Calcium nitrate 16 19 1

Urea 46

UAN solutions 28–32

P and N+P Materials

Superphosphate (ordinary) 20 20 12

Triple superphospate 46 14 1

Diammonium phosphate (DAP) 18 46

Monoammonium phosphate (MAP) 11–13 48–52

K Materials

Potassium chloride 60 47(muriate of potash)

Potassium-magnesium sulfate 22 11 23 2(“Sul-Po-Mag”)

Potassium sulfate 50 1 18 2

Other Materials

Gypsum 23 17

Limestone, calcitic 25–40 0.5–3

Limestone, dolomitic 19–22 6–13 1

Magnesium sulfate 2 11 14

Potassium nitrate 13 44

Sulfur 30–99

Wood ashes 2 6 23 2

—OMAFRA, 1997.


needed to apply them per unit of nutrients. Thetiming of nutrient release is uncertain, becauseit depends both on the type of organic materi-als used and on the action of soil organisms.Their activities change with temperature andrainfall. Finally, the relative nutrient concentra-tions for a particular manure used may notmatch soil needs. For example, manures maycontain high amounts of both nitrogen andphosphorus when your soil already has highphosphorus levels.

Selection of Commercial FertilizerSources

There are numerous forms of commercial fertil-izers, many given in table 16.1. When you buyfertilizers in large quantities, the cheapest sourceis usually chosen. When you buy bulk blendedfertilizer, you usually don’t know what sourceswere used unless you ask. All you know is thatit’s a 10-20-20 (referring to the percent of avail-able N, P2O5, and K2O), or a 20-10-10, or an-

TABLE 16.2Products Used by Organic Growers to Supply Nutrients*

%N %P2O5 %K2O

Alfalfa pellets 2.7 0.5 2.8

Blood meal 13.0 2.0 -

Bone meal 3.0 20.0 0.5

Cocoa shells 1.0 1.0 3.0

Colloidal phosphate - 18.0 -

Compost 1.0 0.4 3.0

Cottonseed meal 6.0 2.0 2.0

Fish scraps, dried & ground 9.0 7.0 -

Granite dust - - 5.0

Greensand - - 7.0

Hoof & horn meal 11.0 2.0 -

Linseed meal 5.0 2.0 1.0

Rock phosphate - 30.0 -

Seaweed, ground 1.0 0.2 2.0

Soybean meal 6.0 1.4 4.0

Tankage 6.5 14.5 -

* 1. Values of P2O5 and K2O represent total nutrients present. For fertilizers listed in table 16.1, the numbers are theamount that are readily available.

2. Organic growers also use potassium-magnesium sulfate (“Sul-Po-Mag” or “K-Mag”), wood ashes, limestone andgypsum listed in table 16.1. Although some use only manure that has been composted, others will use aged manures(see chapter 9). There are also a number of commercial organic products with a variety of trade names.

—PARNES, R. 1990.


other blend. However, below are a number ofexamples where you might not want to applythe cheapest source:

� Although the cheapest nitrogen form isanhydrous ammonia, the problems withinjecting it into a soil with many largestones or the losses that might occur wheninjecting it into very moist clay may call forother nitrogen sources to be used instead.

� If both nitrogen and phosphorus areneeded, diammonium phosphate (DAP) is agood choice because it has approximatelythe same cost and phosphorus content asconcentrated superphosphate and alsocontains 18 percent N.

� Although muriate of potash (potassiumchloride) is the cheapest potassium source,it may not be the best choice under certaincircumstances. If you also need magnesiumand don’t need to lime the field, potassium-magnesium sulfate would be a better choice.

Method and Timing of Application

The timing of fertilizer application is frequentlyrelated to the application method chosen, so inthis section we’ll go over both issues together.

Broadcast application, where fertilizer isevenly distributed over the whole field and thenusually incorporated during tillage, is best usedto increase the nutrient level of the bulk of thesoil. It is especially useful to build phosphorusand potassium when they are very deficient.Broadcasting for incorporation is usually donein the fall or in spring just before tillage. Broad-casting on top of a growing crop, called topdress,is commonly used to apply nitrogen, especiallyto crops that occupy the entire soil surface, suchas wheat or a grass hay crop. [Amendments used

Fertilizer Prices

Costs of fertilizers are currently around 24cents/lb of N, 30 cents/lb of P2O5, and 19cents/lb of K2O. Nitrogen prices fluctuategreatly, while those for phosphorus and po-tassium have been more stable.

in large quantities, like lime and gypsum, are alsobroadcast prior to incorporation into the soil.]

There are various types of localized placementof fertilizer. Banding small amounts of fertilizerto the side and below the seed at planting is acommon application method. It is especiallyuseful for row crops grown in cool soil condi-tions, such as early in the season, on soils withhigh amounts of surface residues, with no-tillmanagement, or on wet soils. It is also usefulfor soils that test low-to-medium in phospho-rus and potassium (or even higher). Banding fer-tilizer at planting, usually called starter fertil-izer, may be a good idea even in warmer climateswhen planting early. It still might be cool enoughto slow root growth and release of nutrients fromorganic matter. Including nitrogen in the bandappears to help roots use fertilizer phosphorusmore efficiently. Starter fertilizer for very low fer-tility soils frequently contains other nutrients,such as sulfur, zinc, boron, or manganese.

Splitting nitrogen applications is a good man-agement practice — especially on sandy soils,where nitrate is easily lost by leaching, or onheavy loams and clays, where it is easily lost bydenitrification. Some nitrogen is applied beforeplanting or in the band as starter fertilizer, andthe rest is applied as a sidedress or topdress dur-ing the growing season. Sometimes, split appli-cations of potassium are recommended for very


sandy soils with low organic matter, especiallyif there has been enough rainfall to cause potas-sium to leach into the subsoil. Unfortunately,relying on sidedressing nitrogen can increase riskof reduced yields if the weather is too wet toapply the fertilizer (and you haven’t put onenough preplant or as starter) or too dry follow-ing an application. Then the fertilizer stays onthe surface instead of washing into the root zone.

Once the soil nutrient status is optimal, tryto balance farm nutrient inflows and outflows.When nutrient levels, especially phosphorus, arein the high or very high range, stop applicationand try to “draw down” soil test levels.

Tillage and Fertility Management:To Incorporate or Not?

With systems that provide some tillage, such asmoldboard plow and harrow, disk harrow alone,chisel plow, zone-till, and ridge-till, it is possibleto incorporate fertilizers and amendments. How-ever, when using no-till production systems, it isnot possible to mix materials into the soil to uni-formly raise the fertility level in that portion ofthe soil where roots are especially active.

in runoff during rain events. Although theamount of runoff is usually lower with reducedtillage systems than with conventional tillage,the concentration of nutrients in the runoff maybe quite a bit higher.

If you are thinking about changing from con-ventional tillage to no-till or other forms of re-duced tillage, you might consider incorporatingneeded lime, phosphate, and potash, as well asmanures and other organic residues, before mak-ing the switch. It’s the last chance to easily changethe fertility of the top 8 or 9 inches of soil.

SOURCES

Ontario Ministry of Agriculture, Food, and Ru-ral Affairs (OMAFRA). 1997. Nutrient Man-agement. Best Management Practices Series.Available from the Ontario Federation of Ag-riculture, Toronto, Ontario (Canada).

Parnes, R. 1990. Fertile Soil: A Grower’s Guide toOrganic and Inorganic Fertilizers. agAccess,Davis, CA.

Soil Tests

Soil tests, one of the key nutrient managementtools, are discussed in detail in chapter 19.

Oxide vs. Elemental Forms?

When talking about using fertilizer phosphateor potash, the oxide form is usually assumed.This is used in all recommendations andwhen you buy fertilizer. When you put 100lbs. of potash per acre, you actually applied100 lbs. of K2O — that’s the equivalent of 83lbs. of elemental potassium. Of course,you’re not really using K2O but rather some-thing like muriate of potash (KCl). It’s the sameidea for phosphate — and 100 lbs. of P2O5

per acre is the same as 44 lbs. of P — andyou’re really using fertilizers like concentratedsuperphosphate (that contains a form of cal-cium phosphate) or ammonium phosphate.

The advantages of incorporating fertilizersand amendments are numerous. Significantquantities of ammonia may be lost by volatil-ization when the most commonly used solidnitrogen fertilizer, urea, is left on the soil sur-face. Also, nutrients remaining on the surfaceafter application are much more likely to be lost

157

17�

Management ofNitrogen and Phosphorus

…an economical use of fertilizers requires that they

merely supplement the natural supply in the soil,

and that the latter should furnish the larger part of

the soil material used by the crop.

—T.L. LYON AND E.O. FIPPIN, 1909

The management of nitrogen and phospho-rus is discussed together because both are

needed in large amounts by plants and both cancause environmental harm when present in ex-cess. We don’t want to do a good jobof managing one and, at the same time,do a poor job with the other. The mainenvironmental concern with nitrogenis the leaching of soil nitrate to ground-water and excess nitrogen in runoff.The drinking of high-nitrate ground-water is a health hazard to infants and younganimals. In addition, nitrate stimulates thegrowth of algae and aquatic plants just like itdoes for agricultural plants. The growth of plantsin many brackish estuaries and saltwater envi-ronments is believed to be limited by nitrogen.So, when nitrate leaches through soil, or runsoff the surface and is discharged into streams,eventually reaching water bodies like the Gulf

of Mexico or the Chesapeake Bay, undesirablemicroorganisms flourish. In addition, the algalblooms that result from excess nitrogen andphosphorus cloud water, blocking sunlight to

important underwater grasses that arehome to numerous species of youngfish, crabs, and other bottom-dwellers.

Phosphorus is the nutrient that ap-pears to limit the growth of freshwa-ter aquatic weeds and algae. Phospho-rus damages the environment when

excess amounts are added to a lake from hu-man activities (agriculture, rural home septictanks, or urban sewage or runoff). This increasesalgae growth, making fishing, swimming, andboating unpleasant or difficult. When excessaquatic organisms die, decomposition removesoxygen from water and leads to fish kills.

All farms should work to have the best nitro-gen and phosphorus management possible —


for economic as well as environmental reasons.This is especially important near bodies of wa-ter that are susceptible to accelerated weed oralgae growth (eutrophication). However, don’tforget that nutrients from farms in the Midwestare contributing to problems in the Gulf ofMexico — over 1,000 miles away.

There are major differences between the waynitrogen and phosphorus behave in soils (seetable 17.1 and figure 17.1 below). Besides fertil-izer sources, nitrogen is readily available to plantsonly from decomposing organic matter, whileplants get their phosphorus from both organicmatter and soil minerals. Nitrate, the primaryform in which plants use nitrogen, is very mo-bile in soils; phosphorus movement in soil is verylimited.

Most unintentional nitrogen loss from soilsoccurs when nitrate leaches or is converted intogases during denitrification, or when surfaceammonium is volatilized. Large amounts of ni-trate may leach from sandy soils, while denitri-fication is generally more important in heavy

loams and clays. On the other hand, almost allunintended phosphorus loss from soils is car-ried away in sediments eroded from fields (seefigure 17.1 for a comparison between relativepathways for nitrogen and phosphorus losses).Except when coming from highly manuredfields, phosphorus losses from healthy grass-lands are usually quite low — mainly as dis-solved phosphorus in the runoff waters — be-cause both runoff water and sediment loss arevery low. Biological nitrogen fixation carried onin the roots of legumes and by some free-livingbacteria actually adds new nitrogen to soil, butthere is no equivalent reaction for phosphorusor any other nutrient.

Improving nitrogen and phosphorus manage-ment can help reduce reliance on commercialfertilizers. A more balanced system with goodrotations and more active organic matter shouldprovide a large proportion of crop nitrogen andphosphorus needs. Better soil structure and at-tention to use of appropriate cover crops canlessen loss of nitrogen and phosphorus by re-

TABLE 17.1Comparing Soil Nitrogen and Phosphorus

NITROGEN PHOSPHORUS

Nitrogen becomes available from vs. Phosphorus becomes available from decom-decomposing soil organic matter. posing soil organic matter and from mineral

forms.

Mostly available to plants as nitrate (NO3-) vs. Available mainly as dissolved phosphate in

— a form that is very mobile in soils. soil water — but little present in solution evenin fertile soils and is not mobile.

Nitrate can be easily lost by leaching to vs. Phosphorus is mainly lost from soils bygroundwater or conversion to gases (N2, N2O). runoff and erosion.

Can add nitrogen to soils by biological vs. No equivalent reaction can add new phos-nitrogen fixation (legumes). phorus to soil, although many bacteria and

some fungi help make phosphorus moreavailable.

MANAGEMENT OF NITROGEN AND PHOSPHORUS 159

ducing leaching, denitrification, and/or runoff.Reducing the loss of these nutrients is an eco-nomic benefit to the farm and, at the same time,an environmental benefit to society. The greaternitrogen availability may be thought of as a“fringe benefit” of a farm with an ecologicallybased cropping system. In addition, the manu-

Figure 17.1 Different pathways for nitrogen andphosphorus losses from soils (relative amountsindicated by width of arrows). Based on anunpublished diagram by D. Beegle.

volatilizationdenitrification

Norganic

NO3-, NH4

+

crop uptake

runofferosion

leaching

Nitrogenvs.

Phosphorus

Porganic

and mineral

crop uptake

leaching

runofferosion

facture, transportation, and application of ni-trogen fertilizers is very energy intensive. Of allthe energy used to produce corn (including themanufacture and operation of field equipment),the manufacture and application of nitrogen fer-tilizer represents close to 40 percent. So relyingmore on biological fixation of nitrogen reducesdepletion of a non-renewable resource. Althoughenergy has been relatively inexpensive for manyyears, it may very well become more expensivein the future. Although phosphorus fertilizersare less energy consuming to produce, a reduc-tion in their use helps preserve this non-renew-able resource.

MANAGEMENT OF NITROGENAND PHOSPHORUS

Nitrogen and phosphorus behave very differentlyin soils, but many of the management strategiesare actually the same or are very similar. Man-agement strategies include the following:

a) Take all nutrient sources into account.� Use soil tests to assess available nutrients.� Use manure tests to determine nutrient

contributions.� Consider nutrients in decomposing crop

residues (for N only).

b) Reduce losses / enhance uptake.� Use nutrient sources more efficiently.� Use localized placement of fertilizers

whenever possible.� Split fertilizer application if leaching or

denitrification losses are a problem (for Nonly).

� Apply nutrients when leaching or runoffthreats are minimal.


� Reduce tillage.� Use cover crops.� Include perennial forage crops in rotation.

c)Balance farm imports and exports once cropneeds are being met.

Taking All Nutrient Sources IntoAccount

Soil testing for nitrogen and phosphorus andinterpreting soil test results are discussed inchapter 19.

Credit nutrients in manures, decomposingsods, and other organic residues. Before apply-ing commercial fertilizers or other off-farm nu-trient sources, you should properly credit thevarious on-farm sources of nutrients. In somecases, there is more than enough fertility in theon-farm sources to satisfy crop needs. If manureis applied before sampling soil, the contribu-tion of much of the manure’s phosphorus andall its potassium should be reflected in the soiltest. One nitrogen soil test, the Pre-SidedressNitrate Test (PSNT), reflects the nitrogen con-tribution of the manure (see chapter 19 for adescription of nitrogen soil tests). The only wayto really know the nutrient value of a particularmanure is to have it tested before applying it tothe soil. Many soil test labs will also analyze ma-nures for their fertilizer value. (Without testingthe manure or the soil following application,estimates can be made based on average ma-nure values, such as those given in table 9.1,p.79.) Because significant nitrogen losses can oc-cur in as little as one or two days after manureapplication, the way to derive the full nitrogenbenefit from manure is to incorporate it as soonas possible. Much of the manure-nitrogen made

available to the crop is in the ammonium form,and losses occur as ammonium is volatilizedwhen manures dry on the soil surface. A sig-nificant amount of the manure’s nitrogen alsomay be lost when application is a long time be-fore crop uptake occurs. About half of the ni-trogen value of a fall manure application — evenif incorporated — may be lost by the time ofgreatest crop need the following year.

Legumes, as either part of rotations or ascover crops, and well-managed grass sod cropscan add nitrogen to the soil for use by the fol-lowing crops (table 17.2). Nitrogen fertilizerdecisions should take into account the amountof nitrogen contributed by manures, decompos-ing sods, and cover crops. If you correctly filledout the form that accompanies your soil sample,the recommendation you receive may take thesesources into account. However, soil testing labsmay not take these into account when making

TABLE 17.2Examples of Nitrogen Credits for

Previous Crops

N CREDITS

PREVIOUS CROP (LBS./ACRE)

corn and most other crops 0

soybeans* 0 to 40*

grass (low level of management) 40

grass (intensively managed) 70

2-yr. stand red or white clover 70

3-yr. alfalfa stand 70(20–60% legume)

3-yr. alfalfa stand 120(>60% legume)

hairy vetch cover crop 110excellent growth

*Some labs give 30 or 40 lbs. N credit for soybeans,while others give no N credit.


their recommendations; most do not even askwhether you’ve used a cover crop. If you can’tfind help deciding how to credit nutrients inorganic sources, take a look at chapters 9 (ani-mal manures), 10 (rotations), and 11 (covercrops). For an example of crediting the nutrientvalue of manure and cover crop, see the section“Making Adjustments to Fertilizer ApplicationRates” in chapter 19 (p. 198).

Rely on legumes to supply nitrogen to fol-lowing crops. Nitrogen is the only nutrient forwhich you can “grow” your own supply. High-yielding legume cover crops, such as hairy vetchand crimson clover, can supply most, if not allthe nitrogen needed by the following crop. Grow-

ing a legume as a forage crop in rotation (alfalfa,alfalfa/grass, clover, clover/grass) also can providemuch, if not all of the nitrogen for row crops.The nitrogen-related aspects of both cover cropsand rotations with forages were discussed in pre-vious chapters (chapters 10 and 11).

Animals on the farm or on nearby farms? Ifyou have ruminant animals on your farm or onnearby farms for which you can grow foragecrops, (and perhaps use the manure on yourfarm), there are many possibilities for actuallyeliminating the need to use nitrogen fertilizers. Aforage legume, such as alfalfa, red clover, or whiteclover or a grass/legume mix, can supply sub-stantial nitrogen for the following crop. Fre-quently, nutrients are imported onto animal farms

TABLE 17.3Comparison of Nitrogen and Phosphorus Management Practices

NITROGEN PHOSPHORUS

Use the PSNT or other reliable nitrogen soil test Soil test regularly (and follow recommendations).(and follow recommendations).

Test manures and credit their nitrogen Test manures and credit their phosphoruscontribution. contribution.

Use legume forage crops in rotation and/or legume No equivalent practice available.cover crops to fix nitrogen for following cropsand properly credit legume nitrogen contributionto following crops.

Time N applications as close to crop uptake as Time P application to reduce runoff potential.possible.

Reduce tillage in order to leave residues on the Reduce tillage in order to leave residues on thesurface and decrease runoff and erosion. surface and decrease runoff and erosion.

Sod-type forage crops in rotation reduce nitrate Sod-type forage crops in rotation reduce theleaching and runoff. amount of runoff and erosion losses of phosphorus.

Grass cover crops, such as winter rye, capture soil Grass cover crops, such as winter rye, protectnitrates left over following the economic crop. soil against erosion.

Make sure that excessive nitrogen is not coming After soil tests are in optimal range, balance farmonto the farm (biological nitrogen fixation phosphorus flow (don’t import much more onto + fertilizers + feeds). farm than is being exported).


as various feeds (usually grains and soybeanmeal mixes). This means that the manure fromthe animals will contain nutrients imported fromoutside the farm and this reduces the need topurchase fertilizers.

No animals? Although land constraints don’tusually allow it, some vegetable farmers grow aforage legume for one or more years as part of arotation, even when they are not planning tosell the crop or feed it to animals. They do so torest the soil and to enhance the soil’s physicalproperties and nutrient status. Also, some covercrops, such as hairy vetch — growing off-sea-son in the fall and early spring — can providesufficient nitrogen for some of the high-demand-ing summer annuals. It’s also possible toundersow sweetclover and then plow it underthe next July to prepare for fall brassica crops.

Reduce Nitrogen andPhosphorus Losses

Use nitrogen and phosphorus fertilizersmore efficiently. If you’ve worked to build andmaintain soil organic matter, you should haveplenty of active organic materials present. Thesereadily decomposable small fragments providenitrogen and phosphorus as they are decom-posed, reducing the amount of fertilizer that’sneeded.

The timing and method of application ofcommercial fertilizers and manures affect theefficiency of use by crops and the amount ofloss from soils — especially in humid climates.In general, it is best to apply fertilizers closeto the time when they are needed by plants.Losses of fertilizer and manure nutrients arealso frequently reduced by soil incorporationwith tillage.

If you’re growing a crop for which a reliablein-season nitrogen test is available (see discus-sion in chapter 19) then you can hold off ap-plying fertilizer until the nitrogen test indicatesa need. At that point, apply nitrogen as asidedress. Otherwise, you may need to broad-cast some nitrogen before planting to supplysufficient nutrition until the soil test indicates ifthere is need for more nitrogen (applied as asidedress). For row crops in colder climates,about 15 to 20 lbs. of starter N per acre (in aband at planting), is highly recommended.

Some of the nitrogen in surface-applied urea,the cheapest and most commonly used solidnitrogen fertilizer, is lost as a gas if it is not rap-idly incorporated into the soil. If as little as 1/4-inch rain falls within a few days of surface ureaapplication, nitrogen losses are usually less than10 percent. However, losses may be as large as30 percent or more in some cases (50 percentloss may occur following surface application toa calcareous soil that is over pH 8). When ureais used for no-till systems, it can be placed be-low the surface. When fertilizer is broadcast asa topdress on grass or row crops, you mightconsider the economics of using ammoniumnitrate. Although it is more costly than urea perunit of nitrogen, nitrogen in ammonium nitrateis generally not lost as a gas when left on thesurface. Anhydrous ammonia, the least expen-sive source of nitrogen fertilizer, causes largechanges in soil pH in and around the injectionband. The pH increases for a period of weeks,many organisms are killed, and organic matteris rendered more soluble. Eventually, the pHdecreases and the band is repopulated by soilorganisms. However, significant nitrogen lossescan occur when anhydrous is applied in a soilthat is too dry or too wet. Even if stabilizers are


used, anhydrous applied long before a cropneeds it significantly increases the amounts ofnitrogen that may be lost in humid regions.

If the soil is very deficient in phosphorus,phosphorus fertilizers are commonly incorpo-rated into the soil to raise the general level ofthe nutrient. Incorporation is not possible withno-till systems and, if the soil was initially verydeficient, some phosphorus fertilizer shouldhave been incorporated before starting no-till.Nutrients accumulate near the surface of re-duced tillage systems when fertilizers or manuresare repeatedly surface-applied.

In soils with optimal phosphorus levels, somephosphorus fertilizer is still recommended, alongwith nitrogen application, for row crops in coolregions. (Potassium is also commonly recom-mended under these conditions.) Frequently, thesoils are cold enough in the spring to slow downboth root development and mineralization ofphosphorus from organic matter, reducing phos-phorus availability to seedlings. This is probablywhy it is a good idea to use some starter phos-phorus in these regions — even if the soil is inthe optimal phosphorus soil test range.

Use perennial forages (sod-forming crops)in rotations. As we’ve discussed a number oftimes, rotations that include a perennial foragecrop help reduce the amount of runoff and ero-sion; build better soil tilth; break harmful weed,insect, and nematode cycles; and build soil or-ganic matter. Decreasing the emphasis on rowcrops in a rotation and including perennial for-ages also helps decrease leaching losses of ni-trate. This happens for two main reasons:

1) There is less water leaching under a sod be-cause it uses more water over the entire grow-

ing season than does an annual row crop(which has a bare soil in the spring and afterharvest in the fall).

2) Nitrate concentrations under sod rarely reachanywhere near those under row crops.

So, whether the rotation includes a grass, a le-gume, or a legume/grass mix, the amount ofnitrate leaching to groundwater is usually re-duced. (A critical step, however, is the conver-sion from sod to row crop. When a sod crop isplowed, a lot of nitrogen is mineralized. If thisoccurs many months before the row crop canuse nitrogen, high nitrate leaching and denitri-fication losses occur.) Using grass, legume, orgrass/legume forages in the rotation also helpswith phosphorus management because of thereduction of runoff and erosion and the effectson soil structure for the following crop.

Use cover crops to prevent nutrient losses.High levels of soil nitrate may be left at the endof the growing season if drought causes a poorcrop year or if excess nitrogen fertilizer or ma-nure has been applied. The potential for nitrateleaching and runoff can be reduced greatly ifyou sow a fast-growing cover crop like winterrye. One option available when using covercrops to help manage nitrogen is to use a com-bination of a legume and grass. The combina-tion of hairy vetch and winter rye works well inthe mid-Atlantic region. When nitrate is scarce,the vetch does much better than the rye and alarge amount of nitrogen is fixed for the nextcrop. On the other hand, the rye competes wellwith the vetch when nitrate is plentiful, and lessnitrogen is fixed (of course, less is needed) andmuch of the nitrate is tied up in the rye andstored for future use.


In general, having any cover crop on the soilduring the off season is helpful for phosphorusmanagement. A cover crop that establishesquickly and helps protect the soil against ero-sion will help reduce phosphorus losses.

Reduce tillage. Because most phosphorus islost from fields by erosion of sediments, envi-ronmentally sound phosphorus managementshould include reduced tillage systems. Leav-ing residues on the surface and maintainingstable soil aggregation and lots of large poreshelps water to infiltrate into soils. When runoffdoes occur, less sediment is carried along withit than if conventional plow-harrow tillage isused. Reduced tillage, by decreasing runoff anderosion, usually decreases both phosphorus andnitrogen losses from fields.

Working Towards BalancingNutrient Imports and Exports

Nitrogen and phosphorus are lost from soils, inmany ways, including runoff that takes both ni-trogen and phosphorus, leaching of nitrate (andsometimes significant amounts of phosphorus),denitrification, and volatilization of ammoniafrom surface applied urea and manures. Even ifyou take all precautions to reduce unnecessarylosses, some loss of nitrogen and phosphoruswill occur anyway. While you can easily overdoit with fertilizers, use of more nitrogen and phos-phorus than needed also occurs on many live-stock farms that import a significant proportionof their feeds. If a forage legume, such as alfalfa,is an important part of the rotation, the combi-nation of biological nitrogen fixation plus im-ported nitrogen in feeds may exceed the farm’sneeds. A reasonable goal for farms with a largenet inflow of nitrogen and phosphorus would

be to try to reduce “imports” of these nutrientson farms (including legume nitrogen), or in-crease exports, to a point closer to balance.

On crop farms, as well as animal farms withlow numbers of animals per acre, it’s fairly easyto bring inflows and outflows into balance byproperly crediting nitrogen from the previouscrop and nitrogen and phosphorus in manure.On the other hand, it is a more challenging prob-lem when there are a large number of animalsfor a given land base and a large percentage ofthe feed must be imported. This happens fre-quently on “factory”-type animal productionfacilities, but can also happen on family-sizedfarms. At some point, thought needs to be given

Reducing tillage usually leads to marked re-ductions in nitrogen and phosphorus loss inrunoff and nitrate leaching loss to groundwa-ter. However, there are two complicating fac-tors that should be recognized:

� If intense storms occur soon afterapplication of surface-applied urea orammonium nitrate, nitrogen is more likelyto be lost via leaching than if it had beenincorporated. Much of the water will flowover the surface of no-till soils, picking upnitrate and urea, before entering worm-holes and other channels. It then easilymoves deep into the subsoil.

� Phosphorus accumulates on the surface ofno-till soils (because there is no incorpo-ration of broadcast fertilizers, manures,crop residues, or cover crops). Althoughthere is less runoff and fewer sedimentsand less total phosphorus lost with no-till,the concentration of dissolved phospho-rus in the runoff may actually be higherthan for conventionally tilled soils.

165

High-phosphorus soils occur because of a his-tory of either excessive applications of phospho-rus fertilizers or — more commonly — applica-tion of lots of manure. This is a problem on live-stock farms with limited land and where a me-dium-to-high percentage of feed is imported.The nutrients imported in feeds may greatly ex-ceed nutrients exported in animal products. Inaddition, where manures or composts are usedat rates required to provide sufficient nitrogento crops, more phosphorus than needed usu-ally is added. It’s probably a good idea to re-duce the potential for phosphorus loss from allhigh-phosphorus soils. However, it is especiallyimportant to reduce the risk of environmentalharm from those high-phosphorus soils that arealso likely to produce significant runoff (becauseof steep slope, fine texture, poor structure, orpoor drainage).

There are a number of practices that shouldbe followed with high-phosphorus soils:

� First, deal with the “front end” and reduceanimal phosphorus intake to the lowestlevels needed. A recent survey found thatthe average dairy herd in the U.S. is fedabout 25 percent more phosphorus thanrecommended by the standard authority(the National Research Council, NRC). Inaddition, research indicates that the NRCrecommendations may be 10 to 15 percenthigher than actually needed. This is costingdairy farmers about $3,500 to feed a 100-cow herd supplemental phosphorus that theanimals don’t need and that only ends up asa potential pollutant!

� Second, reduce or eliminate applying extraphosphorus. For a livestock farm, this may

mean obtaining the use of more land togrow crops and to spread manure over alarger land area. For a crop farm, this maymean using legume cover crops and foragesin rotations to supply nitrogen withoutadding phosphorus. The cover crops andforage rotation crops are also helpful tobuild up and maintain good organic matterlevels in the absence of importing manuresor composts or other organic material fromoff the farm. The lack of imported organicsources of nutrients (to try to reducephosphorus imports) means that a cropfarmer will need more creative use of cropresidues, rotations, and cover crops tomaintain good organic matter levels.

� Third, reduce runoff and erosion to minimallevels. Phosphorus usually is only a problemif it gets into surface waters. Anything thathelps water infiltration or impedes waterand sediments from leaving the fielddecreases problems caused by high-phosphorus soils — reduced tillage, stripcropping along the contour, cover crops,grassed waterways, riparian buffer strips,etc. [Note: Significant phosphorus losses intile drainage water have been observed,especially from fields where large amountsof liquid manure are applied.]

� Fourth, continue to monitor soil phosphoruslevels. Soil test phosphorus will slowlydecrease over the years, once phosphorusimports as fertilizers, organic amendments,or feeds are reduced or eliminated. Testingsoils every two or three years should bedone for other reasons anyway. So, justremember to keep track of soil test phos-phorus to confirm that levels are decreasing.

Managing High Phosphorus Soils


to either expanding the farm’s land base or ex-porting some of the manure to other farms.Another option is to compost the manure —which makes it easier to transport or sell andcauses some nitrogen losses during the com-posting process — stabilizing the remaining ni-trogen before application. On the other hand,the availability of phosphorus in manure is notgreatly affected by composting. That’s why us-ing compost to supply a particular amount of“available” nitrogen usually results in applica-tions of larger total amounts of phosphorus thanplants need.

Phosphorus and Potassium Can Get tooHigh When Using Organic Sources

Manures and other organic amendments are fre-quently applied to soils at rates estimated to sat-isfy nitrogen needs of crops. This commonly addsmore phosphorus and potassium than the cropneeds. After many years of continuous applica-tion of these sources to meet nitrogen needs, soiltest levels for phosphorus and potassium may bein the very high (excessive) range. Although thereare a number of ways to deal with this issue, allsolutions require reduced applications of fertil-izer phosphorus and phosphorus-containing or-ganic amendments. If it’s a farm-wide problem,some manure may need to be exported and ni-trogen fertilizer or legumes relied on to providenitrogen to grain crops. Sometimes, it’s just a ques-tion of better distribution of manure around the

various fields — getting to those fields far fromthe barn more regularly. Changing the rotationto include crops, such as alfalfa, for which nomanure nitrogen is needed can help. However, ifyou’re raising livestock on a limited land base,you should make arrangements to remove someof the manure from the farm.

SOURCES

Brady, N.C., and R.R. Weil. 1999. The Natureand Properties of Soils. 12th ed. MacmillanPubl. Co. New York, NY.

Jokela, B., F. Magdoff, R. Bartlett, S. Bosworth,and D. Ross. 1998. Nutrient Recommendationsfor Field Crops in Vermont. UVM Extension,University of Vermont. Burlington, VT.

Magdoff, F.R. 1991. Understanding the Magdoffpre-sidedress nitrate soil test for corn. Jour-nal of Production Agriculture 4:297–305.

National Research Council. 1988. Nutrient re-quirements of dairy cattle, 6th rev. ed., Na-tional Academy Press, Washington, D.C.

Sharpley, A.N. 1996. Myths about phosphorus.NRAES-96. Proceedings from the Animal Agri-culture and the Environment North AmericanConference, Dec. 11–13, Rochester, NY. North-east Region Agricultural Engineering Service.Ithaca, NY.

Vigil, M.F., and D.E. Kissel. 1991. Equations forestimating the amount of nitrogen mineral-ized from crop residues. Soil Science Societyof America Journal 55:757–761.

167

OTHER NUTRIENTS

Although farmers understandably emphasize ni-trogen and phosphorus — because of the largequantities used and the potential forenvironmental problems — additionalnutrient and soil chemical issues re-main important. In most cases, theoveruse of these other fertilizers andamendments doesn’t cause problemsfor the environment, but inappropri-ate use may waste money and reduce yields.There are also animal health considerations. Forexample, excess potassium in feeds for dry cows(between lactations) results in metabolic prob-lems, and low magnesium availability to dairyor beef cows in early lactation can cause grasstetany. As with most other issues we have dis-cussed, focusing on the management practices

18�

Other Fertility Issues:Nutrients, CEC, Acidity and Alkalinity

The potential available nutrients in a soil,

whether natural or added in manures or fertilizer,

are only in part utilized by plants…

—T.L. LYON AND E.O. FIPPIN, 1909

that build up and maintain soil organic matterwill help eliminate many problems or, at least,make them easier to manage.

Potassium is one of the N-P-K “big three”primary nutrients needed in largeamounts and frequently not present insufficient quantities for plants. Potas-sium availability to plants is sometimesdecreased when liming a soil to in-crease the pH by one or two units. Theextra calcium, as well as the “pull” on

potassium exerted by the new cation exchangesites (see discussion of CEC), contribute to lowerpotassium availability. Problems with low po-tassium levels are usually dealt with easily byapplying muriate of potash (potassium chloride),potassium sulfate, or Sul-Po-Mag or K-Mag (po-tassium and magnesium sulfate). Manures alsousually contain large quantities of potassium.


Magnesium deficiency is easily corrected ifthe soil is acidic by using a high-magnesium(dolomitic) limestone to raise soil pH (see dis-cussion of soil acidity below). Otherwise, Sul-Po-Mag is one of the best choices for correctingsuch a deficiency.

Calcium deficiencies are usually associatedwith low pH soils and soils with low CECs. Thebest remedy is usually to lime and build up thesoil’s organic matter. However, some importantcrops, such as peanuts, potatoes and apples,commonly need added calcium. Calcium addi-tions also may be needed to help alleviate soilstructure and nutrition problems of sodic soils(see below). In general, if the soil does not havetoo much sodium, is properly limed and has areasonable amount of organic matter, there willbe no advantage to adding a calcium source,such as gypsum. However, soils with very lowaggregate stability may sometimes benefit fromthe extra salt concentration associated with sur-face gypsum applications. This is not a calciumnutrition effect, but a stabilizing effect of the dis-solving gypsum salt. Higher soil organic matterand surface residues should do as well as gyp-sum to alleviate this problem.

Sulfur deficiencies are common on soils withlow organic matter. Some soil testing labs aroundthe country offer a sulfur soil test. (Those of youwho grow garlic should know that a good sup-ply of sulfur is important for the full develop-ment of garlic’s pungent flavor — so garlic grow-ers want to make sure there’s plenty available tothe crop.) Much of the sulfur in soils occurs asorganic matter, so building up and maintaininggood amounts of organic matter should resultin sufficient sulfur nutrition for plants. Althoughreports of crop response to added sulfur in theNortheast are rare, it is thought that deficien-cies of this element may become more common

now that there is less sulfur air pollution origi-nating in the Midwest. Some fertilizers used forother purposes, such as Sul-Po-Mag and am-monium sulfate, contain sulfur. Calcium sulfate(gypsum) also can be applied to remedy low soilsulfur. The amounts used on sulfur-deficientsoils are typically 20 to 25 lbs. sulfur/acre.

Zinc deficiencies occur with certain crops onsoils low in organic matter and in sandy soils orthose with a pH near neutral. Zinc problems aresometimes noted on silage corn when manurehasn’t been applied for a while. It also can bedeficient following topsoil removal from partsof fields as land is leveled for furrow irrigation.Sometimes, crops outgrow the problem as thesoil warms up and organic sources become moreavailable to plants. Zinc sulfate (about 35 per-cent zinc) applied to soils is one of the materi-als used to correct zinc deficiencies. If the defi-ciency is due to high pH, or if an orchard cropis zinc-deficient, a foliar application is com-monly used. If a soil test before planting an or-chard reveals low zinc levels, zinc sulfate shouldbe soil-applied.

Boron deficiencies show up in alfalfa whengrowing on eroded knolls where topsoil andorganic matter have been lost. Root crops seemto need higher soil boron levels than many othercrops. Cole crops, apples, celery and spinachare also sensitive to low boron levels. The mostcommon fertilizer used to correct a boron defi-ciency is sodium tetraborate (about 15 percentboron). Borax (about 11 percent boron), a com-pound containing sodium borate, also can beused to correct boron deficiencies. On sandysoils low in organic matter, boron may be neededon a routine basis.

Manganese deficiency, usually associated withsoybeans and cereals on high pH soils and veg-etables grown on muck soils, is corrected with

OTHER FERTILITY ISSUES: NUTRIENTS, CEC, ACIDITY AND ALKALINITY 169

the use of manganese sulfate (about 27 percentmanganese). About 10 lbs. of water-soluble man-ganese per acre should satisfy plant needs for anumber of years. Up to 25 lbs. per acre of man-ganese is recommended, if the fertilizer is broad-cast on a very deficient soil. Natural, as well assynthetic, chelates (at about 5 to 10 percent man-ganese) usually are applied as a foliar spray.

Iron deficiency occurs when blueberries aregrown on moderate to high pH soils. Iron defi-ciency also sometimes occurs on soybeans, wheat,sorghum, and peanuts growing on high pH soils.Iron (ferrous) sulfate or chelated iron are usedto correct iron deficiency. Both manganese andiron deficiencies are frequently corrected byusing foliar application of inorganic salts.

CATION EXCHANGE CAPACITYMANAGEMENT

The CEC in soils is due to well humified (“verydead”) organic matter and clay minerals. Thetotal CEC in a soil is the sum of the CEC due toorganic matter and CEC due to the clays. In fine-

textured soils with medium to high CEC-typeclays, much of the CEC may be due to clays.On the other hand, in sandy loams with littleclay, or in some of the soils of the southeasternU.S. containing clays with low CEC, organicmatter may account for the overwhelming frac-tion of the total CEC.

There are two practical ways to increase theability of soils to hold nutrient cations, such as potas-sium, calcium, magnesium, and ammonium:

� Add organic matter by many of the methodsdiscussed earlier in Part Two.

� If the soil is too acidic, use lime (see below)to raise its pH to the high end of the rangeneeded for the crops you grow.

One of the benefits of liming acid soils is toincrease soil CEC! Here’s why: As the pH in-creases, so does the CEC of organic matter. Ashydrogen (H+) on humus is neutralized by lim-ing, the site where it was attached now has anegative charge and can hold onto Ca++, Mg++,K+, etc.

Estimating Organic Matter’s Contribution to a Soil’s CEC

Example 1: pH = 5.0 and 3% SOM � (5.0 — 4.5) x 3 = 1.5 me/100g




The CEC of a soil is usually expressed in termsof the number of milliequivalents (me) of nega-tive charge per 100 grams of soil. (The actualnumber of charges represented by one me isabout 6 followed by 20 zeros.) A useful rule of

thumb for estimating the CEC due to organicmatter is as follows: for every pH unit above pH4.5, there is 1 me of CEC in 100 gm of soil forevery percent organic matter. (Don’t forget thatthere will also be CEC due to clays.)


Many soil testing labs also will run CEC, ifasked. However, there are a number of possibleways to do the test. Some labs determine whatthe CEC would be if the soil’s pH was 7 orhigher. They do this by adding the acidity thatwould be neutralized if the soil was limed tothe current soil CEC. This is the CEC the soilwould have at the higher pH, but is not the soil’scurrent CEC. For this reason, some labs totalthe major cations actually held on the CEC (Ca++

+ K+ + Mg++) and call it effective CEC. It is moreuseful to know the effective CEC — the actualcurrent CEC of the soil — than CEC determinedat a higher pH.

SOIL ACIDITY

Background

Plants have evolved under specific environ-ments, which in turn influence their needs asagricultural crops. For example, alfalfa origi-nated in a semiarid region where soil pH washigh; alfalfa requires a pH in the range of 6.5 to6.8 or higher (see figure 18.1 for common soilpH levels). On the other hand, blueberries,which evolved under acidic conditions, requirea low pH to provide needed iron (iron is moresoluble at low pH). Other crops, such as pea-nuts, watermelons, and sweet potatoes, do best

in moderately acid soils in the range of pH 5 to6. Most other agricultural plants do best in therange of pH 6 to 7.5.

Several problems may cause poor growth ofacid-sensitive plants in low pH soils. The fol-lowing are three common ones:

� aluminum and manganese are more solubleand can be toxic to plants;

� lack of calcium, magnesium, potassium,phosphorus or molybdenum (especiallyneeded for nitrogen fixation by legumes);and

� a slowed decomposition of soil organicmatter and decreased mineralization ofnitrogen.

The problems caused by soil acidity are usu-ally less severe, and the optimum pH is lower, ifthe soil is well supplied with organic matter.Organic matter helps to make aluminum lesstoxic and, of course, humus increases the soil’sCEC. Soil pH will not change as rapidly in soilsthat are high in organic matter. Soil acidifica-tion is a natural process that is accelerated byacids produced in soil by most nitrogen fertiliz-ers. Soil organic matter slows down acidifica-tion and buffers the soil’s pH because it holdsthe acid hydrogen tightly. Therefore, more acidis needed to decrease the pH by a given amount

Figure 18.1 Soil pH and acid/base status.

pH

acidic neutral basic

Note: Soils at pH 7.5 to 8 frequently contain fine particles of lime (calcium carbonate).Soils above pH 8.5 to 9 usually have excess sodium (sodic, also called alkali soils).

4 5 6 7 8 9 10


when a lot of organic matter is present. Ofcourse, the reverse is also true — more lime isneeded to raise the pH of high-organic mattersoils by a given amount (see “Soil Acidity” boxbelow).

Soil Acidity

BACKGROUND

� pH 7 is neutral.� Soil with pH levels above 7 are alkaline,

those less than 7 are acidic.� The lower the pH, the more acidic is the

soil.� Soils in humid regions tend to be acidic,

those in semi-arid and arid regions tendto be around neutral or are alkaline.

� Acidification is a natural process.� Most commercial nitrogen fertilizers are

acid forming, but many manures are not.� Crops have different pH needs — pro-

bably related to nutrient availability or sus-ceptibility to aluminum toxicity at low pH.

� Organic acids on humus and aluminumon the CEC account for most of the acidin soils.

MANAGEMENT

� Use limestone to raise soil pH (ifmagnesium is also low, use a highmagnesium — or dolomitic — lime).

� Mix lime thoroughly into the plow layer.� Spread lime well in advance of sensitive

crops, if at all possible.� If the lime requirement is high — some

labs say greater than 2 tons and otherssay greater than 4 tons — considersplitting the application over two years.

� Reducing soil pH (making soil more acid)for acid-loving crops is done best withelemental sulfur (S).

Limestone application helps create a morehospitable soil for acid-sensitive plants in manyways, such as:

� neutralizing acids;� adding calcium in large quantities (because

limestone is calcium carbonate, CaCO3);� adding magnesium in large quantities if

dolomitic limestone is used (containingcarbonates of both calcium and magne-sium);

� making molybdenum and phosphorus moreavailable;

� helping to maintain added phosphorus inan available form;

� enhancing bacterial activity; and� making aluminum and manganese less

soluble.

Almost all the acid in acidic soils is held inreserve on the solids, with an extremely smallamount active in the soil water. If all that weneeded to neutralize was the acid in the soil wa-ter, a few handfuls of lime per acre would beenough to do the job, even in a very acid soil.However, tons of lime per acre are needed toraise the pH. The explanation for this is thatalmost all of the acid that must be neutralizedin soils is reserve acidity associated with eitherorganic matter or aluminum.

pH Management

Increasing the pH of acidic soils is usually ac-complished by adding ground or crushed lime-stone. Three pieces of information are used todetermine the amount of lime that’s needed:

� What is the soil pH? Knowing this and theneeds of the crops you are growing tell


whether lime is needed and what target pHyou are shooting for. If the soil pH is muchlower than the pH needs of the crop, youneed to use lime. But the pH value doesn’ttell you how much lime is needed.

� What is the lime requirement needed tochange the pH to the desired level? Thereare a number of different tests used by soiltesting laboratories that estimate soil limerequirements. Most give the results in termsof tons/acre of agricultural grade limestoneto reach the desired pH.

� Is the limestone you use very different fromthe one assumed in the soil test report? Thefineness and the amount of carbonate presentgovern the effectiveness of limestone — howmuch it will raise the soil’s pH. If the limeyou will be using has an effective calciumcarbonate equivalent that’s very differentfrom the one used as the base in the report,the amount applied may need to be ad-justed upward (if the lime is very coarse orhas a high level of impurities) or downward(if the lime is very fine and is high inmagnesium and contains few impurities).

Soils with more clay and more organic mat-ter need more lime to change their pH (see fig-ure 18.2). Although organic matter buffers thesoil against pH decreases, it also buffers againstpH increases when you are trying to raise thepH with limestone. Most states recommend asoil pH around 6.8 only for the most sensitivecrops, such as alfalfa, and about pH 6.2 to 6.5for many of the clovers. As pointed out above,most of the commonly grown crops do well inthe range of pH 6.0 to 7.5.

There are other liming materials in additionto limestone. One of the commonly used onesin some parts of the U.S. is wood ash. Ash froma modern air-tight wood burning stove may havea fairly high calcium carbonate content (80 per-cent or higher). However, ash that is mainlyblack — indicating incompletely burned wood— may have as little as 40 percent effective cal-cium carbonate equivalent. Lime-sludge fromwastewater treatment plants and fly ash sourcesmay be available in some locations. Normally,minor sources like the ones mentioned above arenot locally available in sufficient quantities to putmuch of a dent in the lime needs of a region.Because they might carry unwanted contaminantsto the farm, be sure that liming materials are thor-oughly evaluated for metals and field-tested be-fore you use any new byproduct liming sources.

Testing labs usually use the information youprovide about your cropping intentions andintegrate the three issues (see left column)when recommending limestone applicationrates. There are laws governing the quality oflimestone sold in each state. Soil testing labsgive recommendations assuming the use ofground limestone that meets the minimumstate standard.

“Overliming” Injury

Sometimes there are problems when soils arelimed, especially if a very acidic soil has beenquickly raised to high pH levels. Decreasedcrop growth because of “overliming” injury isusually associated with lowered availability ofphosphorus, potassium, or boron, althoughzinc and copper deficiencies can be pro-duced by liming acidic sandy soils. If therehas been a long history of use of triazine her-bicides, such as atrazine, liming may releasethese chemicals and kill sensitive crops.


Figure 18.2 Examples of approximate lime needs to reach pH 6.8. Modified from Peech, 1961.

Ton

s o

f lim

est

on

e p

er

acre

to

re

ach

pH

6.8 10

8

6

4

2

04.5 5.0 5.5 6.0 6.5 7.0

a

b

c

d

Soil pH before liming

ORGANICMATTER

(%)

a) silty clay loams 5.0b) loams and silt l. 3.0c) sandy loams 2.0d) sands 1.0

ARID REGION PROBLEMS: SODIC(ALKALI) AND SALINE SOILS

Special soil problems are found in arid and semi-arid regions, including soils that are high in salts,called saline soils, and those that have excessivesodium (Na+), called sodic soils. Sometimesthese go together and the result is a saline-sodicsoil. Saline soils usually have good soil tilth, butplants can’t get the water they need because thehigh salt levels inhibit water-uptake. Sodic soilstend to have very poor physical structure be-cause the high sodium levels cause clays to dis-perse, leading to the breaking apart of aggre-gates. As aggregates break down, these soils be-come difficult to work with and very unpleas-ant for plants. Wet sodic soils with an adequateamount of clay end up looking and behavinglike chocolate pudding.

Need to Lower the Soil’s pH?

When growing plants that require low pH,you may want to add acidity to the soil. Thisis probably only economically possible forblueberries and is most easily done with el-emental sulfur (S), which is converted intoan acid by soil microorganisms over a fewmonths. For the examples in figure 18.2, theamounts of S needed to drop the pH by oneunit would be approximately 3/4 ton per acrefor the silty clay loams, 1/2 ton per acre forthe loams and silt loams, 600 lbs. per acrefor the sandy loams, and 300 lbs. per acrefor the sands. Sulfur should be applied theyear before planting blueberries.

Alum (aluminum sulfate) may also be usedto acidify soils. About six times more alumthan elemental sulfur is needed to achievethe same pH change.


Saline and sodic soils are commonly foundin the semi-arid and arid regions in the westernU.S., with pockets of saline soils found near thecoastline. Sometimes, the extra moisture accu-mulated during a fallow year in semi-arid re-gions causes field-seeps, which lead to the de-velopment of saline and sodic patches.

Before embarking on a program to improvesaline or sodic soil, it is important to find outwhat is causing the problem. Do you have a highwater table that contains salty water? Is there asaline or alkali “seep” in a portion of the field?Is it a generalized problem over the entire fieldwithout a high water table? If a high water tableis causing salts to migrate upward to the rootzone, installation of tile drainage may be neces-sary. Water surfacing in saline and alkali seepsusually can be reduced by changing from an al-ternate year fallow to a more intense annualcropping regime.

There are a number of ways to deal with sa-line soils that don’t have shallow salty ground-water. One is to keep the soil continually moist.For example, by using drip irrigation with lowsalt water plus a surface mulch, the salt contentwill not get as high as it would if allowed to con-centrate when the soil dries. Another is to growcrops or varieties of crops that are more tolerantof soil salinity. Saline-tolerant plants include bar-

Saline soil. Electrical conductivity of a soil ex-tract is greater than 4 ds/m, enough to harmsensitive crops.

Sodic soil. Sodium occupies more than 15percent of the CEC. (Soil structure can signifi-cantly deteriorate in some soils at even lowerlevels of sodium.)

Salts are Present in All Soils

Salts of calcium, magnesium, potassium andother cations — along with the commonnegatively charged anions chloride, nitrate,sulfate, and phosphate — are found in allsoils. However, in soils in subhumid and hu-mid climates — with from an inch or two towell over 7 inches of water percolating be-neath the root zone every year — salts don’tusually accumulate to the levels where theycan be harmful to plants. Even when highrates of fertilizers are used, salts usually onlybecome a problem when you place largeamounts in direct contact with seeds orgrowing plants. Salt problems frequently oc-cur in greenhouse potting mixes becausegrowers regularly irrigate their greenhouseplants with water containing fertilizers andmay not add enough water to leach the ac-cumulating salts out of the pot.

ley, bermuda grass, oak, rosemary, and willow.However, the only way to get rid of the salt is toadd sufficient water to wash it below the rootzone. The amount of water needed to do this isrelated to the salt content of the irrigation wa-ter, expressed as electrical conductivity (ECw)and the salt content desired in the drainage water(ECdw). The amount of water needed can becalculated using the following equation:

Water needed = (amount of water neededto saturate soil) x (ECw/ECdw)

The amount of extra irrigation water neededto leach salts is also related to the sensitivity ofthe plants that you’re growing. For example,sensitive crops like onions and strawberries mayhave twice the leaching requirement as moder-


ately sensitive broccoli or tomatoes. Drip irriga-tion uses relatively low amounts of water, so lackof leaching may cause salt build-up even formoderately saline irrigation sources. This meansthat the leaching may need to occur during thegrowing season, but care is needed to preventleaching of nitrate below the root zone.

For sodic soils, a calcium source is added —usually gypsum (calcium sulfate). The calciumreplaces sodium held by the cation-exchangecapacity. The soil is then irrigated so that thesodium can be leached deep in the soil. Becausethe calcium in gypsum easily replaces the so-dium on the CEC, the amount of gypsumneeded can be estimated as follows — for everymilliequilivent of sodium that needs to be re-placed to 1 foot, about 2 tons of agriculturalgrade gypsum is needed per acre. Adding gyp-sum to non-sodic soils doesn’t help physicalproperties if the soil is properly limed, exceptfor those soils containing easily dispersible claythat are also low in organic matter.

SOURCES

Hanson, B.R., S.R. Grattan, and A. Fulton. 1993.Agricultural Salinity and Drainage. Publication3375, Div. of Agriculture and Natural Re-sources, University of California, Oakland,CA.

Magdoff, F.R. and R.J. Bartlett. 1985. Soil pHbuffering revisited. Soil Science Society ofAmerica Journal 49:145–148.

Peech, M. 1961. Lime Requirement vs. Soil pHCurves for Soils of New York State. Mimeo-graphed. Cornell University Agronomy De-partment, Ithaca, NY.

Pettygrove, G.S., S.R. Grattan, T.K. Hartz, L.E.Jackson, T.R. Lockhart, K.F. Schulbach, andR. Smith. 1998. Production Guide: Nitrogen andWater Management for Coastal Cool-SeasonVegetables. Publication 21581, Division of Ag-riculture and Natural Resources, Universityof California, Oakland, CA.

Rehm, G. 1994. Soil Cation Ratios for Crop Pro-duction. North Central Regional ExtensionPublication 533. University of Minnesota Ex-tension Service, St. Paul, MN.

Tisdale, S.L., W.I. Nelson, J.D. Beaton, and J.L.Havlin. 1993. Soil Fertility and Fertilizers.Macmillan Publishing Co., New York, NY.

177

Although fertilizers and other amendments purchased from off the farm are not a

panacea to cure all soil problems, they play animportant role in maintaining soil productivity.Soil testing is the farmer’s best meansfor determining which amendments orfertilizers are needed and how muchshould be used.

The soil test report provides the soil’snutrient and pH levels and, in arid cli-mates, the salt and sodium levels. Rec-ommendations for application of nutrients andamendments accompany most reports. They arebased on soil nutrient levels, past cropping andmanure management, and should be a custom-ized recommendation based on the crop youplan to grow.

Soil tests — and proper interpretation of re-sults — are a very important management toolfor developing a farm nutrient management pro-

19�

Getting the MostFrom Soil Tests

…the popular mind is still fixed on the idea that

a fertilizer is the panacea.


gram. However, deciding how much fertilizerto apply — or the total amount of nutrientsneeded from various sources — is part science,part philosophy, and part art. Understanding soil

tests and how to interpret them canhelp farmers better customize thetest’s recommendations. In this chap-ter, we’ll go over sources of confusionabout soil tests, discuss N and P soiltests, and then examine a number ofsoil tests to see how the information

they provide can help you make decisions aboutfertilizer application.

TAKING SOIL SAMPLES

The usual time to take soil samples for generalfertility evaluation is in the fall or in the spring,before the growing season has begun. Thesesamples are analyzed for pH and lime require-


1. Don’t wait until the last minute. The best timeto sample for a general soil test is usually inthe fall. Spring samples should be taken earlyenough to have results in time to properly plannutrient management for the crop season.

2. Take cores from at least 15 to 20 spots ran-domly over the field to obtain a representa-tive sample. One sample should not repre-sent more than 10 to 20 acres.

3. Sample between rows. Avoid old fence rows,dead furrows, and other spots that are notrepresentative of the whole field.

4. Take separate samples from problem areas, ifthey can be treated separately.

5. In cultivated fields, sample to plow depth.6. Take two samples from no-till fields: one to a

6-inch depth for lime and fertilizer recom-

mendations, and one to a 2-inch depth tomonitor surface acidity.

7. Sample permanent pastures to a 3- to 4-inchdepth.

8. Collect the samples in a clean container.9. Mix the core samplings, remove roots and

stones, and allow to air dry.10. Fill the soil-test mailing container.11. Complete the information sheet, giving all of

the information requested. Remember, the rec-ommendations are only as good as the infor-mation supplied.

12. Sample fields at least every three years. An-nual soil tests will allow you to fine-tune nu-trient management and may allow you to cutdown on fertilizer use.

—MODIFIED FROM THE PENNSTATE AGRONOMY GUIDE, 1999.

Guidelines for Taking Soil Samples

ment as well as phosphorus, potassium, andmagnesium. Some labs also routinely analyzefor selected micronutrients, such as boron, zinc,and manganese.

ACCURACY OFRECOMMENDATIONS

BASED ON SOIL TESTS

Soil tests and their recommendations, althougha critical component of fertility management, arenot 100 percent accurate. Soil tests are an im-portant tool, but need to be used by farmersand farm-advisors along with other informationto make the best decision regarding amounts offertilizers or amendments to apply.

Soil tests are an estimate of a limited numberof plant nutrients, based on a small sample,

which is supposed to represent many acres in afield. With soil testing, the answers aren’t quiteas certain as we might like them. A low potas-sium soil test indicates that you will probablyincrease yield by adding the nutrient. However,adding fertilizer may not increase crop yieldsin a field with a low soil test level. The higheryields may be prevented because the soil test isnot calibrated for that particular soil (and thesoil had sufficient potassium for the crop de-spite the low test level) or because of harmcaused by poor drainage or compaction. Occa-sionally, using extra nutrients on a high-testingsoil increases crop yields. Weather conditionsmay have made the nutrient less available thanindicated by the soil test. So, it’s important touse common sense when interpreting soil testresults.

GETTING THE MOST FROM SOIL TESTS 179

Soil tests are not perfect.

All they indicate is whether or not

adding a nutrient is likely to result in

a yield increase of the crop

growing on that particular soil.

SOURCES OF CONFUSIONABOUT SOIL TESTS

People may be easily confused about the detailsof soil tests, especially if they have seen resultsfrom more than one soil testing laboratory. Thereare a number of reasons for this, including:

� laboratories use a variety of procedures;� labs report results differently; and� different approaches are used to make

recommendations based on soil test results.

Labs Use Varied Procedures

One of the complications with using soil teststo help determine nutrient needs is that testinglabs across the country use a wide range of pro-cedures. The main difference among labs is thesolutions they use to extract the soil nutrients.Some use one solution for all nutrients, whileothers will use one solution to extract potas-sium, magnesium and calcium; another for P;and yet another for micronutrients. The variousextracting solutions have different chemicalcompositions, so the amount of a particularnutrient that lab A extracts may be very differ-ent from the amount extracted by lab B. How-ever, there are frequently good reasons to use aparticular solution. For example, the Olsen testfor phosphorus (see below) is more accurate forhigh-pH soils in arid and semi-arid regions thanare the various acid-extracting solutions com-monly used in more humid regions. Whateverprocedure the lab uses, soil test levels must becalibrated with crop yield response to addednutrients. For example, do the yields really in-crease when you add phosphorus to a soil thattests low in P? In general, university or state labs

in a given region use the same or similar proce-dures that have been calibrated for local soilsand climate.

Labs Report SoilTest Levels Differently

Different labs may report their results in differ-ent ways. Some use part per million (10,000ppm = 1 percent); others use lbs./acre (they dothis usually by using part per two million, whichis twice the part per million level); and othersuse an index (for example, all nutrients are ex-pressed on a scale of 1 to 100). In addition, somelabs report phosphorus and potassium in theelemental form, while others use the oxideforms, P2O5 and K2O.

Most testing labs report results as both a num-ber and a category, such as low, medium, opti-mum, high, very high. However, although mostlabs consider high to be above the amountneeded (the amount needed is called optimum),some labs use optimum and high interchange-ably. If the significance of the various categoriesis not clear on your report, be sure to ask. Labsshould be able to furnish you with the prob-ability of getting a response to added fertilizerfor each soil test category.


Figure 19.1 Percent of maximum yield with different soil test levels.

% o

f m

axim

um

yie

ld

100

90

80

70

60

soil test K

very low low optimum high very high

Different Recommendation Systems

Even when labs use the same procedures, as isthe case in most of the Midwest, different ap-proaches to making recommendations lead todifferent amounts of recommended fertilizer.Three different philosophies are used to makefertilizer recommendations based on soil tests.One approach — the sufficiency level system —suggests there is a point, the sufficiency or criti-cal soil test value, above which there is little like-lihood of response to an added nutrient. Its goalis not to produce the highest yield every year,but, rather, to produce the highest average re-turn over time from using fertilizers. Experi-ments that relate yield increases with added fer-tilizer to soil test level provide much of the evi-dence supporting this approach. As the soil testlevel increases from optimum to high, yieldswithout adding fertilizer are closer to the maxi-mum obtained by adding fertilizer (figure 19.1).Of course, farmers should be shooting for the

maximum economic yields, which are slightlybelow the highest possible yields.

Another approach used by soil test labs —the build-up and maintenance system — calls forbuilding up soils to high levels of fertility andthen keeping them there by applying enoughfertilizer to replace nutrients removed in har-vested crops. It is used mainly for phosphorus,potassium, and magnesium recommendations.

The basic cation saturation ratio system, amethod of estimating calcium, magnesium, andpotassium needs, is based on the belief that cropsyield best when calcium (Ca++), magnesium(Mg++), and potassium (K+) — usually the domi-nant cations on the CEC — are in a particularbalance. Although there are different versionsof this system, most call for calcium to occupyabout 60 to 80 percent of the CEC, whereasmagnesium should be from 10 to 20 percentand potassium from 2 to 5 percent of the CEC.Great care is needed when using the base cationsaturation ratio system. For example, the ratios


To estimate the percentages of the variouscations on the CEC, the amounts need to beexpressed in terms of quantity of charge.Some labs give both concentration by weight(ppm) and by charge (me/100g). If you wantto convert from ppm to milliequivalent per100 grams (me/100g), you can do it as follows:

(Ca in ppm)/200 = Ca in me/100g(Mg in ppm)/120 = Mg in me/100g

(K in ppm)/390 = K in me/100g

As discussed previously (chapter 18), add-ing up the amount of charge due to calcium,magnesium, and potassium gives a very goodestimate of the CEC for most soils for most soilsabove pH 5.5.

of the nutrients can be within the guidelines,but there may be such a low CEC (such as witha sandy soil that is very low in organic matter),that the amounts present are insufficient for crops.In addition, when there is a high CEC, there maybe plenty of the nutrient, but the cation ratiosystem will call for adding more. This can be aproblem with soils that are naturally high inmagnesium, because the recommendations maycall for high amounts of calcium and potassiumto be added when none are really needed.

Research indicates that plants do well over abroad range of cation ratios, as long as there aresufficient supplies of potassium, calcium, andmagnesium. However, there are occasions whenthe calcium-magnesium-potassium ratios arevery out of balance. For example, when magne-sium occupies more than 50 percent of the CECin soils with low aggregate stability, using cal-cium sulfate may help restore aggregation. Asmentioned previously, liming very acidic soilssometimes results in decreased potassium avail-

ability and this would be apparent when usingthe cation ratio system. The sufficiency systemwould also call for adding potassium because ofthe low potassium levels in these very acid soils.

The sufficiency level approach is used by mostfertility recommendation systems for potassium,magnesium, and calcium. It generally calls forlower application rates and is more consistentwith the scientific data than the cation ratio sys-tem. The cation ratio system can be used suc-cessfully, if interpreted with care and commonsense — not ignoring the total amounts present.

Labs sometimes use a combination of thesesystems, something like a hybrid approach.Some laboratories that use the sufficiency sys-tem will have a target for magnesium, but thensuggest adding more if the potassium level ishigh. Others suggest that higher potassium lev-els are needed as the soil CEC increases. Theseare really hybrids of the sufficiency and cationratio systems. At least one lab uses the sufficiencysystem for potassium and a cation ratio systemfor calcium and magnesium. Also, some labs as-sume that soils will not be tested annually. Therecommendation that they give is, therefore, acombination of the sufficiency system (what isneeded for this crop) with a certain amountadded for maintenance. This is done to be surethere is enough fertility in the following year.

Crop Value, Fertilizer Rates,and Recommendation System

The value of your crop can have a major impacton the economics of over-applying fertilizer. Asa general rule, the lower the per acre value ofyour crop, the greater the economic penalty forapplying extra fertilizer (see box on p. 183).Farmers growing agronomic crops should takespecial care not to over-apply fertilizer.


Recommendation SystemComparison

Most university testing laboratories use thesufficiency level system, but some make po-tassium or magnesium recommendations bymodifying the sufficiency system to take intoaccount the portion of the CEC occupied bythe nutrient. The build-up and maintenancesystem is used by some state university labsand many commercial labs. An extensiveevaluation of different approaches to fertil-izer recommendations for agronomic cropsin Nebraska found that using the sufficiencylevel system resulted in using less fertilizerand gave higher economic returns than thebuild-up and maintenance system. Otherstudies in Kentucky, Ohio, and Wisconsin in-dicate that the sufficiency system is superiorto the build-up and maintenance or cationratio systems.

As the soil test level of a particular nutrientincreases, there is less chance that adding thenutrient will result in a greater yield. However,it may be worth adding fertilizer to high-valuecrops grown on soils with the same test levelsthat call for no fertilizer use low-value crops (fig-ure 19.2). This difference should be reflectedin the recommendations provided by soil test-ing laboratories.

Plant Tissue Tests

Soil tests are the most common means of as-sessing fertility needs of crops, but plant tissuetests are especially useful for nutrient manage-ment of perennial crops, such as apple, citrusand peach orchards, and vineyards. For mostannuals, including agronomic and vegetablecrops, tissue testing is not widely used, but canhelp diagnose problems. The small samplingwindow available for most annuals and an in-

chance ofadded

fertilizerincreasingyield by

enough tocover costs

100%

75%

50%

25%

0%

soil test level

low value/acrecrop

high value/acre crop

Figure 19.2 The chance of an economic return at different soil test levels.


Crop Value and Care withFertilizer Rates

Most agronomic crops grown on large acre-ages are worth around $200 to $400 per acreand the fertilizer used may represent 30 to 40percent of out-of-pocket growing costs. So,if you use 100 lbs. of N you don’t need, that’sabout $30/acre and represents a major eco-nomic loss. Some years ago, one of the au-thors worked with two brothers who operateda dairy farm in northern Vermont that had highsoil test levels of N, P, and K. Despite his rec-ommendation that no fertilizer was needed,the normal practice was followed and $70 peracre worth of fertilizer N, P, and K was ap-plied to their 200 acres of corn. The yields on40 feet wide, no-fertilizer strips that they leftin each field were the same as where fertil-izer had been applied, so the $14,000 for fer-tilizer was wasted.

When growing fruit or vegetable crops —worth thousands of dollars per acre — fertil-izers represent about 1 percent of the valueof the crop and 2 percent of the costs. Butwhen growing specialty crops (medicinalherbs, certain organic vegetables for directmarketing) worth over $10,000 per acre, thecost of fertilizer is dwarfed by other costs,such as hand labor. A waste of $30/acre in un-needed nutrients for these crops would causea minimal economic penalty — assuming youmaintain a reasonable balance between nu-trients — but there may be environmental rea-sons against applying too much fertilizer.However, there may be a justification for us-ing the build-up and maintenance approachfor phosphorus and potassium on high-valuecrops because: a) the extra costs are such asmall percent of total costs and b) there mayoccasionally be a higher yield because of thisapproach that would more than cover theextra expense of the fertilizer.

ability to effectively fertilize them once they arewell established, except for N during earlygrowth stages, limits the usefulness of tissueanalysis for annual crops. However, leaf petiolenitrate tests are sometimes done on potato andsugar beets to help fine-tune in-season N fertili-zation. Petiole nitrate is also helpful for N man-agement of cotton and for help managing irri-gated vegetables, especially during the transi-tion from vegetative to reproductive growth.With irrigated crops, in particular when the dripsystem is used, fertilizer can be effectively de-livered to the rooting zone during crop growth.

What Should You Do?

After reading the discussion above you may besomewhat bewildered by the different proceduresand ways of expressing results, as well as thedifferent recommendation approaches. The factis that it is bewildering! Our general suggestionsof how to deal with these complex issues are:

1.Send your soil samples to a lab that usestests evaluated for the soils and crops ofyour state or region. Continue using thesame lab or another that uses the sameprocedures and recommendation system.

2.If you’re growing low value-per-acre crops(wheat, corn, soybeans, etc.), be sure thatthe recommendation system used is basedon the sufficiency approach. This systemusually results in lower fertilizer rates andhigher economic returns for low valuecrops. [It is not easy to find out what systema lab uses. Be persistent and you will get toa person that can answer your question.]

3.Dividing the same sample in two andsending it to two labs may result in confu-sion. You will probably get different recom-


proper management

high

optimal soiltest range

low

over-fertilization

under-fertilization

high

low

low

high

corn/hay rotation

high

lowcorn cornhay hay

Figure 19.3 Soil test phosphorus and potassiumtrends under different fertility managementregimes. Modified from The PennState AgronomyGuide, 1999.

mendations and it won’t be easy to figureout which is better for you, unless you arewilling to do a comparison of recommenda-tions. In most cases you are better offstaying with the same lab and learning howto fine-tune recommendations for yourfarm. However, if you are willing to experi-ment, you can send duplicate samples to

two different labs, with one going to yourstate-testing laboratory. In general, therecommendations from these labs call forless, but enough, fertilizer. If growing cropsover large acreage, set up a demonstrationor experiment in one field where you applythe fertilizer recommended by each lab overlong strips and see if there is any yielddifference. A yield monitor for grain cropswould be very useful for this purpose. Ifyou’ve never set up a field experimentbefore, you should ask your extension agentfor help, and might find the brochure Howto Conduct Research on Your Farm or Ranch ofuse (see Sources at end of chapter).

4.Keep a record of soil tests for each field, sothat you can track changes over the years(figure 19.3). If records show a build up ofnutrients to high levels, reduce nutrientapplications. If you’re drawing nutrientlevels down too low, start applying fertiliz-ers or off-farm organic nutrient sources. Insome rotations, such as the corn-corn-4years of hay shown at the bottom of figure19.3, it makes sense to build up nutrientlevels during the corn phase and draw themdown during the hay phase.

SOIL TESTING FOR N

Soil samples for nitrogen tests are usually takenat a different time and using a different methodthan for the other nutrients (which are typicallysampled to plow depth in the fall or spring). Be-fore the mid-1980s, there was no reliable soil testfor N availability in the humid regions of the US.

The nitrate test commonly used for corn inhumid regions was developed during the 1980sin Vermont. It is usually called the Pre-Sidedress

2 4 6 8 10 12years

optimal soiltest range


Nitrate Test (PSNT), but also goes under othernames (see box). All of these names refer to thesame test — a soil sample is taken to 1 footdepth, when corn is between 6 inches and 1foot tall. The original idea behind the test wasto wait as long as possible before sampling, be-cause soil and weather conditions in the earlygrowing season may reduce or increase N avail-ability for the crop later in the season. After thecorn is 1 foot tall, it is difficult to get samples toa lab and back in time to apply any neededsidedress N fertilizer. The PSNT is now used onboth field corn and sweet corn and research isunderway in the northeastern U.S. to extend itsuse to pumpkins and cabbage. Although thePSNT is widely used, there are some situations,such as the sandy coastal plains soils of the deepsouth, where it is not very accurate.

Different approaches to using the PSNT workfor different farms. In general, using the soil testallows a farmer to avoid adding excess amountsof “insurance fertilizer.” Two contrasting ex-amples follow:

� For farms using rotations with legumeforages and applying animal manuresregularly (so there’s a lot of active soilorganic matter), the best way to use the testto your advantage is to apply only theamount of manure necessary to provide

sufficient N to the plant. The PSNT willindicate whether or not you need to side-dress any additional N fertilizer. It will alsoshow you whether you’ve done a good jobof estimating N availability from manures.

� For farms growing cash grains withoutusing legume cover crops, it’s best to applya conservative amount of fertilizer N beforeplanting and then use the test to see if moreis needed. This is especially important inregions where rainfall cannot always berelied upon to quickly bring fertilizer intocontact with roots. The PSNT test providesa backup and allows the farmer to be moreconservative with preplant applications,knowing that there is a way to make up anypossible deficit.

Other nitrogen soil tests. In the drier parts ofthe country, a nitrate soil test, requiring samplesto 2 feet or more, has been used with successsince the 1960s. The deep-soil samples can betaken in the fall or early spring, before the grow-ing season, because of low leaching and deni-trification losses and low levels of active organicmatter (so hardly any nitrate is mineralized fromorganic matter). Soil samples can also be takenat the same time for analysis for other nutrientsand pH. A few states in the upper Midwest offera preplant nitrate test, which calls for samplingto 2 feet in the spring.

SOIL TESTING FOR P

Soil test procedures for phosphorus are differ-ent than for nitrogen. When testing for phos-phorus, the soil is usually sampled to plowdepth at a different time — in the fall or in theearly spring before tillage — and the sample isusually analyzed for phosphorus, potassium,

Names Used For The PSNT

Pre-Sidedress Nitrate Test (PSNT)Magdoff PSNTLate Spring Nitrate Test (LSNT)June Nitrate TestVermont N Soil Test


sometimes other nutrients (such as calcium,magnesium, and micronutrients) and pH. Themethods used to estimate available P vary fromregion to region, and sometimes, from state tostate within a region (Table 19.1). Althoughthe relative test value for a given soil is usuallysimilar when using different soil tests (for ex-ample, a high P-testing soil by one procedureis generally also high by another procedure),the actual numbers can be different (table19.2).

The various soil tests for P take into accounta large portion of the available P contained inrecently applied manures and the amount thatwill become available from the soil minerals.However, if there is a large amount of active or-ganic matter in your soils from crop residues ormanure additions made in previous years, theremay well be more available P for plants thanindicated by soil test. (On the other hand, thePSNT reflects the amount of N that may becomeavailable from decomposing organic matter.)

TABLE 19.2 Interpretation Ranges for Different P Soil Tests

LOW OPTIMUM HIGH VERY HIGH

Olsen 0 to 7 7 to 15 15 to 21 >21

Morgan 0 to 4 4 to 7 7 to 20 >20

Bray 1 (Bray P-1) 0 to 15 15 to 24 24 to 31 >31

Mehlich 1 0 to 25 25 to 50 >50

Mehlich 3 0 to 15 15 to 24 24 to 31 >31

AB—DTPA (for irrigated crops) 0 to 7 8 to 12 12 to15 >15

Note: units are in parts per million phosphorus (ppm P) and ranges used for recommendations may varyfrom state to state.

TABLE 19.1Phosphorus Soil Tests Used in Different Regions.

REGION SOIL TEST SOLUTIONS USED FOR P

Arid and semi-arid Midwest, West and Northwest OlsenAB-DTPA

Humid Midwest, mid-Atlantic, Southeast, and Mehlich 3eastern Canada

North Central and midwest Bray 1 (also called Bray P-1 or Bray-Kurtz P)

Southeast and mid-Atlantic Mehlich 1

Northeast (New York and most of New England),some labs in Idaho and Washington Morgan or modified-Morgan

—MODIFIED FROM ALLEN ET AL. (1994)


Unusual Soil Tests?

From time to time we’ve come across unusualsoil test results. A few examples and their typi-cal causes are given below.

Very high phosphorus levels. High poultryor other manure application over many years.

Very high salt concentration in humid re-gion. Recent application of large amounts ofpoultry manure or immediately adjacent toroad where de-icing salt was used.

Very high pH and high calcium levels, rela-tive to potassium and magnesium. Largeamounts of lime-stabilized sewage sludgewere used.

Very high calcium levels given the soil’s tex-ture and organic matter content. Using anacid solution, such as the Morgan, Mehlich 1,or Mehlich 3, to extract soils containing freelimestone causes some of the lime to dissolve,giving artificially high calcium test levels.

TESTING SOILS FORORGANIC MATTER

A word of caution when comparing your soiltest organic matter levels with those discussedin this book. If your laboratory reports organicmatter as “weight loss” at high temperature, thenumbers may be higher than if the lab uses thetraditional wet chemistry method. A soil with 3percent organic matter by wet chemistry mighthave a weight-loss value of between 4 and 5percent. Most labs use a correction factor toapproximate the value you would get by usingthe wet chemistry procedure. Although eithermethod can be used to follow changes in yoursoil, when you compare soil organic matter ofsamples run in different laboratories, it’s best tomake sure the same methods were used.

There is now a laboratory that will determinevarious forms of living organisms in your soil.Although it costs quite a bit more than tradi-tional testing for nutrients or organic matter, youcan find out the amount (weight) of fungi andbacteria in a soil, as well as analysis for otherorganisms. (See the Resources section at the backof the book for laboratories that run tests inaddition to basic soil fertility analysis.)

INTERPRETINGSOIL TEST RESULTS

Below are five soil test examples, including dis-cussion about what they tell us and the types ofpractices that should be followed. Suggestionsare provided for conventional farmers and or-ganic producers. These are just suggestions —there are other satisfactory ways to meet theneeds for crops growing on these soils. The soiltests were run by different procedures, to give

examples from around the US. Interpretationsfor a number of commonly used soil tests —relating test levels to general fertility categories— are given later in the chapter (tables 19.3and 19.4). Many labs estimate the cation ex-change capacity that would exist at pH 7 (oreven higher). Because we feel that the soil’s cur-rent CEC is of most interest (see chapter 18),the CEC is estimated by summing the exchange-able bases. The more acidic a soil, the greaterthe difference between its current CEC and theCEC it would have near pH 7.

Following the five soil tests is a section onmodifying recommendations for particular situ-ations.


Soil Test #1 Report Summary*

SOIL TEST RECOMMENDATION

LBS./ACRE PPM CATEGORY SUMMARY

P 4 2 low 50–70 lbs. P2O5/acre

K 100 50 low 150–200 lbs. K2O/acre

Mg 60 30 low lime (see below)

Ca 400 200 low lime (see below)

pH 5.4 2 tons dolomitic limestone/acre

CEC** 1.4 me/100g

OM 1% add organic matter: compost, cover crops,animal manures

PSNT 5 low sidedress 80–100 lbs. N/acre

* Nutrients extracted by modified Morgan’s Solution (see table 19.3a for interpretations).

** CEC by sum of bases. The estimated CEC would probably double if “exchange acidity” were determined and addedto the sum of bases.

— SOIL TEST #1 —(New England)

Field name: NorthSample date: September (PSNT sample taken

the following June)Soil type: loamy sandManure added: noneCropping history: mixed vegetablesCrop to be grown: mixed vegetables

What can we tell about soil #1 based onthe soil test?

❑ It is too acidic for most agricultural crops,so lime is needed.

❑ Phosphorus is low, as are potassium,magnesium, and calcium. All should beapplied.

❑ This low organic matter soil is probablyalso low in active organic matter (indicatedby the low PSNT test, see table 19.4a) andwill need an application of nitrogen. (ThePSNT is done during the growth of thecrop, so it is difficult to use manure tosupply extra N needs indicated by the test.)

❑ The coarse texture of the soil is indicatedby the combination of low organic matterand low CEC.

General recommendations:

1. Apply dolomitic limestone, if available, inthe fall at about 2 tons/acre (and work it intothe soil and establish a cover crop if pos-sible). This will take care of the calcium andmagnesium needs at the same time the soil’spH is increased.


2. Because no manure is to be used after thetest was taken, broadcast significant amountsof phosphate (probably around 50 to 70 lbs.phosphate (P2O5)/acre) and potash (around150 to 200 lbs. potash (K2O)/acre). Somephosphate and potash can also be appliedin starter fertilizer (band applied at plant-ing). Usually N is also included in starterfertilizer, so it might be reasonable to useabout 300 lbs. of a 10-10-10 fertilizer, whichwill apply 30 lbs. of N, 30 lbs. of phosphate,and 30 lbs. of potash per acre. If that rate ofstarter is to be used, then broadcast 400 lbs.per acre of a 0-10-30 bulk blended fertil-izer. The broadcast plus the starter will sup-ply 30 lbs. of N, 70 lbs. of phosphate, and150 lbs. of potash per acre.

3. If only calcitic (low magnesium) limestoneis available, use Sul-Po-Mag as the potassiumsource in the bulk blend to help supply mag-nesium.

4. Nitrogen should be sidedressed at around80 to 100 (or more) lbs./acre for N-demand-ing crops, such as corn or tomatoes. About300 lbs. of ammonium nitrate or 220 lbs. ofurea per acre will supply 100 lbs. of N.

5. Use various medium-to-long-term strategiesto build up soil organic matter, including useof cover crops and animal manures.

Most of the nutrient needs of crops on thissoil could have been met by using about 20 tonswet weight of solid cow manure/acre or itsequivalent. It is best to apply it in the spring,before planting. If the manure had been applied,the PSNT test would probably have been quitea bit higher, perhaps around 25 ppm.

Recommendations for organicproducers:

1. Use dolomitic limestone to increase the pH(as recommended for the conventionalfarmer above).

2. Apply 2 tons/acre of rock phosphate, orabout 5 tons of poultry manure for phos-phorus, or — better yet — a combination of1 ton rock phosphate and 21/2 tons of poul-try manure. If the high level of rock phos-phate is applied, it should supply some phos-phorus for a long time, perhaps a decade.

3. If the poultry manure is used to raise thephosphorus level, add 2 tons of compost peracre to add some longer lasting nutrients andhumus. If rock phosphate is used to supplyphosphorus, then use livestock manure andcompost (to add N, potassium, magnesium,and some humus).

4. Establish a good rotation with soil-buildingcrops and cover crops.

5. Care is needed with manure use. Althoughthe application of uncomposted manure isallowed by organic certifying organizations,there are restrictions. For example, three tofour months may be needed between appli-cation of uncomposted manure and eitherharvest of root crops or planting of cropsthat accumulate nitrate, such as leafy greensor beets. A two-month period may be neededbetween uncomposted manure applicationand harvest of other food crops.





P 174 87 high none

K 360 180 high none

Mg 274 137 high none

Ca 3880 1940 high none

pH 7.2 no lime needed

CEC 11.7 me/100g

OM 3% add organic matter: compost, cover crops,animal manures

N No N soil test little to no N needed

* Soil sent to a commercial lab. P using Bray-1 solution. This is probably the equivalent of over 20 ppm by usingMorgan or Olsen procedures. Other nutrients extracted with pH 7 ammonium acetate (see table 19.3d).

— SOIL TEST #2 —(Pennsylvania, New York)

Field name: Smith upperSample date: November (no sample for PSNT

will be taken)Soil type: silt loamManure added: none this year (some last year)Cropping history: legume cover crops used

routinelyCrop to be grown: corn


❑ The high pH indicates that this soil doesnot need any lime.

❑ Phosphorus is high, as are potassium,magnesium, and calcium (see table 19.3d).

❑ The organic matter is very good for a siltloam.

❑ There was no test done for nitrogen, butthis soil probably supplies a reasonableamount of N for crops, because the farmeruses legume cover crops and allows themto produce a large amount of dry matter.


1. Continue building soil organic matter.2. No phosphate, potash, or magnesium needs

to be applied. The lab that ran this soil testrecommended using 38 lbs. potash (K2O)and 150 lbs. of magnesium (MgO) per acre.However, with a high K level, 180 ppm(about 8 percent of the CEC) and a high Mg,137 ppm (about 11 percent of the CEC),there is a very low likelihood of any increasein yield or crop quality from adding eitherelement.

3. Nitrogen fertilizer is probably needed in onlysmall to moderate amounts (if at all), but we





P 20 10 very low 30 lbs. P2O5/acre

K 58 29 very low 200 lbs. K2O/acre

Mg 138 69 high none

Ca 3168 4084 high none


CEC 21.1 me/100g

OM 4.3% rotate to forage legume crop

N No N soil test 100-130 lbs. N/acre

*all nutrients determined using Mehlich 3 solution (see table 19.3c).

need to know more about the details of thecropping system or run a nitrogen soil testto make a more accurate recommendation.

Recommendations for organic producers:

1. A good rotation with legumes will providenitrogen for other crops.

— SOIL TEST #3 —(Humid Midwest)

Field name: #12Sample date: December (no sample for PSNT

will be taken)Soil type: clay (somewhat poorly drained)Manure added: noneCropping history: continuous cornCrop to be grown: corn


❑ The high pH indicates that this soil doesnot need any lime.

❑ Phosphorus and potassium are low.❑ The organic matter is relatively high.

However, considering that this is a some-what poorly drained clay, it probablyshould be even higher.


❑ About half of the CEC is probably due tothe organic matter with the rest probablydue to the clay.

❑ Low potassium indicates that this soil hasprobably not received high levels ofmanures recently.

❑ There was no test done for nitrogen, butgiven the field’s history of continuous cornand little manure, there is probably a needfor nitrogen. A low amount of activeorganic matter that could have suppliednitrogen for crops is indicated by pasthistory (the lack of rotation to perenniallegume forages and lack of manure use)and the moderate percent organic matter(considering that it is a clay soil).


1. This field should probably be rotated to aperennial forage crop.

2. Phosphorus and potassium are needed.Probably around 30 lbs. of phosphate (P2O5)and 200 or more lbs. of potash (K2O) ap-plied broadcast, preplant, if a forage crop isto be grown. If corn will be grown again, allof the phosphate and 30 to 40 lbs. of thepotash can be applied as starter fertilizer atplanting. Although magnesium, at about 3percent of the effective CEC, would be con-sidered low by relying exclusively on a ba-sic cation ratio saturation recommendationsystem, there is little likelihood of an increasein crop yield or quality by adding magne-sium.

3. Nitrogen fertilizer is probably needed in largeamounts (100 to 130 lbs./acre) for high N-demanding crops, such as corn. If no in-sea-son soil test (like the PSNT) is done, somepreplant N should be applied (around 50

lbs./acre), some in the starter band at plant-ing (about 15 lbs./acre) and some side-dressed (about 50 lbs.).

4. One way to meet the needs of the crop is asfollows:

a) broadcast 500 lbs. per acre of an 11-0-44bulk blended fertilizer;

b) use 300 lbs. per acre of a 5-10-10 starter;and

c) sidedress with 150 lbs. per acre of ammo-nium nitrate.

This will supply approximately 120 lbs. ofN, 30 lbs. of phosphate and 210 lbs. of potash.


1. 2 tons/acre of rock phosphate (to meet Pneeds) or about 5 to 8 tons of poultry ma-nure (which would meet both phosphorusand nitrogen needs), or a combination of thetwo (1 ton rock phosphate and 3 to 4 tonsof poultry manure).

2. 400 lbs. of potassium sulfate per acre broad-cast preplant. (If poultry manure is used tomeet phosphorus and nitrogen needs, useonly 200 to 300 lbs. of potassium sulfate peracre.)

3. Care is needed with manure use. Althoughthe application of uncomposted manure isallowed by organic certifying organizations,there are restrictions. For example, three tofour months may be needed between appli-cation of uncomposted manure and eitherharvest of root crops or planting of cropsthat accumulate nitrate, such as leafy greensor beets. A two-month period may be neededbetween uncomposted manure applicationand harvest of other food crops.


*Soil Test #4 Report Summary



P 102 51 very high none

K 166 83 high none


Ca 1158 579 none

pH 6.5 moderate no lime needed

CEC 4.2 me/100 g

OM not requested use legume cover crops, consider crop rotation

N No N soil test 70–100 lbs. N/acre

*all nutrients determined using Mehlich 1 solution (see table 19.3b).

— SOIL TEST #4 —(Alabama)

Field name: River ASample date: OctoberSoil type: sandy loamManure added: noneCropping history: continuous cottonCrop to be grown: cotton


❑ With a pH of 6.5, this soil does not needany lime.

❑ Phosphorus is very high, and potassiumand magnesium are sufficient.

❑ Magnesium is high, compared with calcium(Mg occupies over 26 percent of the CEC).

❑ The low CEC at pH 6.5 indicates that theorganic matter content is probably around1 to 1.5 percent.


1. No phosphate, potash, magnesium, or limeis needed.

2. Nitrogen should be applied, probably in asplit application totaling about 70 to 100 lbs.N/acre.

3. This field should be rotated to other cropsand cover crops used regularly.

Recommendations for organic producer:

1. Although poultry or dairy manure can meetthe crop’s needs, that means applying phos-phorus on an already high-P soil. If there isno possibility of growing an overwinter le-gume cover crop (see below), then about 15to 20 tons of bedded dairy manure (wetweight) should be sufficient.

2. If time permits, this soil can use a high-Nproducing legume cover crop, such as hairyvetch or crimson clover, to provide nitrogento cash crops.





P 14 7 low 20–40 lbs. P2O5

K 716 358 very high none


Ca not determined none


CEC not determined

OM 1.8% use legume cover crops, consider rotation toother crops that produce large amounts ofresidues

N 5.8 ppm 170 lbs. N/acre

*K and Mg extracted by neutral ammonium acetate, P by Olsen solution (see table 19.3d).

3. Develop a good rotation so that all theneeded nitrogen will be supplied to non-le-gumes between the rotation crops and covercrops.

4. Although the application of uncompostedmanure is allowed by organic certifying or-ganizations, there are restrictions whengrowing food crops. Check with the persondoing your certification to find out what re-strictions apply to cotton.

—SOIL TEST #5—(Semi-arid Great Plains)

Field name: HillSample date: AprilSoil type: silt loamManure added: none indicatedCropping history: not indicatedCrop to be grown: corn


❑ The pH of 8.1 indicates that this soil ismost likely calcareous.

❑ Phosphorus is low, there is sufficientmagnesium, and potassium is very high.

❑ Although calcium was not determined,there will be plenty in a calcareous soil.


❑ The organic matter at 1.8 percent is low fora silt loam soil.

❑ The nitrogen test indicates a low amount ofresidual nitrate (table 19.4b) and, given thelow organic matter level, a low amount ofN mineralization is expected.


1. No potash, magnesium, or lime is needed.2. About 170 lbs. of N/acre should be applied.

Because of the low amount of leaching inthis region, most can be applied pre-plant,with perhaps 30 lbs. as starter (applied atplanting). Using 300 lbs. per acre of a 10-10-0 starter would supply all P needs (seebelow) as well as give some N near the de-veloping seedling. Broadcasting and incor-porating 300 lbs. of urea or 420 lbs. of am-monium nitrate will provide 140 lbs. of N.

3. About 20 to 40 lbs. of phosphate (P2O5) isneeded per acre. Apply the lower rate asstarter, because localized placement resultsin more efficient use by the plant. If phos-phate is broadcast, apply at the 40 lb rate.

4. The organic matter level of this soil shouldbe increased. This field should be rotated toother crops and cover crops used regularly.


1. Because rock phosphate is so insoluble in highpH soils, it would be a poor choice for add-ing P. Poultry (about 6 tons per acre) or dairy(about 25 tons wet weight per acre) manurecan be used to meet the crop’s needs for bothN and P. However, that means applying moreP than is needed, plus a lot of potash (whichis already at very high levels).

2. A long-term strategy needs to be developedto build soil organic matter — better rota-tions, use of cover crops, and importing or-ganic residues onto the farm.

3. Care is needed with manure use. Althoughthe application of uncomposted manure isallowed by organic certifying organizations,there are restrictions. For example, threemonths may be needed between applicationof uncomposted manure and either harvestof root crops or planting of crops that accu-mulate nitrate, such as leafy greens or beets.A two-month period may be needed betweenuncomposted manure application and har-vest of other food crops.


TABLE 19.3Soil Test Categories for Various Extracting Solutions

A.Modified Morgan’s Solution (Vermont)

CATEGORY LOW MEDIUM OPTIMUM HIGH EXCESSIVELY HIGH

PROBABILITY OF RESPONSE

TO ADDED NUTRIENTVERY HIGH HIGH LOW VERY LOW

Available P (ppm) 0–2 2–4 4–7 7–20 >20

K (ppm) 0–50 51–100 101–130 131–160 >160

Mg (ppm) 0–35 35–50 51–100 >100 –

B.Mehlich 1 Solution (Alabama)*

CATEGORY VERY LOW LOW MEDIUM HIGH EXCESSIVELY HIGH



Available P (ppm) 0–12 13–25 26–50 51–125 >125

K (ppm) 0–45 46–90 91–180 >180

Mg (ppm)** 0–25 26–50 >50

Ca for tomatoes (ppm)*** 0–150 151–250 >250

* From Procedures Used by State Soil Testing Laboratories in the Southern Region of the United States, 1998.** for corn, legumes, and vegetables on soils with CECs greater than 4.6 me/100g***for corn, legumes, and vegetables on soils with CECs from 4.6 to 9.0 me/100g

C.Mehlich 3 Solution (North Carolina).*




Available P (ppm) 0–12 13–25 26–50 51–125 >125

K (ppm) 0–17 18–44 45–87 88–174 >175

Mg (ppm)** 0–30 31–60 >60

* From Procedures Used by State Soil Testing Laboratories in the Southern Region of the United States, 1998.** percent of CEC is also considered


TABLE 19.4Soil Test Catagories for Nitrogen Tests

A.Presidress Nitrogen Test (PSNT)*

CATEGORY LOW MEDIUM OPTIMUM HIGH EXCESSIVE


TO ADDED NUTRIENTVERY HIGH HIGH LOW VERY LOW NONE

Nitrate-N (ppm) 0–10 11–22 23–28 29–35 >35

*Soil sample taken to 1 ft when corn is 6 to 12 inches tall.

B.Deep (4ft) Nitrate Test (Nebraska)

CATEGORY LOW MEDIUM OPTIMUM HIGH EXCESSIVE


TO ADDED NUTRIENTVERY HIGH HIGH LOW VERY LOW NONE

Nitrate-N (ppm) 0–6 7–15 15–18 19–25 >25

D.Neutral Ammonium Acetate Solution

for K and Mg and Olsen or Bray-1 for P (Nebraska (P and K), Minnesota (Mg))



TO ADDED NUTRIENTVERY HIGH HIGH LOW VERY LOW VERY LOW

P (Olson,ppm) 0–3 4–10 11–16 17–20 >20

P (Bray-1,ppm) 0–5 6–15 16–24 25–30 >30

K (ppm) 0–40 41–74 75–124 125–150 >150

Mg (ppm) 0–50 51–100 >101


Worksheet for Adjusting Fertilizer RecommendationsN P2O5 K2O

SOIL TEST RECOMMENDATION 120 40 140Accounts for contributions from the soil. Accounts fornutrients contributed from manure and previous croponly if information is included on form sent with soil sample.

CREDITS(Use only if not taken into account in recommendationreceived from lab.)

Previous crop (already taken into account) -0

Manure (10 tons @ 6 lbs. N–2.4 lbs. P2O5–9 lbs. K2O per ton, -60 -24 -90assuming that 60% of the nitrogen, 80% of the phosphorusand 100% of the potassium in the manure will be availablethis year.)

Cover Crop (medium growth crimson clover) -50

TOTAL NUTRIENTS NEEDED FROM FERTILIZER 10 16 50

Making Adjustments to Fertilizer Application Rates

If information about cropping history, covercrops, or manure use is not provided to the soiltesting laboratory, the report containing the fer-tilizer recommendation cannot take these factorsinto account. Below is an example of how youcan modify the report’s recommendations.

Past crop = cornCover crop = crimson clover, but small tomedium amount of growth.Manure = 10 tons of dairy manure that testedat 10 lbs. N, 3 lbs. of P2O5, and 9 lbs. of K2Oper ton. (A decision to apply manure wasmade after the soil sample was sent, so therecommendation could not take those nutrientsinto account.)

ADJUSTING A SOIL TESTRECOMMENDATION

Specific recommendations must be tailored tothe crops you want to grow, as well as other char-acteristics of the particular soil, climate, andcropping system. Most soil test reports use in-formation that you supply about manure use

and previous crop to adapt a general recommen-dation for your situation. However, once youfeel comfortable with interpreting soil tests, youmay also want to adjust the recommendationsfor a particular need. What happens if you de-cide to apply manure after you sent in the formalong with the soil sample? Also, you usuallydon’t get credit for the nitrogen produced by


TABLE 19.5Amounts of Available Nutrients

from Manures and LegumeCover Crops

NLEGUME COVER CROPS

1LBS./ACRE

Hairy vetch 70–140

Crimson clover 40–90

Red and white clovers 40–90

Medics 30–80

N P2O5 K2OMANURES

2LBS. PER TON MANURE

Dairy 6 4 10

Poultry 20 15 10

Hog 6 3 9

1Amount of available N varies with amount ofgrowth.

2Amount of nutrients varies with diet, storage,and application method. Quantities given inthis table are somewhat less than for the totalamounts given in table 9.1.

legume cover crops because most forms don’teven ask about their use. The amount of avail-able nutrients from legume cover crops and frommanures is indicated in table 19.5. Another com-mon situation occurs because most farmers don’ttest their soil annually and the recommenda-tions they receive are only for the current year.Under these circumstances, you need to figureout what to apply the next year or two, untilthe soil is tested again.

No single recommendation, based only onthe soil test, makes sense for all situations. Forexample, your gut feeling might tell you that atest is too low (and fertilizer recommendationsare too high). Let’s say that you broadcast 100lbs. N/acre before planting, but a high rate of N

fertilizer is still recommended by the in-seasonnitrate test (PSNT), even though there wasn’tenough rainfall to leach out nitrate or causemuch loss by denitrification. In this case, youmay not want to apply the full amount recom-mended.

Another example: a low potassium level in asoil test (let’s say around 40 ppm) will certainlymean that you should apply potassium. But,how much should you use? When/how shouldyou apply it? The answer to these two questionsmight be quite different on a low-organic mat-ter, sandy soil where high amounts of rainfallnormally occur during the growing season (inwhich case potassium may leach out if appliedthe previous fall, or early spring) versus a high-organic matter clay loam soil that has a higherCEC and will hold onto potassium added in thefall. This is the type of situation that dictatesusing labs whose recommendations are devel-oped for soils and cropping systems in yourhome state or region. It also is an indication thatyou may need to modify a recommendation foryour specific situation.

SOURCES

Allen, E.R., G.V. Johnson, and L.G. Unruh. 1994.Current approaches to soil testing methods:Problems and solutions. pp. 203–220. In SoilTesting: Prospects for Improving Nutrient Rec-ommendations (J.L. Havlin et al., eds). Soil Sci-ence Society of America. Madison, WI.

Cornell Cooperative Extension. 2000. Cornell Rec-ommendations for Integrated Field Crop Produc-tion. Cornell Cooperative Extension, Ithaca, NY.

Herget, G.W., and E. J. Penas. 1993. New Nitro-gen Recommendations for Corn. NebFacts NF93–111, University of Nebraska Extension.Lincoln, NE.


Jokela, B., F. Magdoff, R. Bartlett, S. Bosworth,and D. Ross. 1998. Nutrient Recommendationsfor Field Crops in Vermont. University of VermontExtension. Brochure 1390. Burlington, VT.

Penas, E.J., and R.A. Wiese. 1987. Fertilizer Sug-gestions for Soybeans. NebGuide G87-859-A.University of Nebraska Cooperative Exten-sion. Lincoln, NE.

Hanlon, E. (ed.). 1998. Procedures Used by StateSoil Testing Laboratories in the Southern Regionof the United States. Southern CooperativeSeries Bulletin No. 190–Revision B. Univer-sity of Florida. Immokalee, FL.

How to Conduct Research on Your Farm or Ranch.1999. Available from SARE regional offices.Also available at: www.sare.org/san/htdocs/pubs

Recommended Chemical Soil Test Procedures for theNorth Central Region. 1998. North Central Re-gional Research Publication No. 221 (re-vised). Missouri Agricultural Experiment Sta-tion SB1001. Columbia, MO.

Rehm, G., 1994. Soil Cation Ratios for Crop Pro-duction. North Central Regional ExtensionPublication 533. University of MinnesotaExtension. St. Paul, MN.

Rehm, G., M. Schmitt, and R. Munter. 1994. Fer-tilizer Recommendations for Agronomic Cropsin Minnesota. University of Minnesota Exten-sion. BU-6240-E. St Paul, MN.

The PennState Agronomy Guide. 1999. ThePennsylvania State University. UniversityStation, PA.

P A R T T H R E E

Putting It AllTogether

203

20�

How Good are Your Soils?Assessing Soil Health

…the Garden of Eden, almost literally,

lies under our feet almost anywhere on the earth

we care to step. We have not begun to tap the

actual potentialities of the soil for producing crops.

—E.H. FAULKNER, 1943

By now, you should have some ideas aboutpractices for increasing soil health on your

farm. Most farmers know that soil health is im-portant. However, when you work to improveyour soils, how can you tell that they’reactually getting any better? We’re allused to taking soil samples and havingthem analyzed for available nutrients,pH, lime requirement, and total organicmatter. Those tests are important piecesof information when used to adjust nu-trient management practices. But the real issueis not how well a soil does in a lab test, but howwell it does in the field. Does it do all the thingswe expect from a healthy soil? Of course, wewant a soil to supply nutrients in adequateamounts but not in such excess that mightcause plant health or environmental problems.

Does your soil also…� Allow water to infiltrate easily during a

downpour and drain afterward?� Provide sufficient water to plants during dry

spells?� Allow crops to fully develop healthy root systems?� Suppress root diseases and parasitic nematodes?

We can evaluate the quality of oursoils in many different ways, just as we

do with human health. We can assess humanhealth with a variety of diagnoses or procedures,ranging from “you look a little pale today” totaking a person’s temperature to doing a simpleblood pressure test to computerized body im-aging. For soils, we are limited in our ability todiagnose problems because we do not have the

204 BUILDING SOILS

equivalent of the extensive medical knowledgebase that is available for humans. We also havesome additional challenges with soils. For ex-ample, a single blood sample will assess the en-tire human body, because the blood circulatesrapidly through the entire vascular system. Soilsdo not function as a single organism, but as partof an ecosystem. Therefore, to obtain a goodassessment of the soil’s health, we need to makemultiple observations — at different locationsin a field and over a period of time.

A simple but very good place to start assess-ing a soil’s health is to look at its general perfor-mance as you go about your normal practices.It’s something like wondering about your ownperformance during the course of a day: Do youfeel more tired than usual? Are you concentrat-ing less well on tasks than you commonly do?Do you look paler than normal? These are indi-cations that something isn’t quite right. Like-wise, there are signs of poor soil health youmight notice as part of the normal process ofgrowing crops:

� Are yields declining?� Do crops perform as well as those on

neighboring farms with similar soils?� Do your crops quickly show signs of stress

or stunted growth during wet or dryperiods?

� Does the soil plow up cloddy, and is itdifficult to prepare a good seedbed?

� Does the soil crust over easily?� If you are no-tilling, is it difficult to get the

planter to penetrate?

The next step should be a little more quanti-tative. In a few states, farmers and researchershave developed “soil health score cards” that you

fill out for each field. Card choices may varyfrom state to state. The differences in soils andclimates suggest that there is no uniform soilhealth card that can be used everywhere. Nor isthere a magic number or index value for soilhealth. The goal of any evaluation is to help youmake changes and improve your soil’s healthover time by identifying key limitations or prob-lems. Perfect soil can’t be created everywhere,but you want to help the land reach its fullestpotential.

Whenever you try to become more quantita-tive in assessing your soils, you should be awarethat measurements may naturally vary consid-erably within a field, or may change over thecourse of the year. For example, if you decideto evaluate soil hardness with a penetrometer,or by using a thin metal rod, your results de-pend on the soil moisture conditions at the timeof measurement. If you use a penetrometer inJune of a year with low early-season rainfall, youmay find the soil quite hard. If you go back thenext year following a wet spring, the soil maybe much softer. You shouldn’t conclude that yoursoil’s health has dramatically improved becauseyou really measured the effect of variable soilmoisture on soil strength. Similarly, earthwormswill be abundant in the plow layer when it’smoist, but tend to go deeper into the soil dur-ing dry periods. This type of variability with timeof year or climatic conditions should not dis-courage you from starting to evaluate your soil’shealth — just keep in mind the limitations ofcertain measurements. Also, you can take ad-vantage of the fact that soil health problems tendto be more obvious during extreme conditions.It’s a good idea to spend some extra time walk-ing your fields and digging in the soil after ex-tended wet or dry periods.

HOW GOOD ARE YOUR SOILS? 205

In the next section, we use ideas and expres-sions developed for soil health or soil qualitycards in Maryland, Oregon, and Wisconsin.

SOIL HEALTH INDICATORS

The indicators are not discussed below in anyspecial order — all are important to help youassess soil health as it relates to growing crops.

Soil testing is a very common way to assessyour soil’s health from a chemical perspective(see detailed discussion in chapter 19). Thisprovides information on potential nutrient andpH imbalances. To get the most benefit fromsoil tests, sample soils frequently and keep goodrecords. If you change soil management prac-tices, look at soil test trends and evaluatewhether your soil is improving. Evaluate whetheryour soil test values are remaining in the opti-mal range, without adding large amounts of fer-tilizers. Also, make sure that you do not end upwith excessive nutrient levels, especially phos-phorus and potassium, due to over-applicationof organic materials. If your soil test report in-cludes information on cation exchange capac-ity (CEC), you should expect it to increase withincreasing organic matter levels.

Soil color is an indicator of soil organic mat-ter content, especially within the same generaltextural class. The darkness is an indicator ofthe amount of humus (see chapter 2) in the soil.We generally associate black soils with high qual-ity. The Illinois color chart, relating color to or-ganic matter content, is proving useful in otherparts of the country. However, don’t expect dra-matic color change when you add organic mat-ter; it may take years to notice a difference.

Soil organisms such as ants, termites, andearthworms are “ecosystem engineers” that aid

the initial organic matter breakdown that allowsother species to thrive. They are easily recog-nized, and their general abundance is stronglyaffected by temperature and moisture levels inthe soil. Their presence is best assessed in mid-spring, after considerable soil warming, and inmid-fall, before the soils become cold. Springand fall assessment are practical during moist,but not excessively wet, conditions. Just take aspadeful of soil from the surface layer and siftthrough it looking for bugs and worms. If thesoil is teeming with life, this suggests that thesoil is healthy. If few invertebrates are observed,then the soil may be a poor environment forsoil life and organic matter processing is prob-ably low. Earthworms are often used as an indi-cator species of soil biological activity (see table20.1). The most common worm types, such asthe garden and red worms, live in the surfacelayer when soils are warm and moist and feedon organic materials in the soil. The longnightcrawlers dig almost vertical holes that ex-tend well into the subsoil, but they feed on resi-due at the surface. Look for the worms them-selves as well as their casts (on the surface, fornightcrawlers) and holes to assess their presence.If you dig out a square foot of soil down to 1foot depth and find 10 worms, the soil has a lotof earthworm activity.

Soil tilth and hardness can be assessed withan inexpensive penetrometer (the best tool), atile finder, a spade, or a stiff wire (like thosethat come with wire flags). Tilth characteristicsvary greatly during the growing season due totillage, packing, settling (dependent on rainfall),crop canopy closure, and travel over the field tocultivate, apply pesticides, and harvest. It is,therefore, best to assess soil hardness severaltimes during the growing season. If you do it

206 BUILDING SOILS

only once, the best time is when the soil is moist,but not too wet — it should be in the friablestate. Soil is generally considered too hard forroot growth if the penetrometer resistance isgreater that 300 psi. Note also whether the soilis harder below the plow layer. We cannot bevery quantitative with tile finders and wire, butthe soil is generally too hard when you cannoteasily push them in. If you use a spade whensoil is not too wet, evaluate how hard the soil isand also pay attention to the structure of thesoil. Is the plow layer fluffy and does it mostlyconsist of granules of about quarter-inch size?Or does the soil dig up in large clumps? A goodway to evaluate that is by lifting a spade full ofsoil and slowly turning it over. Does the soilbreak apart into granules or does it drop in largeclumps? When you dig below the plow layer,take a spade full of soil and pull the soil clumpsapart. They should generally come apart easilyin well-defined aggregates of several inches insize. If the soil is compacted, it does not easilycome apart in distinct units.

Root development also can be evaluated bydigging and is best done when the crop is in itsrapid phase of growth — generally during latespring. Have the roots properly branched andextend in all directions to their fullest potentialfor the particular crop? Look for obvious signsof problems: short stubby roots, abrupt changesin direction when hitting hard layers, signs of rotor other diseases. Make sure to dig deep enoughto get a full picture of the rooting environment.

Crusting, ponding, runoff, and erosion canbe observed from the soil surface. However, theextent of their occurrence depends on whetherthere was an intense rainstorm. The presence ofthese symptoms are a sign of poor soil health,but the lack of visible signs doesn’t necessarily

mean that the soil is in good health — it mustrain hard for signs to occur. Try to get out intothe field after heavy rainstorms, especially in theearly growing season. Crusting is recognized bya dense layer at the surface, which may becomehard after it dries. Ponding is recognized eitherdirectly when the water is still in a field depres-sion, or afterwards by small areas where the soilhas slaked (aggregates have disintegrated ). Ar-eas that were ponded often show cracks afterdrying. Slaked areas going down the slope arean indication that runoff and early erosion haveoccurred. When rills and gullies are present, asevere erosion problem is at hand. Another idea:put on your raingear and go out during a rain-storm (not during lightning, of course) and ac-tually see runoff and erosion in action. Com-pare fields with different crops, management,or soil types. This might give you ideas aboutchanges you can make to reduce runoff and ero-sion.

You also can easily get an idea about stabilityof soil aggregates, especially those near the sur-face. If the soil crusts readily, you already knowthe answer — the aggregates are not very stableand break down completely when wet. If thesoil doesn’t usually form a crust, you might takea sample of aggregates from the top 3 or 4 inchesof soil from a number of different fields that seemto have different soil quality. Gently drop a num-ber of aggregates from each field into separatecups that are half-filled with water — the ag-gregates should be covered with water. See ifthey hold up or if they break apart. You can swirlthe water in the cups to see if that helps to breakup the aggregates. Very turbid water indicatesthat the aggregates have broken down. If thewater stays fairly clear, the aggregates are verystable.

TABLE 20.1Qualitative Soil Health Indicators

BEST

INDICATOR ASSESSED POOR MEDIUM GOOD

Earthworms Spring/Fall. 0–1 worms in shovel- 2–10 in shovelful. 10+ in top foot of soil. LotsGood soil ful of top foot of soil. Few casts, holes, or of casts and holes in tilledmoisture. No casts or holes. worms. clods. Birds behind tillage.

Organic Matter Moist soil. Topsoil color similar Surface color closer Topsoil clearly defined,Color to subsoil color. to subsoil color. darker than subsoil.

Organic Matter Anytime. No visible residues. Some residues. Residues on most of soilResidues surface.

Root Health Late spring Few, thick roots. Off color (staining) Roots fully branched and(rapid growth No subsoil inside root. extended, reaching intostage). penetration. subsoil. Root exterior and

interior is white.

Subsurface Pre-tillage Wire breaks or bends Have to push hard, Flag goes in easily withCompaction or post harvest. when inserting flag. need fist to push fingers to twice the depth of

Good soil flag in. plow layer.moisture.

Soil Tilth Good soil Looks dead. Like Somewhat cloddy, Soil crumbles well, canMellowness moisture. brick or concrete, balls up, rough slice through, like cuttingFriability cloddy. Either blows pulling seedbed. butter. Spongy when you

apart or hard to pull walk on it.drill through.

Erosion After heavy Large gullies over 2 Few rills or gullies, No gullies or rills, clearrainfall. inches deep joined to gullies up to 2 or no runoff.

others, thin or no inches deep. Sometopsoil, rapid runoff swift runoff, coloredthe color of soil. water.

Water Holding After rainfall. Plant stress two days Water runs out after Holds water for a longCapacity During growing after a good rain. a week or so. period of time without

season. signs of drought stress.

Drainage After rainfall. Water lays for a long Water lays for short No ponding, no runoff,Infiltration time, evaporates more period, eventually water moves through

than drains, always drains. soil steadily. Soil not toovery wet ground. wet, not too dry.

Crop Condition Growing Problem growing Fair growth, spots in Normal healthy dark green(How well season. throughout season, field different, color, excellent growth allit grows) Good soil poor growth, yellow medium green color. season, across field.

moisture. or purple color.

pH Anytime, but at Hard to correct for Easily correctable. Proper pH for crop.same time of desired crop.year each time.

Nutrient Over a five-year Soil tests dropping Little change or slow Soil tests trending up inHolding period always into “low” category. down trend. relation to fertilizer appliedCapacity at same time of and crop harvested but not

year. into “very high” category.

—MODIFIED FROM USDA. 1997.

208 BUILDING SOILS

The effects of soil health problems on gen-eral crop performance are most obvious dur-ing extreme conditions. That’s why it is worth-while to occasionally walk your fields during awet period (when a number of rains have fallenor just after a long, heavy rain) or during anextended drought. During prolonged wet pe-riods, poor soils often remain saturated forextended periods. The lack of aeration stuntsthe growth of the crop, and leaf yellowing in-dicates loss of available N by denitrification.This may even happen with high-quality soilsif the rainfall is very excessive, but it is cer-tainly aggravated by poor soil conditions. Dense,no-tilled soil may also show greater effects.Purple leaves indicate a phosphorus deficiencyand are also often an indirect sign of stress onthe crop. This may be related to soil health, butalso can be brought on by other causes, suchas cold temperatures.

Watch for stunted crop growth during dryperiods and also look for the onset of droughtstress — leaf curling or sagging leaves (depend-ing on the crop type). Crops on soils that are ingood health generally have delayed occurrenceof drought stress. Poor soils, especially, mayshow problems when heavy rainfall, causing soilsettling after tillage, is followed by a long dry-ing period. Soils may hardset and completelystop crop growth under these circumstances. Ex-

treme conditions are good times to look at cropperformance and, at the same time, evaluate soilhardness and root growth.

Using the simple tools and observations sug-gested above, you can evaluate your soil’s health.Soil health cards or soil quality books provide aplace to record field notes and assessment in-formation to allow you to compare changes thatoccur over the years. You also can make up yourown assessment sheets.

OTHER TOOLS

More “scientific” measurements can be made:infiltration capacity, bulk density, the volume oflarge pores, soil strength, etc. However, makingthese measurements in a meaningful way is chal-lenging and you should get a soil scientist orextension agent involved if you want to pursuemore sophisticated measurements.

Soils also can be tested for their biological char-acteristics — for potentially harmful organisms(usually for specific species of nematodes) or, morebroadly, for large organisms and microbiology.Laboratories that do these types of tests are listedin Resources on p.221.

SOURCE

U.S. Department of Agriculture (USDA). 1997.Maryland Soil Quality Assessment Book.

209

In this chapter, we’ll give some guidelines onhow you can promote high quality soils by

adopting practices that maintain or increase soilorganic matter, develop and maintain optimalsoil physical conditions, and promotetop-notch nutrient management. Inearlier chapters of Part Two, we dis-cussed many different ways to man-age soils, crops, and residues, but welooked at each one as a separate strat-egy. In the real world, we need to com-bine a number of these approaches and use themtogether. In fact, each practice is related to, orimpacts, other soil heath promoting practices.The real key is to modify and combine them inways that make sense for your farm.

We hope that you don’t feel as confused asthe person on the left in the drawing on the nextpage. If the thought of making changes on yourfarm is overwhelming, you can start with only

21�

Putting It All Together

… generally, the type of soil management that gives

the greatest immediate return leads to a deterioration

of soil productivity, whereas the type that provides

the highest income over the period of a generation leads

to the maintenance or improvement of productivity.

—CHARLES KELLOGG, 1936

one or two practices that improve soil health.Not all of these suggestions are meant to be usedon every farm.

Decisions on the farm need to support theeconomic bottom line. Researchshows that the practices that improvesoil health generally also improve theeconomics of the farm, in some casesdramatically. However, you need toconsider the fact that the increased re-turns may not be immediate. After

implementing new practices, soil health mayimprove at a slow rate and it may take a fewyears to see improved yields. A “learning pe-riod” is probably needed to make the new man-agement practices work on your farm. Permityourself to make a few mistakes. Changing man-agement practices may involve an investmentin new equipment. For example, changing till-age systems requires an investment in new till-


age tools and planters, and the bottom line maynot improve immediately. For many farmers, theshort-term limitations may keep them frommaking these changes, even though they arehurting the long-term viability of the farm. Bigchanges are probably best implemented at stra-tegic times. For example, when you are readyto buy a new planter, consider a whole new ap-proach to tillage as well. Also, take advantage offlush times, when you receive high prices forproducts, to invest in new management ap-proaches. However, don’t wait until that timeto make decisions. Plan ahead, so you are readyto make the move at the right time.

GENERAL APPROACHES

There are many options for making soil man-agement changes in different types of farmingsystems. Let’s go over the general approachesthat can be used for most types of agriculture. If

at all possible, use rotations that utilize grass,legume, or a combination of grass and legumesod crops, or crops with large amounts of resi-due as important parts of the system. Leave resi-dues from annual crops in the field or, if youremoved them for composting or to use as bed-ding for animals, return them to the soil as ma-nure or compost. Use cover crops when soilswould otherwise be bare to add organic matter,capture residual plant nutrients, and reduce ero-sion. Cover crops also help maintain soil organicmatter in resource-scarce regions that lack pos-sible substitutes to using crop residues for fuelor building materials.

Raising animals or having access to animalwastes from nearby farms gives you a widerchoice of economically sound rotations. Rota-tions that include perennial forages make hayor pasture available for use by dairy and beefcows, sheep, and goats. In addition, on mixedcrop-livestock farms, animal manures can be

Reduced tillage

Rotations

Erosion control

Cover crops

Manures, composts

Nutrients

Prevent compaction

Manures, compost

Cover crops

Erosion control

Rotations

Reduced tillage

Nutrients

PUTTING IT ALL TOGETHER 211

applied to cropland. It’s easier to maintain or-ganic matter on a diversified crop-and-livestockfarm, where sod crops are fed to animals andmanures returned to the soil. However, grow-ing crops with high quantities of residues plusfrequent use of green manures and compostsfrom vegetative residues helps maintain soil or-ganic matter even without animals.

You can maintain or increase soil organic mat-ter more easily when you use reduced-tillage sys-tems, especially no-till, instead of the conven-tional moldboard plow and disk system. Thedecreased soil disturbance under reduced tillageslows the rate of organic matter decompositionand helps to maintain a soil structure that allowsrainfall to infiltrate rapidly. Leaving residue onthe surface encourages the development of earth-worm populations, which also improves soilstructure. Compared with conventional tillage,

It’s the Combination…

Farmers are learning that the combination ofreduced tillage, cover crops, and better ro-tations can have a dramatic effect on theirsoil and the health of crops. They are findingthat, by combining practices, they are reduc-ing pest damage, improving soil tilth, vastlyreducing runoff and erosion, increasing soilorganic matter, and producing better cropgrowth. Each practice by itself is worthwhile.However, the greatest strengths and benefitsare derived from combining a number of keypractices.

For example, on the Groff farm (p. 145–146),it’s the combination of good rotations, inte-grating livestock with crops, using no-till andcover crops that all work together to producehigh-quality soil and crops.

soil erosion is greatly reduced under minimum-tillage systems, which helps keep the organic mat-ter and rich topsoil in place. Any practice thatreduces soil erosion, such as contour tillage, strip-cropping along the contours, and terracing, alsohelps maintain soil organic matter.

Even if you use minimum-tillage systems toleave significant quantities of residue on thesurface and decrease the severity of erosion, youalso should use sound crop rotations. In fact, itmay be more important to rotate crops whenlarge amounts of residue remain on the surface.Decomposing residues harbor many insect anddisease organisms. These problems may beworse in monoculture with no-till practices thanwith conventional tillage.

Test your soils regularly and apply lime andfertilizers only when they are needed. Testingsoils every two or three years on each field isone of the best investments you can make. Makesure that you properly credit the N contribu-tion of a decomposing sod or the N, P, and Kcontributions from manures. If you keep thereport forms, or record the results, you will beable to follow the fertility changes over the years.Monitoring soil test changes will help you fine-tune your practices. Soil testing laboratoriesusually charge extra for an organic matter de-termination, but it’s worth the money every fewyears just to track changes. Also, if you’re inter-ested in soil microorganisms, there is now a labo-ratory that can help you. In dry areas, salt accu-mulation may be a problem. You may need touse gypsum or other leaching salts. Also, main-tain your pest scouting efforts and keep recordsover the years. This allows you to evaluate im-provements in this area.

There is no substitute for taking a little timeeach year to observe your soils for such things


as indications of compaction, the presence ofearthworms, the health of roots or other indica-tors we discussed in chapter 20. The saying “Thefarmer’s footprint is the best fertilizer” can bemodified to “The farmer’s footprint is the bestpath to improved soil health.” If you don’t al-ready, begin to regularly observe and record thevariability in crop yield across your fields. Asequipment changes are made, you might con-sider buying a yield monitor that allows you totrack yields on a field. Or, simply take the timeto track production from the various sections ofyour fields that seem different. Compare yourobservations with your soil sampling plan, soyou can be sure that the various areas within afield are receiving optimum management. Per-haps the hilltop or sideslope would benefit fromadditional manure or compost, while none isneeded in other portions of the field.

WHAT MAKES SENSEON YOUR FARM?

What makes sense on any individual farmdepends on the soils, the climate, the nature ofthe farm enterprise itself and the surroundingregion, potential markets, and the family’s needsand goals. The specific details of implementinggeneral management approaches depend prima-rily on the type of farm enterprise: grain or veg-etable crops only, integrated crop-livestock, or-ganic or not, etc.

Most grain crop farms export a lot of nutri-ents and are managed with a net loss of organicmatter. However, these farms provide a great dealof flexibility in adopting alternative soil man-agement systems because there is a wide rangeof equipment available for grain production sys-tems. You can promote soil health easily with

reduced-tillage systems, especially no-till andzone-till, instead of the conventional moldboardplow and disk system. Well-drained, coarse-tex-tured soils are especially well adapted to no-tilland zone-till systems, and the finer-textured soilsdo well with ridge-tillage or zone-tillage systems.Regardless of the tillage system that is used, youshould try to travel on soils only when they’redry enough to resist compaction. However,managing no-till cropping on soils that are eas-ily compacted is quite a challenge because thereare few options to relieve compaction once itoccurs. Maintaining controlled traffic zones orusing some tillage to break up compacted lay-ers may be necessary on such soils.

Even if you use minimum-tillage systems thatleave significant quantities of residue on thesurface and decrease the severity of erosion, youalso should use sound crop rotations. Considerrotations that utilize grass, legume, or a combi-nation of grass and legume perennial foragecrops. Raising animals on what previously wereexclusively crop farms, cooperating on rotationsand manure management with a nearby farm,or growing forage crops for sale to a beef or dairyfarm gives you a wider choice of economicallysound rotations and at the same time helps tocycle nutrients better. Incorporating these in-novations into a conventional grain farm oftenrequires investment in new equipment and cre-atively looking for new markets for your prod-ucts. There also are many opportunities to usecover crops on grain farms, even in reduced till-age systems.

Organic grain crop farms do not have the flex-ibility in soil management that conventionalfarms have. Tillage choices are limited becauseof the reliance on mechanical methods insteadof herbicides to control weeds. On the positive

PUTTING IT ALL TOGETHER 213

side, organic farms already rely heavily on or-ganic inputs through green and animal manuresand composts to provide adequate nutrients totheir crops. A well-managed organic farm usu-ally uses many aspects of ecological soil man-agement. However, erosion may remain a con-cern because many organic farms use clean andintensive tillage. It is important to think aboutreducing tillage intensity and perhaps invest ina better planter. New mechanical cultivators cangenerally handle higher residue and mulch lev-els and may still provide adequate weed con-trol. Try to look into ways to increase surfacecover, although this is a challenge without theuse of chemical weed control. Alternatively, youshould consider conventional erosion controlpractices, such as strip cropping, as they workwell with rotations involving sod and covercrops.

corporate manure requires at least some type oftillage. You should still consider minimizing till-age by trying to inject the manure or chiselingit in, rather than plowing it under. Also, mini-mize soil pulverization by reducing secondarytillage and establishing the crops with no-till-age (or zone-tillage) planters.

Preventing soil compaction is important onmany livestock-based farms. Manure spread-ers are typically heavy and frequently go overthe land at very unfavorable times, doing a lotof compaction damage. Think about ways tominimize this. In the spring, allow the fieldsto dry adequately (do the ball test) before tak-ing spreaders out. If there is no manure stor-age, building a structure to hold it temporarilyallows you to avoid the most damaging soilconditions.

Livestock farms require special attention tonutrient management, making sure that the or-ganic nutrient sources are optimally used aroundthe farm and that no negative environmentalimpacts occur. This requires a comprehensivelook at all nutrient flows on the farm, findingways to most efficiently use them, and prevent-ing problems with excesses.

Soil quality management is especially diffi-cult on vegetable farms. Many vegetable cropsare sensitive to soil compaction and often posegreater challenges in pest management. Thesecropping systems, therefore, can greatly benefitfrom improved soil health. Most vegetable farmsare not integrated with livestock production, andit is difficult to maintain a continuous supply offresh organic matter. Bringing manure, compost,or other locally available sources of organic ma-terials to the farm should be seriously consid-ered. In some cases, vegetable farms can eco-nomically use manure from nearby livestock

Diversified crop-and-livestock farms

have an inherent advantage for

improving soil health.

Diversified crop-and-livestock farms have aninherent advantage for improving soil health.Crops can be fed to animals and manures re-turned to the soil, thereby providing a continu-ous supply of organic materials. For many live-stock operations, perennial forage crops are alogical part of the cropping system, thereby re-ducing erosion potential and improving soilphysical properties. Livestock-based farms alsohave some disadvantages. It is more difficult toadopt minimum tillage practices when sod cropsare rotated with row crops, and the need to in-


operations or swap land with them in a rota-tion. Farms near urban areas may benefit fromleaves and grass clippings and municipal or foodwaste composts, which are increasingly becom-ing available. In such case, care should be takento insure that the compost does not contain con-taminants.

Vegetable cropping systems are generally welladapted to the use of cover crops because themain cropping season is generally shorter thanthose for grain and forage crops. There is usu-ally sufficient time for growth of cover crops inthe pre- or post-season to gain real benefits, evenin colder climates. Using the cover crop as amulch (or importing mulch materials from offthe farm) appears to be a good system for cer-tain fresh market vegetables, as it keeps the cropfrom direct contact with the ground, therebyreducing the potential for rot or disease.

The need to harvest crops during a very shortperiod before quality declines — regardless ofsoil conditions — often results in severe com-paction problems on large vegetable farms us-ing large-scale equipment. Controlled trafficsystems, including permanent beds, should be

given serious consideration. Limiting compac-tion to narrow lanes and using other soil-build-ing practices in between them is the best way toavoid severe compaction damage under thoseconditions.

THE FUTURE

Each of the farming systems discussed above hasits limitations and opportunities for buildingbetter soils. Although there are ways to improvesoil health in any system, the details may differ.Whatever crops you grow, when you creativelycombine a reasonable number of practices thatpromote high quality soils, most of your farm’ssoil fertility problems should be solved alongthe way. The health and yield of your cropsshould improve. The soil will have more avail-able nutrients, more water for plants to use, andbetter tilth. There should be fewer problems withdiseases, nematodes, and insects. By concentrat-ing on the practices that build high quality soils,you also will leave a legacy of land stewardshipfor your children and their children to inheritand follow.

215

Glossary

Alkaline soil. A soil with a pH above 7, contain-ing more base than acid.

Allelopathic effect. The effect that some plantshave in suppressing the germination orgrowth of other plants. The chemicals respon-sible for this effect are produced during thegrowth of a plant or during the decomposi-tion of its residues.

Acid. A solution containing free hydrogen ions(H+) or a chemical that will give off hydro-gen ions into solution.

Acidic soil. A soil that as a pH below 7. Thelower the pH, the more acidic is the soil.

Aggregates. The structures, or clumps, formedwhen soil minerals and organic matter arebound together with the help of organic mol-ecules, plant roots, fungi, and clays.

Anion. A negatively charged element or moleculesuch as chloride (Cl-) or nitrate (NO3

-).Ammonium (NH4

+). A form of nitrogen that is

available to plants and is produced in theearly stage of organic matter decomposition.

Available nutrient. The form of a nutrient that aplant is able to use. Nutrients that the plantneeds are commonly found in the soil in formsthat the plant can’t use (such as organic formsof nitrogen) and must be converted into formsthat the plant is able to take into the roots anduse (such as the nitrate form of nitrogen).

Ball test. A simple field test to determine soilreadiness for tillage. A handful of soil is takenand squeezed into a ball. If the soil moldstogether, it is in the plastic state and too wetfor tillage or field traffic. If it crumbles, it isin the friable state.

Base. Something that will neutralize an acid,such as hydroxide or limestone.

Beds. Small hilled-up, or raised, zones wherecrops (usually vegetables) are planted. Theyprovide better-drained and warmer soil con-

216 BUILDING SOILS

ditions. Similar to ridges, but are generallybroader, and are usually shaped after con-ventional tillage has occurred.

Buffering. The slowdown or inhibition ofchanges. A substance that has the ability tobuffer a solution is also called a buffer. Buff-ering can slow down pH changes by neutral-izing acids or bases.

Bulk density. The mass of dry soil per unit vol-ume. It is an indicator of the compactness ofthe soil.

Calcareous soil. A soil in which finely dividedlime is naturally distributed, usually has a pHfrom 7 to slightly more than 8.

Cation. A positively charged ion such as cal-cium (Ca++) or ammonium (NH

4+).

Cation exchange capacity (CEC). The amountof negative charge that exists on humus andclays, allowing them to hold onto positivelycharged chemicals (cations).

Chelate. A molecule that uses more than onebond to attach strongly to certain elementssuch as iron (Fe++) and zinc (Zn++). These el-ements may later be released from the che-late and used by plants.

Coarse textured. Soil dominated by large min-eral particles (sand size). May also includegravels. Previously called “light soil.”

Colloid. A very small particle, with a high sur-face area, that can stay in a water suspensionfor a very long time. The colloids in soils, theclay and humus molecules, are usually foundin larger aggregates and not as individual par-ticles. These colloids are responsible for manyof the chemical and physical properties ofsoils, including cation exchange capacity, che-lation of micronutrients, and the develop-ment of aggregates.

Compost. Organic material that has been welldecomposed by organisms under conditions

of good aeration and high temperature, of-ten used as a soil amendment.

Controlled traffic. Field equipment is restrictedto limited travel or access lanes in order toreduce compaction on the rest of the field.

Conventional tillage. Preparation of soil forplanting by using the moldboard plow fol-lowed by disking or harrowing. It usuallybreaks down aggregates, buries most crop resi-dues and manures, and leaves the soil smooth.

Coulter. A fluted or rippled disk mounted onthe front of a planter to cut surface crop resi-dues and perform minimal soil looseningprior to seed placement. Multiple coultersare used on zone-till planters to provide awider band of loosened soil.

Cover crop. A crop grown for the purpose ofprotecting the soil from erosion during thetime of the year when the soil would other-wise be bare. It is sometimes called a greenmanure crop.

Crumb. A soft, porous, more-or-less round soilaggregate. Generally indicative of good soil tilth.

Crust. A thin, dense layer at the soil surface thatbecomes hard upon drying.

C:N ratio. The amount of carbon in a residuedivided by the amount of nitrogen. A highratio results in low rates of decompositionand can also result in a temporary decreasein nitrogen nutrition for plants, as micro-or-ganisms use much of the available nitrogen.

Deep tillage. Tillage practices that loosen thesoil at greater depths (usually greater than12 inches) than during regular tillage.

Disk. An implement for harrowing, or breakingup, the soil. It is commonly used following amoldboard plow, but is also used by itself tobreak down aggregates, help mix fertilizersand manures with the soil, and smooth thesoil surface.

GLOSSARY 217

Drainage. The process of soil water loss by per-colation through the profile as a result of thegravitational force. Also: Removal of excesssoil water through the use of channels, ditches,soil shaping and subsurface drain pipes.

Element. All matter is made up of elements, sev-enteen of which are essential for plant growth.Elements such as carbon, oxygen, and nitro-gen combine to form larger molecules.

Erosion. Wearing away of the land surface byrunoff water (water erosion), wind shear(wind erosion) or tillage (tillage erosion).

Field capacity. Water content of a soil follow-ing drainage by gravity.

Fine textured. Soils dominated by small min-eral particles (silt and clay). Previously called“heavy soil.”

Friable. Consistency status when soil crumblesinstead of being molded when a force is ap-plied.

Frost tillage. Tillage practices performed whena shallow (2–4 inches) frozen layer exists atthe soil surface.

Full-field (full-width) tillage. Any tillage systemthat results in soil loosening over the entirewidth of the tillage pass. For example, mold-board plowing, chisel tillage, and disking.

Green manure. A crop grown for the main pur-pose of building up or maintaining soil or-ganic matter. It is sometimes called a covercrop.

Heavy soil. Nowadays usually called “fine tex-ture” soil, it contains a lot of clay and is usu-ally more difficult to work than coarser texturesoil. It normally drains slowly following rain.

Humus. The very well decomposed part of thesoil organic matter. It has a high cation ex-change capacity.

Infiltration. The process of water entering intothe soil at the surface.

Inorganic. Chemicals that are not made fromchains or rings of carbon atoms (for example,soil clay minerals, nitrate, and calcium).

Least-limiting water range. See optimum wa-ter range.

Legume. A group of plants including beans, peas,clovers, and alfalfa that forms a symbiotic re-lationship with nitrogen-fixing bacteria livingin their roots. These bacteria help to supplyplants with an available source of nitrogen.

Lignin. A substance found in woody tissue andin stems of plants that is difficult for soil or-ganisms to decompose.

Lime, or limestone. A mineral that can neutral-ize acids and is commonly applied to acidsoils, consisting of calcium carbonate (CaCO3).

Loess soils. Soils formed from windblown de-posits of silty and fine-sand-size minerals.They are easily eroded by wind and water.

Micronutrient. An element, such as zinc, iron,copper, boron, and manganese, needed byplants only in small amounts.

Microorganism. Very small and simple organ-ism such as bacteria and fungi.

Mineralization. The process by which soil or-ganisms change organic elements into the“mineral” or inorganic form as they decom-pose organic matter (for example, organicforms of nitrogen are converted to nitrate).

Moldboard plow. A commonly used plow thatcompletely turns over the soil and incorpo-rates any surface residues, manures, or fer-tilizers deeper into the soil.

Monoculture. Production of the same crop inthe same field year after year.

Mycorrhizal relationship. The mutually benefi-cial relationship that develops between plantroots of most crops and fungi. The fungi helpplants obtain water and phosphorus by act-ing like an extension of the root system and

218 BUILDING SOILS

in return receive energy-containing chemi-cal nutrients from the plant.

Nitrate (NO3-). The form of nitrogen that is most

readily available to plants. It is the nitrogenform normally found in the greatest abun-dance in agricultural soils,

Nitrogen fixation. The conversion of atmo-spheric nitrogen by bacteria to a form thatplants can use. A small number of bacteria,which include the rhizobia living in the rootsof legumes, are able to make this conversion.

Nitrogen immobilization. The transformation ofavailable forms of nitrogen, such as nitrateand ammonium, into organic forms that arenot readily available to plants.

No-till. A system of planting crops without till-ing the soil with a plow, disk, chisel, or othertillage implement.

Optimum tillage water range. The range of soilwater contents in which plants do not expe-rience stress from drought, high soil strength,or lack of aeration.

Organic. Chemicals that contain chains or ringsof carbon connected to one another. Most ofthe chemicals in plants, animals, microorgan-isms, and soil organic matter are organic.

Oxidation. The combining of a chemical suchas carbon with oxygen, usually resulting inthe release of energy.

Penetrometer. A device measuring soil resistanceto penetration, an indicator of the degree ofcompaction. Includes a cone-tipped metalshaft that is slowly pushed into the soil whilethe resistance force is measured.

Perennial forage crops. Crops such as grasses,legumes, or grass/legume mixtures that formcomplete soil cover (sods) and are grown forpasture or to make hay and haylage for ani-mal feed.

pH. A way of expressing the acid status, or hy-drogen ion (H+) concentration, of a soil or asolution on a scale where 7 is neutral, lessthan 7 is acidic, and greater than 7 is basic.

Photosynthesis. The process by which greenplants capture the energy of sunlight anduse carbon dioxide from the atmosphere tomake molecules needed for growth and de-velopment.

Plastic. State of a soil when it molds easily whena force is applied. Compare to friable.

Plastic limit. Water content of soil at the transi-tion from the plastic to the friable state. Up-per limit of soil moisture where tillage andfield traffic do not result in excessive com-paction damage.

Polyculture. Growth of more than one crop in afield at the same time.

PSNT. The Pre-Sidedress Nitrate Test is a soiltest for nitrogen availability where the soil issampled to 1 ft depth during the early cropgrowth.

Respiration. The biological process that allowsliving things to use the energy stored in or-ganic chemicals. In this process, carbon di-oxide is released as energy is made availableto do all sorts of work.

Restricted tillage. Any tillage system that in-cludes only limited and localized soil distur-bance in bands where plant rows are to beestablished. For example, no-till, zone-till,strip-till and ridge-till. Compare with full-field tillage.

Rhizobia bacteria. Bacteria that live in the rootsof legumes and have a mutually beneficial re-lationship with the plant. These bacteria fixnitrogen, providing it to the plant in an avail-able form, and in return receive energy-richmolecules that the plant produces.

GLOSSARY 219

Ridge tillage. Crops are planted on top of a smallridge (usually 2–4 inches height) which isgenerally re-formed annually with a specialcultivator.

Rotation effect. The crop-yield benefit from rota-tions that includes better nutrient availability,fewer pest problems, and better soil structure.

Runoff. Water lost by flow over the soil surface.Saline soil. A soil that contains excess free salts,

usually sodium and calcium chlorides.Saturation, soil. When all soil pores are water-

filled (a virtual absence of soil air).Silage. A feed produced when chopped-up corn

plants or wilted hay are put into air-tight stor-age facilities (silos) and partially fermentedby bacteria. The acidity produced by the fer-mentation and the lack of oxygen help pre-serve the quality of the feed during storage.

Slurry (manure). A manure that is between solidand liquid. It flows slowly and has the con-sistency of something like a very thick soup.

Sod crops. Grasses or legumes such as timothyand white clover that tend to grow very closetogether and form a dense cover over the en-tire soil surface.

Sodic soil. A soil containing excess amounts ofsodium. If it is not also saline, clay particlesdisperse and the soil structure may be poor.

Strip cropping. Growing two or more crops inalternating strips, usually along the contour orperpendicular to the prevailing wind direction.

Structure. The physical condition of the soil. Itdepends upon the amount of pores, the ar-rangement of soil solids into aggregates, andthe degree of compaction.

Texture. A name that indicates the relative sig-nificance of a soil’s sand, silt, and clay con-tent. The term “coarse texture” means that a

soil has a high sand content, while “fine tex-ture” means that a soil has a high clay content.

Thermophilic bacteria. Bacteria that live andwork best under high temperatures, around110° to 140° F. They are responsible for themost intense stage of decomposition that oc-curs during composting.

Tillage. The mechanical manipulation of soil,generally for the purpose of creating soilloosening, a seedbed, controlling weeds, andincorporation of amendments. Primary till-age (moldboard plowing, chiseling) is amore rigorous practice, primarily for soilloosening and incorporation of amendments.Secondary tillage (disking, harrowing) is aless rigorous practice following primary till-age that creates a seedbed containing fineaggregates.

Tillage erosion. The downslope movement of soilthrough the action of tillage implements.

Tilth. The physical condition, or structure, ofthe soil as it influences plant growth. A soilwith good tilth is very porous and allows rain-fall to infiltrate easily, permits roots to growwithout obstruction, and is easy to work.

Wilting point. The moisture content when a soilcontains only water that is too tightly held tobe available to plants.

Zone tillage. A restricted tillage system that es-tablishes a narrow (4–6 inches width) bandof loosened soil with surface residues re-moved. This is accomplished using multiplecoulters and row cleaners as attachments ona planter. May also include a separate “zone-building” practice that provides deep, nar-row ripping without significant surface dis-turbance. A modification of no tillage, gen-erally better adapted to cold and wet soils.

221

Resources

General Information Sources

ATTRA (Appropriate Technology Transfer forRural Areas), the sustainable farming informa-tion center funded by the U.S. Department ofAgriculture, provides assistance, publicationsand resources, including sustainable soil man-agement, cover crops and green manures, farm-scale composting, and nutrient cycling in pas-tures, free of charge. P.O. Box 3657, Fayetteville,AR 72702; (800) 346-9140; www.attra.org

The Alternative Farming Systems Informa-tion Center (AFSIC) of USDA’s National Agri-cultural Library compiles bibliographies andresource lists on topics of current interest, suchas Soil Organic Matter: Impacts on Crop Produc-tion QB 91-24, Compost: Application and Use, QB97-01, Legumes in Crop Rotations, QB 94-38, andDairy Farm Manure Management, QB 95-02.National Agricultural Library, 10301 Baltimore

Ave., Beltsville, MD, 20705-2351; (301) 504-6559; www.nal.usda.gov/afsic

How to Conduct Research on Your Farm orRanch, an informational bulletin from the Sus-tainable Agriculture Network (SAN), providespractical tips for laying out a research trial. On-farm research can help you evaluate new prac-tices such as fertilizer rates or cover crop spe-cies. Available from SARE (see next page) or at:www.sare.org/san/htdocs/pubs/

Most state Cooperative Extension offices pub-lish leaflets and booklets on manures, soil fer-tility, cover crops, and other subjects describedin this book. Request a list of publications fromyour county extension office. A number of statesalso have sustainable agriculture centers thatpublish newsletters.

USDA’s Sustainable Agriculture Research andEducation (SARE) program studies and spreadsinformation about sustainable agriculture via a


nationwide grants program. SARE funds publi-cations through its Sustainable Agriculture Net-work (SAN), and maintains a database of morethan 1,600 projects. For information about pub-lications, funded projects and how to apply fora grant, call (301) 405-3186 or visit www.sare.org.

The Sustainable Farming Connection website— managed by former staff members of The NewFarm magazine — offers practical informationto farmers through a diverse collection of re-sources and web links on soil health, covercrops, composts, and related topics. metalab.unc.edu/farming-connection/soilhlth/home.htm

The “Soil Biology Primer” presents an intro-duction to the living soil system for natural re-source specialists, farmers, and others. This setof eight units describes the importance of soilorganisms and the soil food web to soil produc-tivity and water and air quality. 1-888-LAND-CARE; landcare@swcs. org.

Manures, Fertilizers, Tillage,and Rotations

Best Management Practices Series: Soil Manage-ment, Nutrient Management, and No-Till.Ontario Ministry of Agriculture and Rural Af-fairs with Agriculture and Agri-Food Canada.Provides practical information on these sub-jects to farmers and crop advisers. Availablefrom Ontario Federation of Agriculture, Attn.Manager, BMP, 40 Eglinton Ave. E., 5th Floor,Toronto, Ontario, M4P 3B1, Canada.

“Crop Rotations in Sustainable Production Sys-tems.” Francis, C.A., and M.D. Clegg. 1990.pp. 107–122. In Sustainable Agricultural Sys-tems (C.A. Edwards, R. Lal, P. Madden, R.H.Miller, and G. House, eds.). Soil and WaterConservation Society, 7515 NE Ankeny Road,

Ankeny, Iowa, 50021; (515) 289-2331;www.swcs.org/f_publications.htm

The Farmer’s Fertilizer Handbook. Cramer, Craig,and the editors of The New Farm. 1986. Re-generative Agriculture Association. Emmaus,PA. This handbook contains lots of very goodinformation on soil fertility, soil testing, useof manures, and use of fertilizers.

Fertile Soil: A Growers Guide to Organic and Inor-ganic Fertilizers. Parnes, R. 1990. FertileGround Books, P.O. Box 2008, Davis, CA95617; 800-540-0170.

Michigan Field Crop Ecology: Managing biologicalprocesses for productivity and environmentalquality. 1998. Cavigelli, M.A., S.R. Deming,L.K. Probyn, and R. R. Harwood (eds.). Michi-gan State University Extension Bulletin E-2646. East Lansing, MI.

No-Till Vegetables: A Sustainable Way to IncreaseProfits, Save Soil and Reduce Pesticides.SteveGroff. This video covers the basics of sustain-able no-till vegetable production, detailingmethods to control weeds and improve soilusing cover crops or plant residue on his 175-acre Cedar Meadow Farm. $21.95 plus $3 s/hto Cedar Meadow Farm, 679 Hilldale Rd.,Holtwood, PA 17532; (717) 284-5152. www.cedarmeadowfarm .com

Soil Fertility and Organic Matter as Critical Com-ponents of Production Systems. Follett, R. F., J.W. B. Stewart, and C. V. Cole (eds.). 1987.SSSA Special Publication No. 19. Soil ScienceSociety of America, American Society of Agro-nomy. Madison, WI.

Soils for Management of Organic Wastes and Waste-waters. Elliott, L.F., and F. J. Stevenson (eds.).1977. Soil Science Society of America. Madi-son, WI.

Soil Management for Sustainability. Lal, R., and F.J. Pierce (eds.). 1991. Soil and Water Con-

RESOURCES 223

servation Society, 7515 NE Ankeny Road, An-keny, Iowa, 50021; (515) 289-2331; www.swcs.org/f_publications.htm

USDA Natural Resources Conservation Service/SoilQuality Institute - Agronomy Technical NotesSeries. The NRCS technical note series pro-vides an excellent introduction to cover crops,effect of conservation crop rotation on soilquality, effects of residue management & no-till on soil quality, legumes and soil quality,and related topics. Free from NRCS Soil Qual-ity Institute, 2150 Pammel Drive, Ames, IA50011; (515) 294-4592. www.statlab.iastate.edu/survey/SQI/agronomy.shtml

Soils, Soil Organisms, and Composting

Ecology of Compost. Dindal, D. 1972. Office ofNews and Publications, 122 Bray Hall, SUNYCollege of Environmental Science and For-estry, 1 Forestry Drive, Syracuse, NY, 13210-2778; (315) 470-6644.

Effects of Conversion to Organic Agricultural Prac-tices on Soil Biota. Werner, M.R., and D.L. Din-dal. 1990. American Journal of AlternativeAgriculture 5(1):24-32.

The Field Guide to On-Farm Composting. Dough-erty, M. (ed.). 1999. NRAES-114. Natural Re-source, Agriculture, and Engineering Service.Ithaca, NY. NRAES, 152 Riley Robb Hall,Cooperative Extension, Ithaca, NY, 14853-5701. www.nraes.org

NRCS Soil Quality Website. The Soil Quality In-stitute identifies soil quality research findingsand practical technologies that help conserveand improve soil, and enhance farming,ranching, forestry, and gardening enterprises.USDA-NRCS Soil Quality Institute, 2150Pammel Drive, Ames, Iowa, 50011; 515-294-4592; www.statlab.iastate.edu/survey/SQI/

The Nature and Properties of Soils. 12th ed. Brady,N.C., and R.R. Weil. 1999. Macmillan Pub-lishing Co. New York, NY.

On Farm Composting. Rynk, R. (ed.). 1992. NRAES-54. Natural Resource, Agriculture, and Engi-neering Service. Ithaca, NY. Contact NRAES,152 Riley Robb Hall, Cooperative Extension,Ithaca, NY 14853-5701 or www. nraes.org

The Pedosphere and Its Dynamics: A Systems Ap-proach to Soil Science. University of Alberta,Canada. An award-winning website on soilscience, www.pedosphere.com

Phytohormones in Soils: Microbial Production andFunction. Frankenberger, Jr., W.T., and M. Ars-had. 1995. Marcel Dekker, Inc. New York, NY.

The Rodale Book of Composting: Easy Methods forEvery Gardener. Martin, D.L., and G. Gershuny(eds.). 1992. Rodale Press. Emmaus, PA.

Soil Microbiology: An Exploratory Approach. Coyne,M.S. 1999. Delmar Publishers. Albany, NY.

Soil Microbiology and Biochemistry. Paul, E.A., andF.E. Clark. 1989. Academic Press. San Diego,CA.

Cover Crops

University of California’s SAREP (SustainableAgriculture Research and Education Program)The UC-SAREP Cover Crops Resource Pageprovides access to a host of on-line and in-print educational materials, including thevery informative UC-SAREP Cover Crop Da-tabase. www.sarep.ucdavis.edu/ccrop/

Cover Crops for Clean Water. Hargrove, W.L.(ed.). 1991. Soil and Water ConservationSociety. 7515 NE Ankeny Road, Ankeny,Iowa, 50021; 515-289-2331; www. swcs.org/f_publications.htm

Green Manuring Principles and Practices. Pieters,A.J. 1927. John Wiley & Sons. New York, NY.


An oldie but goody. This is an out-of-printbook that some readers may enjoy siftingthrough. It can be located in college librar-ies, or borrowed through Inter-Library Loan.

Managing Cover Crops Profitably, 2nd Edition.1998. Sustainable Agriculture Network,Handbook Series, No. 3. USDA SustainableAgriculture and Education Program. An ex-cellent source of practical information aboutcover crops. $19 plus $3.95 s/h to Sustain-able Agriculture Publications, Rm. 10, HillsBldg., University of Vermont, Burlington, VT05405-0082; www.sare.org

Northeast Cover Crop Handbook. Sarrantonio, M.1997. Soil Health Series, Rodale Institute.Kutztown, PA.

The Role of Legumes in Conservation Tillage Sys-tems. Power, J.F. (ed.). 1987. Soil & WaterConservation Society, 7515 NE Ankeny Road,Ankeny, Iowa, 50021; 515-289-2331; www.swcs.org/f_publications.htm

Dynamics and Chemistryof Organic Matter

Building Soils for Better Crops. 1st Edition. Mag-doff, F. 1992. University of Nebraska Press,Lincoln, NE. The last two chapters of the firstedition contain information on the chemis-try and dynamics of soil organic matter.

Humic, Fulvic, and Microbial Balance: Organic SoilConditioning. Jackson, William R. 1993. Jack-son Research Center. Evergreen, CO.

Humus Chemistry: Genesis, Composition, Reactions.

2nd Edition. Stevenson, F.J. 1994. Wiley &Sons. New York, NY.

“Soil carbon dynamics and cropping practices.”Lucas, R.E., J.B. Holtman, and J.L. Connor.1977. pp. 333–351. In Agriculture and En-ergy (W. Lockeretz, ed.). Academic Press. NewYork, NY.

Soil Organic Matter. Schnitzer, M., and S.U. Kahn(eds.). 1978. Developments in Soil Science8. Elsevier Scientific Publishing Co. Amster-dam, Holland.

“Soil organic matter and its dynamics.” Jenkin-son, D.S. 1988. pp. 564–607. In Russell’s SoilConditions and Plant Growth (A. Wild, ed.).John Wiley & Sons. New York, NY.

Soil Testing Laboratories

Most state land grant universities have soil test-ing laboratories. A number of commercial labo-ratories (such as Brookside and A&L Laborato-ries) also perform routine soil analyses. TheATTRA publication, Alternative Soil Testing Labo-ratories, is available on line (www. attra. org/attra-pub/soil-lab.html) as well as in print.

Publications

Soil Testing: Prospects for Improving Nutrient Rec-ommendations. Havlin, J.L., et al. (eds). 1994.Soil Science Society of America. Madison, WI.

Soil Testing: Sampling Correlation, Calibration, andInterpretation. Brown, J.R., T.E. Bates, andM.L. Vitosh. 1987. Special Publication 21. SoilScience Society of America. Madison, WI.

225

Aacidity, 27, 58, 148, 169–173,

187, 207aluminum solubility and, 28bacteria and, 15crop rotations and, 108manures, use of, 81sludges, use of, 72soil conditions, effect on, 35

aeration, 9, 22, 25, 41–44, 64,208. See also compaction

of compost, 112–113cover crops, effect of, 97decomposition rate and, 34earthworms and, 18

aggregates/aggregation, 9, 10, 26,44, 206

compaction and, 48–49, 125–126, 128, 136

erosion and, 75rainfall, dispersal by, 47–48residues and, 68–69soil organisms and, 10, 17tillage and, 36, 121, 136–137,

140

Index

alfalfa, 15–16, 86, 145, 172compaction and, 130–131as cover crop, 89–91in rotations, 37, 85, 102–105,

161, 164algae, 9, 17allelopathic effects, 95aluminum, 9, 25, 28ammonia, 83, 164ammonium, 23–24, 30, 31, 70,

78, 82animals, 9–10, 17–19. See also

livestock farms

Bbacteria, 4, 9, 14–16, 20, 39, 44, 84,

99. See also nitrogen fixationbarley, 89, 102berseem clover, 89–90biological diversity, 74, 102buckwheat, 89, 92, 95, 105bulking materials, 112

Ccabbage, 52, 92, 103, 105

calcium, 9, 17, 23, 24, 58–59,72, 75, 79, 108, 126, 148,149, 168, 175, 187

carbon, 23. See also C:N ratioin compost, 110–112inorganic/organic, 11

carbon cycle, 28–29carbon dioxide, 71, 115

atmosphere, buildup in, 28–29as plant carbon source, 23respiration, production during,

9, 11, 13, 42soil aeration and, 9, 25

carrots, 84, 105catch crops. See cover cropscation exchange capacity (CEC),

24, 169–170, 175, 180–181

CEC. See cation exchangecapacity

cereal rye. See ryechelates, 25, 115, 149, 168chisel tillage, 137–138, 140clay, 9, 34, 49, 169, 173, 175clay soils, 11, 24, 41–43, 64–65,


clay soils continued86, 130, 155, 172

clovers, 15, 101, 102, 108, 161.See also specific clovers

C:N ratio, 69–70, 91, 110–112compaction, 4, 40, 44, 63, 136,

138, 207, 212, 213cover crops and, 88, 97

load distribution, effect of,129–130

plow layer and subsoil com-paction, 126–133, 148

preventing/lessening, 6, 11,25–26, 125–133

roots and, 19tillage practices and, 141–144traffic on soil, effect of, 129,

132–133types of, 47–52water range for, 54

compost, 59, 64–66, 68–69, 73,105, 109–119, 147, 152,211

advantages, 115–116animals, composting of, 112C:N ratio, 110–112compaction and, 132curing stage, 114diseases suppressed by, 114,

115making, 110–114manures, 78, 109, 116–118,

151moisture in, 110, 111, 113pile size, 112soil biodiversity, effect on, 74starting materials, 110–112turning pile, 112–113use of, 115

controlled traffic, 129, 132–133corn, 86

continuously grown, 120cover crops mixed with, 145grain, 57, 65–66, 72manure application rates, 80,

82, 83nitrogen for, 80, 82, 83, 159residue from, 72in rotations, 99–105

corn continuedsilage, 37, 38, 65, 72, 80, 132stalk rot, 148sweet, 52, 97

cotton, 99cover crops, 15–16, 65, 73, 87–

97, 126, 136, 145, 210,212. See also specific crops

compaction and, 130–131effects of, 88for erosion reduction, 122incorporation of, 131intercrops, 95–96mixtures of, 92mulching of, 131, 142mycorrhizal fungi and, 74nutrients from, 75, 86, 148,

160, 163–164, 199residue from, 64, 67–69in rotations, 37–38, 103,

107–108selection of, 88–89soil biodiversity, effect on, 74timing growth of, 92–95types of, 89–92for weed control, 74, 142

cowpeas, 90, 107crimson clover, 89–90, 97, 107,

161crop residue, 40, 64–68, 88, 152,

210–212accumulations, effects of, 72–73application rate, 72availability, 102burning, 67characteristics, 68–73C:N ratio in, 69–70composting, 109mulches, use as, 67–68nutrients in, 56, 159reduced tillage practices, 121,

126removal of, 66–67from rotations, 100–102soil biodiversity, effect on, 74

crop rotations. See rotationscrown vetch, 91, 95crust (soil), 47–49, 125–127,

136, 206

Ddeep tillage, 128–129disc harrow, 137–139diseases, 4, 14, 17, 20, 67

compaction and, 74, 127composts and, 74, 114, 115earthworms and, 18rotations and, 99, 102tillage practices and, 143

diversion ditches, 122drainage, 4, 41, 43, 75, 122,

148, 207humus, effect of, 11improving, 130, 141topographical position and,

34–35of topsoil, 22

drought stress, 53–54

EE. coli, 84earthworms, 4, 9–10, 14, 17–18,

204, 205, 207, 212pores created by, 18, 27rotations, effect of, 102surface residues, effect of, 44tillage practices, effect of, 146

erosion, 4, 6, 44–47cover crops and, 95, 97nitrates, loss of, 31organic matter levels and, 65reduction of, 119–123rotations and, 101, 104, 107soil organisms, effect of, 10,

19tillage, effects of, 46–47, 211,

212tilth, effects of, 26, 44topsoil, loss of, 35, 45water, caused by, 45, 75, 206,

207wind, caused by, 45, 75, 123

Ffertilizers, commercial, 150–156.

See also nitrogenapplication, 136, 147, 148,

155–156, 159, 181–183,211

INDEX 227

fertilizers, commercial, continuedcost of, 145, 155nutrient flows, effect of, 55–

56, 58–59, 158organic materials compared,

152–154selecting correct, 154–155

field capacity, 43, 53fresh residues. See organic matter,

activefriable soil, 48–49, 207frost tillage, 143fungi, 4, 9–10, 16–17, 20, 26,

31, 39, 44, 74, 99

Ggrassed waterways, 122grasses, 57, 89, 91–92

compaction and, 130–131for erosion reduction, 38, 122as livestock food, 210–211manures, use of, 82, 83nitrogen from, 91, 160, 161,

163no–till, use of, 141in rotations, 37–38, 93, 97,

99, 101–105, 210green manures. See cover cropsgroundwater, 31, 104, 147

Hhairy vetch, 15, 89–90, 161–163.

See also cover crops;legumes

in cover crop mixtures, 92,145, 163

in rotations, 104–105, 107weed problems from, 95

hay. See grasseshealth, soil, 4–6, 63–76hogs, 86humus, 10–11, 22, 63–64, 109,

169. See also carbon;organic matter

cation exchange capacity,source of, 24

root development and, 27rotations, effect of, 101

hydrologic cycle, 31

hyphae, 16, 44. See also fungi

Iinsects, 9–10, 18, 20

compaction and, 127cover crops and, 88earthworms and, 18mulches, control by, 67–68nutrient management and, 74,

82, 148pores created by, 27, 44rotations and, 99, 102, 104,

163tillage practices, effects of,

143, 145intercrops, 73, 95–96iron, 23, 25, 169irrigation, 82, 126, 174. See also

runoff

Lleaching. See nitrate; runofflegumes, 15, 75, 89–91, 148,

151, 160–163, 199. Seealso nitrogen fixation;specific plants

bacteria inoculation of, 89for erosion reduction, 122incorporation of, 138manures, use of, 82no–till for, 141root systems, 101in rotations, 37–38, 86, 92,

99, 102–105, 210lettuce, 65, 84, 107lignin, 16, 68–70, 72, 80, 111–

112lime/limestone, 11, 59, 72, 75, 81,

156, 168, 169, 171–172,211

livestock farms, 73, 131. See alsomanures

composting of animals, 112groundwater pollution, 147nutrient cycling, 55–59, 150–

151, 161–162, 164–165,213

rotations, 103–105, 210–211tillage, 138, 143, 213

Mmagnesium, 9, 17, 23, 24, 58–

59, 72, 75, 79, 108, 147,149, 167, 168

manganese, 23, 25, 147, 149,168–169

manures, 38, 67, 70, 77–86, 166accumulations, effects of,

72–73application, 65, 73, 82–84,

105, 210–211chemical characteristics, 78–79compaction and, 132composting, 109, 116–118,

151decomposition rate, 64,

80–82green (See cover crops)handling systems, 78incorporation, 122, 138, 148,

164nutrient content, 55–56, 75,

77, 152, 160, 162, 167,199

pollution from, 147residues, characteristics of,

68–73soils, effects on, 58, 59, 74,

80–82storage of, 78testing, 75, 79, 148, 159

micronutrients, 23–25, 147, 149,152

microorganisms, 15–17, 34, 44,64, 101. See also specificorganisms

C:N ratio, 69, 70in composting, 109–111root development and, 27tillage practices and, 37

millet, 105moldboard plows, 37, 101–102,

137–138, 143, 156. Seealso tillage

mucigel, 20mulches, 65, 67–68, 73, 75, 122,

126, 142mycorrhizae (mycorrhizal fungi),

16–17, 26, 44, 74, 92


Nnematodes, 17, 20, 74, 99, 102,

163nitrate

in atmosphere, 30availability, 24, 70, 75in composts, 115leaching, 31, 43, 55–56, 72, 82,

92, 104, 157–159, 163, 164in leafy crops, 82sod, use of, 163

nitrogen. See also C:N ratioapplication rate, 72in compost, 110–113, 115,

116from cover crops, 88, 90–92,

95deficiency, 69denitrification, 31, 158–159,

164, 208excess, 4, 148, 157fertilizers, 30, 152–156, 162–

163immobilization, 70inorganic, 30management of, 75, 147, 149,

157–166from manures, 72–73, 78–79,

82–83, 85mineralization, 17, 23–24,

131, 135nutrient flow patterns, 57–59rotations and, 99, 102–105soil tests for, 160, 162, 184–

185, 211tillage practices and, 143use of, 122

nitrogen cycle, 29–31nitrogen fixation, 15–16, 24, 25,

30, 59, 82, 89–91, 99,158, 159

no–till, 18, 101, 103, 108, 120–122, 127–128, 131–132,140–142, 145–146, 156,211–212

Ooats, 89, 91–92, 97, 101–105, 107onions, 57, 103, 105, 174

organic farms, 143, 151, 152, 154,212–213

organic matteractive, 9, 10, 39, 64, 101, 116,

149adding, 38, 122, 126amount in soil, 33–40, 44, 148application rates, 72–73C:N ratio in, 69–70distribution in soil, 38–39fertilizers vs., 152–154importance of, 21–33levels (See organic matter

levels)living, 9–10, 13–20, 39–40,

103 (See also soil organisms)management of, 5–6, 63–73,

104, 150parts of, 9–11use of, 65–68

organic matter levels, 6, 33–38,64–65

maintenance of, 122nutrient availability, 149rotations, effect of, 100–102

oxidation, 11oxygen, 4, 9, 11, 13, 23, 25, 75,

112

Ppeas, 15–16, 86, 108penetrometer, 133, 204, 206peppers, 105, 145pH. See acidityphosphorous, 23

cation exchange capacity and,24–25

in composts, 115excess, 4, 72, 157–158, 166–

167fertilizers, 152–156, 162–163management of, 147–149,

157–166in manures, 78–85mineralization, 15–17, 23–24,

149nutrient flow patterns, 57–59reducing losses of, 162–164soil tests for, 108, 185–187, 211

photosynthesis, 11physical condition of soil, 41–54,

63, 115, 150Phytophthora, 127plant tissue tests, 182–183plastic (soil), 48–49plastic limit, 48–49plow layer, 47–49, 126–133pores, 9, 26–27, 34, 40–44, 52,

81potassium, 9, 23, 167

cation exchange capacity and,24

in composts, 115excess, 166, 167fertilizers, 152–156management of, 147–149in manures, 78–79, 81, 83nutrient flow patterns, 57–59,

75in sewage sludge, 72soil tests for, 108, 211

potatoes, 65, 84, 103, 105, 132protozoa, 9, 17, 20

Rrainfall, 4, 34, 82, 121, 125, 126,

140, 208. See also runoff;water

rape, 89, 92, 97red clover, 86, 89, 91, 92, 105respiration, 11, 13rhizobia, 15, 89. See also legumes;

nitrogen fixationridge tillage, 140–141roots, 19–20, 44, 75, 148

cells, respiration of, 9, 25compaction and, 52–53, 126,

130–131in degraded soils, 41diseases, 74, 114, 115for erosion reduction, 122evaluation of, 206–207fungi and, 16left after harvest, 66–67nutrients and, 70, 150old channels of, 27rotations and, 100–103of sods/grasses, 35, 38

INDEX 229

roots continuedsoil organisms, interactions

with, 10, 20, 27stimulation of, 27

rotations, 99–108, 145, 160,161, 163, 210, 212

biodiversity of soil, effect on,74

compaction and, 126, 130–131economics of, 102erosion, effect on, 124examples, 103–105farm labor and, 102general principles of, 102–103incorporation of, 131, 138manure use and, 85mulching of, 131organic matter levels, effect on,

37–38, 40, 63, 73, 75,100–102

rooting periods, 102species richness for, 102

runoff, 4, 45, 58–59, 122, 148,206. See also nitrate,leaching

cover crops/rotations and, 75,87

manure incorporation, 82–84surface crusts, effect of, 47tillage practices and, 120

rye (winter, cereal), 74, 88, 89,91, 92, 94, 104–105, 107,130, 131, 145, 163

ryegrass (annual), 92, 95, 97

Ssafflower, 103saline seep, 105saline soils, 6–7, 173–175salt damage, 82, 84, 105, 187,

211sand, 9, 34, 41, 64–65sandy soil, 11, 28, 34, 41–43,

49, 65, 86, 125, 168, 169sewage sludge, 59, 70–73, 132,

147snap beans, 52sod. See also grasses; legumes

continuous growth of, 100

sod continuederosion, effect on, 120in rotations, 99–102, 104,

105, 120tillage, 131

sodic (alkali) soils, 168, 173–175sodium. See saline soils; salt

damage; sodic (alkali) soilssoil aggregates. See aggregates/

aggregationsoil color, 28, 205, 207soil consistency, 48–49soil degradation, 6–7, 41, 45, 46soil hardness, 204–206soil loss tolerance, 119–120soil organisms, 4, 69, 102, 205.

See also microorganismsbeneficial effects of, 25classification of, 13–14cultivation, effect of, 40size of, 14

soil quality, 4–6, 63–76soil solution (water), 9soil strength, 52soil structure, 4, 9–10, 44. See

also aggregates/aggregationsoil testing, 60, 108, 156, 159–

161, 177–199, 205, 208,211

accuracy of, 178adjusting recommendations,

198–199cation exchange capacity, 170examples, 188–197interpreting results, 187manures (See manures)nitrogen, 184–185for organic matter, 187phosphorous, 185–187samples for, 177–178variations in, 179–184

soil texture, 34, 49, 52, 64sorghum, 65–66, 103, 132soybeans, 15, 86, 90, 168. See

also legumesresidues from, 65, 68–69, 132in rotations, 37, 99, 104, 107,

145spiders, 18

squash, 105, 107strip cropping, 103, 122–123strip tillage, 97subsoils, 35, 39, 103

compaction, 47, 50–52, 126–133, 207

deep tillage, 128–129subterranean clover, 89–90sudangrass, 92, 97, 107, 131sulfur, 23, 24, 79, 83, 147, 149,

168sunflower, 103, 105surface crusting. See crust (soil)surface sealing, 125–126sustainable agriculture, 5sweet clover, 89, 91, 162

Tterraces, 45, 123tillage, 36–37, 39, 48–52, 73,

97. See also moldboardplows; no–till

compaction and, 127–129conventional, 137–140, 143erosion and, 120fertilizer/amendment incorpo-

ration, 156frost tillage, 143organic matter levels and, 65reduced, 121, 126, 135–146,

160, 164, 211–213rotation of systems, 143selecting best practice, 141–

143, 145soil check prior to tilling, 50soil degradation and, 46–47timing of field operations,

143–144tilth, 5, 9, 22, 25–27, 44, 63, 122,

148, 163, 173, 205–207timothy, 101tomatoes, 57, 68, 105, 107, 145,

148, 175topographical position, 35, 46–47topsoil, 15, 35, 39, 65

erosion and, 119–120, 122organic matter content, 21–22properties of, 22rotations, effect of, 100–101


traffic, controlled, 129, 132–133

V, W, Zvetch, 86, 97. See also crown

vetch; hairy vetchwater, 9. See also groundwater;

rainfall; runoffcycle, 31optimum range for plant

growth, 53–54organisms living in, 17in soil pores, 42–44

water infiltration, 44–45, 150, 207compaction, effect of, 47composts and, 117

water infiltration continuedcover/rotation crops, use of,

103, 131earthworms and, 18organic matter levels and, 65salts, movement of, 105soil structure and, 4, 10, 31

water storage, 4, 22, 36, 65, 207water supply, 75, 148

cover crops and, 89, 95manure composting, effect of,

116mulches and, 67

weed control, 4, 67, 74, 87, 163compaction and, 127, 131

weed control continuedcover crops and, 88–90, 92nutrient management and, 148rotations and, 99, 103–105,

108tillage practices, 135–136,

138, 142, 143wheat, 37, 86, 89, 97, 101–102,

104–105, 107, 120white clover, 89, 91wilting point, 43, 53–54winter rye. See rye (winter, cereal)zinc, 23, 25, 79, 83, 147, 149, 168zone tillage, 129, 131, 132, 138–

142

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