Chapter 5 · streams into freshwater lakes or ponds is ... CHAPTER 5 IDAHO MASTER GARDENER PROGRAM HANDBOOK 5 ... A good example of silt is talcum powder. III. Soil Texture

CHAPTER 5 IDAHO MASTER GARDENER PROGRAM HANDBOOK 5 - 1

I. The Five Soil Forming Factors 2

A. Parent Material 2

B. Climate 2C. Topography 2D. Living Organisms (Biota) 3E. Time 3

II. Major Components of a Mineral Soil 3III. Soil Texture 4

A. Sand 4B. Silt 4C. Clay 5D. Texture and Soil Management 5

IV. Soil Structure 5

V. Organic Matter 6VI. Carbon:Nitrogen Ratio 6

VII. Soil/Water Relationships 6A. Water-Holding Capacity 6B. Water Infiltration Rate 7C. Permeability Rate 7D. Soil Compaction 7E. Water & Air 7F. Nutrient Leaching 7G. Soluble Salts 7

VIII. pH 8IX. Plant Nutrients 8

A. Essential Nutrients 8B. Functions of Macronutrients 9C. Functions of Micronutrients 9D. General Nutrient Deficiency 10

Symptoms

X. Mulches 10A. Organic Mulches 10B. Inorganic Mulches 11C. Seasonal Mulches 11D. Problems with Mulches 11

XI. General Information on Fertilizers 11

XII. Fertilizer Terminology 11XIII. Nutrient Sources & Fertilizer Types 12XIV. Fertilizer Application and Timing 13

A. Fertilizer Timing 13B. Fertilizer Application 13C. Salt Accumulation and Soil 13

LeachingXV. Green Manure and Cover Crops 13

A. Green Manure 13B. Cover Crops 14

XVI. Organic Fertilizers 14

Further Reading and Resources 15

MASTER GARDENERIDAHOUNIVERSITY OF IDAHO EXTENSION

Chapter 5SOILS AND FERTILIZERS

5 - 2 SOILS AND FERTILIZERS CHAPTER 5

I. The Five Soil Forming FactorsWhat is soil? Soil is basically weathered rock,

decaying remains of living organisms—plantsand animals—and microorganisms. Soil is alsocommonly described as a medium for plantgrowth because it provides physical support andnutrients for plants.

Have you ever noticed how soils can varywithin a short distance as well as regionally? Fivesoil forming factors are at work on all soils anddetermine their physical and chemical properties.Where all five factors are the same we can expectthe soils will be very similar. The following is adescription of the soil forming factors: parent ma-terial, climate, topography, living organisms, andtime.

A. Parent MaterialThe term parent material refers to where thesoil came from. The source of the soil has alarge influence on soil texture, or particlesize, and minerals present. Soil surveys in-clude the parent materials in the soil de-scriptions; therefore, it is important to learnthe common terms.There are six basic types of parent material:1. Alluvium—Parent material that has been

transported by water, such as in floodplains and washes.

2. Colluvium—Parent material that has beentransported by gravity (talus), such as intoeslopes or debris at the base of a cliff.

3. Eolian—Parent material composed ofsand-sized particles that have been trans-ported by the wind.

4. Loess (pronounced “luss”)—Parent mate-rial composed of silt sized particles thathave been transported by the wind. ThePalouse area of northern Idaho and east-

ern Washington is a good example of aloess deposit.

5. Lacustrine and marine sediments—Parentmaterial that has been deposited bystreams into freshwater lakes or ponds islacustrine. Parent material that has beendeposited by oceans or seas, sometimesfound in salt-water basins, is marine.

6. Residual/residuum—parent material thathas weathered, in place, from the bedrockbelow. These materials have not beentransported.

B. ClimateTemperature and precipitation are majorfactors influencing the rate of weathering ofa soil. They also control the rate of chemicaland physical processes. Water is the me-dium by which things are moved into andthrough a soil profile.Less annual precipitation means that solublecomponents, such as calcium carbonate(lime), will accumulate, which is why soilsin arid regions are more alkaline. Con-versely, soils in high-moisture areas willhave faster rates of leaching, or removal, ofsoluble components, which is why soils inhumid regions are more acidic.

C. TopographyTopography includes both the gradient(steepness) of a slope and the aspect (direc-tion) of the slope. The gradient influenceshow quickly water enters the soil or runsoff, which directly influences the amount ofsoil loss or erosion. The aspect of the slopeinfluences the amount of solar radiation andtemperature fluctuations in the soil, whichdirectly influences the type of plants thatwill grow in the soil.

Chapter 5

Soils and FertilizersValdasue Steele, Extension Educator, Benewah County


D. Living Organisms (Biota)Living organisms include plants, animals,and microorganisms that live in and on thesoil. Biota have a significant impact on soilformation due to factors such as nutrientcycling, production of organic matter, andvegetative cover of the soil surface. Humanactivities such as farming and constructionalso impact the soil.

E. TimeHow “old” a soil is makes a difference in itsdevelopment. Soils are dynamic in that theyare “a work in progress,” constantly underthe influence of the soil forming factors. Theolder a soil is, the more “developed” it is.Therefore, “young” soils are very differentfrom “old” soils chemically and physically.An example of a young soil would be newsoil deposits from a flood.Each of the five soil forming factors has apowerful influence on the characteristics ofa soil. This helps explain why soils a veryshort distance apart can be different. It takesa change in only one of the five soil formingfactors to differentiate the classification ofone soil from another’s.The consistency of the soil forming factorsis demonstrated in a soil survey. Most soilsurveys are done by county, and they are agood source of general soils information,including information on climate and land-use suitability.The first step in developing a soil survey isto gather topographic data on the area to besurveyed. Lines are drawn to delineate areasof similar slope, and major changes in veg-etation are noted. Each delineated area, ormapping unit, is then given a soil classifica-tion designation or name. A soil scientistwill do field work to verify a few of thesemapping units. Extrapolations can be madewithout field verification because the soilforming factors have such consistent effectson soil characteristics.

II. Major Components of a Mineral SoilOnly 48% of the soil is made up of minerals;

the other 52% is organic matter and pore spacefilled with water or air (fig. 1). Water is the me-dium of transport for nutrients to reach the plant

roots. Air is required for chemical processes inthe plant roots as well as for the microorganismsthat live in the soil. Too much water can not onlycause the plants to die but can kill microorgan-isms as well. Generally, the mineral soils in Idahohave about 2% organic matter, plus or minus 2 to3% in some areas of the state. That 2% organicmatter affects the soil’s water-holding capacity,soil structure, and fertility. Every bit of organicmatter counts.

There is a vast array of microorganisms livingin the soil. Microorganisms play a major role innutrient cycling—the retention and release of nu-trients in the soil. The main categories of micro-organisms are listed below, with a conservativeestimate of their concentration in soil:• Bacteria –> 500 million per gram of soil• Actinomycetes –> 1 to 20 million per gram of

soil• Fungi –> large variation up to 1 million per

gram of soil• Algae –> up to 500,000 per gram of soil• Protozoa –> up to 500,000 per gram of soil• Nematodes –> 50 or more per gram of soil

Have you ever noticed the smell of freshlytilled soil? That is the smell of microorganisms atwork!

Fig. 1. Major components of a mineral soil. The per-centage will vary depending on the soil. Underwet conditions, there is less air. Under dry con-ditions there is less water.


stone, cobble, etc.), not part of the soil. Sandparticles provide the most stable mediumfor engineering purposes. Sand particles arerelatively inactive chemically. The largespaces between the particles (pore space)mean that water and some nutrients cannotbe retained very long and readily move outof the soil profile. This is why sandy soil isoften referred to as a “droughty” soil.

B. SiltSilt particles range in size from 0.02 to0.002 mm. Silt particles carry a very weaknegative charge and are capable of holdingsmall amounts of plant nutrients. Silty soilshold more water than sand, and water move-ment through silty soils is generally slower.A good example of silt is talcum powder.

III. Soil TextureThe texture (particle size distribution) of a soil

is determined by the relative amounts of sand,silt, and clay present. There are many possiblecombinations of these particles and distinct tex-tural classes to describe all possible particle sizecombinations, or distributions. See the soil tex-ture triangle (fig. 2) to determine the texturalclass of a particular soil. The three major particlesize classes are sand, silt, and clay.

A. SandSand particles range in size from 2.0 to 0.02millimeters (mm). Sand particles are thelargest particles in the soil. Any particlelarger than the sand particle (>2 mm) isconsidered part of the rock fraction (gravel,

Fig. 2. Soil texture triangle.


Soils that have organic matter and are properlymanaged will have good soil structure. If soils arecultivated when they are too wet, they can be-come very compacted and lose structure until re-aggregation occurs. Plant roots and the additionof organic matter to the soil will help improvesoil structure or tilth (soil workability).

C. ClayClay particles are smaller than 0.002 mm,and there are many types of clays. Clay par-ticles carry negative charges capable of at-tracting all positively charged ions in thesoil; as a consequence, clays attract posi-tively charged plant nutrients. The very tiny,flat clay particles lead to high water-holdingcapacity and can result in slow movement ofwater through the soil. Although there maybe more total pore space in a given volumeof clay soil than in sandy soil, most of thepore space in a clay soil will hold water. Incontrast, most of the pore space in sand islarge so water moves through rapidly ratherthan being held. Compaction can be a prob-lem in clay soils more so than in sandysoils.

D. Texture and Soil ManagementBy knowing the texture of a soil and under-standing the influences of sand, silt, andclay on the soil, you can make inferencesabout the management of that soil. Soilswith a high sand content will need to be wa-tered more often than soils with a high claycontent. Soils with a high clay content haveto be watered less often or there is a risk ofwaterlogging the soil.A loam does not have equal percentages ofthe three soil separates (sand, silt, and clay)but is influenced by them equally. A loamhas the characteristics of a clay for water-and nutrient-holding capacities, while thesand and silt provide pore space for air andwater movement. Notice that a smallamount of clay can strongly influence thesoil texture class (fig. 2).

IV. Soil StructureThe arrangement of soil particles or aggregates

is referred to as soil structure. Soil structure is theaggregation of sand, silt, and clay into shapeswith distinct sizes and strengths (fig. 3). Soilstructure provides additional pore space and openchannels for movement of water, nutrients, air,and plant roots. Soil structure can give an indica-tion of the age of the soil, parent material, vegeta-tion, and land use.

Fig. 3. Various types of soil structures.


Organic matter in soil is made up of the re-mains of plants and animals. Residue from previ-ous crops must be broken down to provide soilfertility and structure benefits. Moist and warmsoil is ideal for microbes to work at breakingdown plant tissue.

The organic matter content of a soil is an im-portant factor related to overall productivity forthe following reasons:• Contributes to well-structured soil• Is a source of three nutrients—nitrogen,

phosphorus, and sulfur• Increases water-holding capacity• Increases soil aeration• Provides an energy source for soil microorgan-

isms (plants and animals)

VI. Carbon:Nitrogen (C:N) RatioThe carbon:nitrogen (C:N) ratio is an important

consideration whenever you add organic materialto your production system (table 1). Plant residuesand manures are made up largely of the follow-ing:• Sugars, starches, and simple proteins that de-

compose rapidly• Crude proteins• Hemicelluloses• Celluloses• Lignin, fats, waxes, etc., that decompose

slowlyTheir rates of decay and release of nutrients to

the soil vary greatly, as do the demands of livingsoil microorganisms as they “break down” plantresidue.

In order to break down the carbon compoundsin the plant tissue, microorganisms consume N. Ifthe C:N ratio in the organic material is too high,greater than 25:1, there will be a net loss of nitro-gen available for plant growth in the short termbecause the microorganisms will consume any Nadded with the organic material. On the otherhand, if the C:N ratio is low, less than 20:1, suffi-cient N will be available to meet themicroorganism’s needs with some left over forplant growth. Thus organic material such as straw(C:N = 80:1) added to the soil will need to have Nadded with it or the plants will suffer.

V. Organic Matter Table 1. Carbon: Nitrogen ratio of common organicmaterials.

Material C:N

Wheat straw 80:1

Pine needles 90:1Sawdust 625:1

VII. Soil/Water RelationshipsA. Water-Holding Capacity

One of the main functions of soil is to storemoisture and supply it to plants betweenrainfalls or irrigations. Water in the soil isheld in pores, the spaces between soil par-ticles. If the soil’s water content becomestoo low, plants become stressed.The water-holding capacity of a soil, andthe amount of water available for plants touse, is dependent on the number and size ofits pore spaces, which is directly related tosoil texture and organic matter content. Wa-ter is held by the soil in various ways, andnot all water in the soil is available to plants(table 2).Capillary water is held in pores that aresmall enough to hold water against gravitybut not so tightly that roots cannot absorb it.This water occurs as a film around soil par-ticles and in the pores between them and isthe main source of moisture for plants. Asthis water is withdrawn, the larger poresdrain first. The finer the pores, the more re-sistant they are to removal of water. Capil-lary water can move in all directions forseveral feet as the particles and pores of thesoil act like a wick.When soil is saturated, all the pores are fullof water and the water that drains out of thesoil in the first few hours is called gravita-tional water. Gravitational water is avail-able to plants only for a short time. Whenthe gravitational water is gone, the soil is atfield capacity. Plants then draw water outof the capillary pores until no more can bewithdrawn and the only water left is in themicropores. The soil is then at the wiltingpoint, and if water is not added to the soil,plants will die.


D. Soil CompactionCompacted soils have low water infiltration/permeability rates. When the soil air spacesare compacted there is less space for air andwater. Compacted soils also make root pene-tration and plant growth more difficult.Compaction can best be avoided by keepingheavy equipment off the soil when it has ahigh moisture content. Soil compaction canbe remedied mechanically with deep tillageequipment.

E. Water & AirUnder irrigated conditions, regulating thesoil water (moisture level) is an importantmanagement consideration. Excessive soilmoisture, or saturated conditions, can be asharmful as limited-water conditions. Soilpore space and soil temperature are directlyrelated to soil water content. For instance, awet soil takes longer to warm up and willhave a greater incidence of fungal and bac-terial plant diseases.Plant roots need oxygen and get it from theair in the soil pore space. When a soil issaturated, water displaces all the air in thepore space. If the wet condition persists,plant roots will die from lack of oxygen.

F. Nutrient LeachingLeaching can best be described as the“flushing” of water and soluble nutrients outof the soil profile, specifically, out of theplant root zone. Factors that affect the rateof leaching in soils include:• Amount of rainfall/irrigation• Intensity (rate) of rainfall/irrigation• Soil texture• Quantity and type of clay minerals

present• Amount of organic matter

G. Soluble SaltsSoluble salts are described in terms of soilsalinity and are measured by determiningthe electrical conductivity of a soil extract(EC). Salinity problems can occur wherethere are excessive applications of fertilizer,fresh manure, wood ash, or irrigation wateror in areas with high water evaporation.High soil salinity is detrimental to plantgrowth.

Table 2. Water-holding capacity of main soil texturegroups (inches per foot).

Field Wilting AvailableTexture capacity point water

Sand 1.3 0.6 0.7

Sandy Loam 2.5 1.3 1.2

Loam 3.6 1.8 1.8

Silt Loam 4.4 2.3 2.1

Clay Loam 4.6 2.6 2.0

Clay 4.7 2.8 1.9

Note: Figures are averages and vary with structure andorganic matter.

A soil dominated by large particles (sand)has a lower water-holding capacity than asoil dominated by small particles (clay). Al-though a soil dominated by fine particlescan hold more water, that water is not nec-essarily available (wilting point) because ittakes more energy for plants to remove thewater from the tiny pores.

B. Water Infiltration RateInfiltration specifically refers to watermovement into the soil from the surface.The rate of infiltration is measured in inches(or cm) per hour. The finer the soil texture(more clay), the slower the infiltration rateand vice versa. The infiltration rate of a soilwill determine how much and how often toapply water. The infiltration rate also af-fects how much rainfall or irrigation waterwill enter the soil or run off, which relatesto erosion hazard.

C. Permeability RatePermeability specifically refers to watermovement within and through the soil pro-file (after infiltration). The permeability rateis the speed at which water moves in thesoil profile and like infiltration is measuredin inches (or cm) per hour. Soil compaction,plow layers, hard pans, clay layers, rock, orchanges in soil texture all can influence thepermeability rate of the soil. When a home-owner seeks a permit to have an on-site sep-tic leach field a “perc test” is required. Thisis a test of the permeability rate of the soil.If the rate is too slow then the site is notsuitable for a septic leach field.


VIII. pHThe term pH refers to the concen-

tration of hydrogen ions (H+) presentin the soil. As the concentration of H+

increases, the soil becomes moreacidic. As the concentration de-creases, the soil becomes more basic(alkaline). The pH scale is 0 (acidic)to 14 (alkaline). A pH of 7 is consid-ered neutral. The pH scale is logarith-mic:• pH 8—10 times more alkaline than

pH 7• pH 7—Neutral• pH 6—10 times more acidic than

pH 7• pH 5—100 times more acidic than

pH 7• pH 4—1,000 times more acidic

than pH 7Plant species have various adapta-

tions to specific acidic, neutral, or al-kaline soil conditions. When plantsfail to thrive, even after a fertilizerapplication, it may be an indicationthat there is a pH problem.

When pH is too low, applied limecan raise the pH. When pH is toohigh, applied sulfur can lower the pH.The change will be only temporary,however. Eventually, the soil pH willgo back to what it was originally ifamendments are not applied regu-larly. This ability of the soil to chemi-cally revert to the way it was is re-ferred to as buffering capacity. A soilthat is high in clay content has agreater buffering capacity and willrequire a larger quantity of an amend-ment, such as lime, to change the pH.A sandy soil will require less lime tochange the pH.

Soil pH greatly affects the avail-ability of nutrients in the soil forplants to utilize. When pH is tooacidic or alkaline then nutrients be-come unavailable to plants (fig. 4).

IX. Plant NutrientsA. Essential nutrients

Plants obtain from the soil 14 of the 17 elementsessential to their growth. The other three ele-ments—carbon, hydrogen, and oxygen—comefrom the water and from the air.1. Macronutrients— Large quantities are re-

quired. Nitrogen generally is for leaf or vegeta-tive growth, phosphorus is for root and fruitproduction, and potassium is for cold hardi-ness, disease resistance, and general durability.a. Primary macronutrients

• Nitrogen (N)• Phosphorus (P or in fertilizers, designated

as phosphate, P2O

5)

• Potassium (K or in fertilizers, designatedas potash, K

2O)

b. Secondary macronutrients• Calcium (Ca)• Magnesium (Mg)• Sulfur (S)

Fig. 4. Relative availability of nutrients as affected by soil pH.The bar width for each nutrient indicates its relativeavailability to plants at a particular pH. The wider thebar, the more available the nutrient.


2. Micronutrients—Small quantities are re-quired. Deficiencies in these nutrients areless common.• Boron (B)• Iron (Fe)• Molybdenum (Mo)• Chlorine (Cl)• Copper (Cu)• Manganese (Mn)• Zinc (Zn)• Nickel (Ni)

B. Functions of Macronutrients1. Nitrogen (N)—Nitrogen can be taken up

by plants as ammonium (NH4

+) or nitrate(NO

3-). Nitrogen is essential for the syn-

thesis of proteins. It is essential to chloro-phyll, which gives green color to plants;induces rapid vegetative growth; in-creases yields of leaf, fruit, or seed; im-proves quality of leaf crops; increasesprotein content of food and feed crops;and feeds soil microorganisms. Nitrogentends to extend the length of the plant’smaturity period, but too much will causeplants to fall over.

2. Phosphorus (P)—Phosphorus plays animportant role in the metabolic processesof the cells such as cell division and ex-pansion, respiration, and photosynthesis.In addition, phosphorus is important forearly root growth and development. Phos-phorus is significant in plant reproductivefunctions such as reducing the maturityperiod and stimulating flowering and seedformation. For some species, phosphorusimproves winter hardiness.

3. Potassium (K)—Potassium is vital to wa-ter relations in the plant. It is responsiblefor movement of water in and out of theguard cells that open and close the sto-mata and the movement of water in andout of the plant leaf. It also serves as anutrient regulator; increases vigor,strength, and disease resistance; makesstalks and stems stronger; helps earlyroots form and grow; and improves win-ter hardiness.

4. Sulfur (S)—As part of several amino ac-ids, sulfur is essential for protein synthe-

sis. It is also involved in nodule forma-tion and nitrogen fixation in legumes.

5. Calcium (Ca)—Calcium aids in the devel-opment of leaves and roots. It is an essen-tial part of the cell wall structure andmust be present for the formation of newplant cells.

6. Magnesium (Mg)—Magnesium is essen-tial for photosynthesis because it is thecentral atom in the chlorophyll molecule.Magnesium is also involved in phosphatemetabolism and plant respiration andserves as an activator for many plant en-zymes required in growth processes.

C. Functions of MicronutrientsThe content of micronutrients in the soil isvariable, as is their availability to plants.Soil pH is a significant factor in micronutri-ent availability. In very acidic soils, micro-nutrients can be toxic to plants.1. Boron (B)—Boron is probably the most

commonly deficient micronutrient. Boronis essential for germination of pollengrains, growth of pollen tubes, and for-mation of seeds and cell walls. Boronmay also be involved in carbohydratetransport.

2. Chlorine (Cl)—Chlorine, usually as thechloride ion, is active in the energy reac-tions of the plant, specifically in thebreakdown of water during photosynthe-sis. Chloride is present in the stomatalguard cells that regulate the loss of waterfrom leaves through transpiration. Chlo-ride has also been linked to increased re-sistance to fungal diseases in roots.

3. Copper (Cu)—Copper is necessary forchlorophyll production and may play apart in vitamin A production. Copper isalso a component of several plant en-zymes.

4. Iron (Fe)—Iron is a catalyst of chloro-phyll formation and acts as a carrier ofoxygen. It is also involved in the forma-tion of some respiratory enzymes.

5. Manganese (Mn)—Manganese is part ofthe enzyme systems and metabolic reac-tions of the plant. It is also directly in-volved in the synthesis of chlorophyll.


6. Molybdenum (Mo)—Molybdenum is re-quired in the smallest quantity of all theessential nutrients. Plants need it to usenitrogen, particularly for nitrogen fixationin the root nodules found on legumes.

7. Nickel (Ni)—Nickel is the most recentnutrient to be added to the essential nutri-ent family. It is an important componentin nitrogen metabolism, particularly inthe conversion of urea to ammonia.

8. Zinc (Zn)—Zinc is necessary for the pro-duction of chlorophyll and carbohydrates.Zinc is involved in the synthesis of plantgrowth hormones and in some metabolicreactions.

9. NOTE: Cobalt (Co) is not considered anessential nutrient, but root-nodule-form-ing bacteria in legumes need it for fixingnitrogen.

D. General Nutrient Deficiency SymptomsNutrient deficiency symptoms are an indi-cation of severe starvation. A nutrient defi-ciency will limit plant production beforedeficiency symptoms actually show. Defi-ciency symptoms are sometimes difficultto distinguish visually and may resembledisease or insect problems.General nutrient deficiency symptoms arecategorized here according to whether ornot the nutrient is translocated in the plant.Deficiencies of translocated nutrients ex-hibit symptoms in the lower, or older,leaves because the nutrients are mobilizedand moved to new, growing parts of theplant.1. Translocated Nutrients

a. Nitrogen—Plants are light green incolor; older leaves yellow starting atthe leaf tips.

b. Phosphorus—Plants are small anddark green with purple coloration.

c. Potassium—Yellow or brown discol-oration appears along the outer mar-gins of the older leaves.

d. Magnesium—Yellow discolorationoccurs between the leaf veins. Red-dish-purple discoloration extendsfrom the outer edge of the leaf in-ward.

2. Non-Translocated Nutrients—TerminalBud Diesa. Calcium—Primary leaf emergence

is delayed, and terminal buds dete-riorate.

b. Boron—Leaves near the growingpoint (meristem) are yellow, andbuds look like white or light browndead tissue.

3. Non-Translocated Nutrients–TerminalBud Remains Alivea. Sulfur—The whole leaf turns pale

green to yellow starting with theyounger leaves.

b. Zinc—Distinctive yellowing ap-pears between the leaf veins; someplants show a broad band of discol-oration on each side of the midrib.The plant is stunted and has shortinternodes.

c. Iron—Leaves are pale yellow orwhite between leaf veins.

d. Manganese—Leaves are yellowishgray or reddish gray with greenveins.

e. Copper—Young leaves are paleyellow and/or are wilted or with-ered; seedheads may not form.

f. Chlorine—Upper leaves wilt thenyellow.

g. Molybdenum—Young leaves wiltand die along the margins; olderleaves yellow due to their inhibitedability to utilize N.

4. Nickel—The ExceptionDeficiency symptoms have not beenobserved in field conditions, only inresearch settings, but include yellow-ing of young leaves and death of mer-istem tissue.

X. MulchesMulch is any material, organic or inorganic,

that is spread upon the surface of the soil to pro-tect it and plant roots from the impact of rain-drops, crusting, freezing, and evaporation.

A. Organic MulchesOrganic mulches include grass clippings,


hay, straw, bark, sawdust, wood shavings,leaves, and newspapers. You can use al-most any plant material for mulching aslong as it allows air and water to penetrateto the soil below. Coarse-textured material,such as coarse-textured hay, straw, woodshavings, and chips, are more desirable thanfine textured materials such as leaves, pineneedles, and sawdust. When using fine ma-terials, loosen them occasionally to keepthem from sealing the soil surface.

B. Inorganic MulchesInorganic mulches include plastic films,mat-type weed barriers, aluminum foil, andeven old carpet. Although inorganicmulches provide some of the same benefitsas organic mulches, they cannot be incorpo-rated into the soil at the end of the growingseason and must be removed where youplant annual crops. Perforated plastic film orspun-bound material, such as landscapecloth, allows water and air to easily reachthe soil.

C. Seasonal MulchesA thin layer of mulch will conserve soilmoisture, and 2 or more inches of mulchwill control most weeds. Mulch effective-ness depends upon the material you are us-ing and the weed species to be controlled.1. Summer Mulches—Use summer mulches

to control weeds, reduce water evapora-tion from the soil, stabilize water tem-perature, and reduce fruit rot on bare soil.Incorporate organic summer mulches inthe fall to improve soil structure.

2. Winter Mulches—Use winter mulches toreduce water loss from evergreen planttissue and to stabilize soil temperatures.Stable soil temperatures will minimizesoil heaving caused by alternate freezingand thawing. Winter mulch applied tooearly in the fall can cause more winterinjury than none at all.

D. Problems with MulchesOrganic mulches, such as cereal grain straw,can introduce weed seeds. The mulch mayattract rodents, insects, and other pests as anoverwintering site. Mulching too soon in thespring can prolong cool soil temperatures,which will delay the growth of warm-season

crops. Material with a high C:N ratio such asbark, wood shavings, sawdust, or straw maytemporarily reduce soil nitrogen available toplants unless you incorporate additional ni-trogen fertilizer into the soil.

XI. General Information on FertilizersFertilizer is defined as any substance added to

the soil, or sprayed on plants as a foliar fertilizer,to supply one or more plant nutrients. Everymixed fertilizer or individual material sold has aguaranteed analysis written on the bag. The analy-sis gives the amounts of available nitrogen (N),available phosphate (P

2O

5), and soluble potash

(K2O), in that order. The three numbers are always

percentages by weight. Certain secondary macro-nutrients and micronutrients may also be includedin the analysis.

Many brands and formulas of fertilizer are onthe market. Select a brand that supplies nitrogen(N), phosphate (P

2O

5), and potash (K

2O) in ap-

proximately the same ratio your soil test indicates.For example, if your test indicates you should use1 pound of nitrogen (N), 2 pounds of phosphate(P

2O

5), and 1 pound of potash (K

2O), the ratio in-

dicated is 1-2-1. You could use a 10-20-10, a 5-10-5, a 6-10-4, or an 8-17-7 analysis fertilizer. Allof these are in approximate 1-2-1 ratios.

XII. Fertilizer Terminology• Mixed fertilizer—A fertilizer that contains two

or more of the macronutrients (N, P, K).• Complete fertilizer—A fertilizer that contains

all three macronutrients (N, P, K).• Incomplete fertilizer—A fertilizer that is miss-

ing one, or more, of the major componentsfound in a complete fertilizer.

• Grade—The guaranteed minimum analysis, inpercent, of plant nutrients in a fertilizer, ex-pressed as total N, available P

2O

5, and soluble

K2O.

• Chelates—The word chelate comes from theGreek word for “claw.” Chelates are organicsubstances, or chemicals, that act like claws andhelp hold metal ions in solution, in an availableform, so that plants can absorb them. The solu-bility of metals, particularly Cu, Fe, Mn, andZn, is greatly increased when they are held bychelating agents.

• Soil amendment—A substance added to the soil


to change its pH or physical properties. Acommon example is the use of lime to increasesoil pH.

XIII. Nutrient Sources and Fertilizer TypesCommon nutrient sources and contents of fer-

tilizers appear in table 3.

Table 3. Common sources and nutrient contents offertilizers.

Nutrient content ofNutrient and source fertilizer (% by weight)

Nitrogen (N)

Anhydrous ammonia (gas) 82Ammonium nitrate 33-34Ammonium sulfate (24% sulfur) 21Urea 46Urea-ammonium nitrate solution (UAN) 28-32Sulfur coated urea (slow release) 39Monoammonium phosphate (MAP) 10-11Diammonium phosphate (DAP) 18Potassium nitrate 13Calcium nitrate 15

Phosphorus (P2O5) 1

Normal or single superphospate (NSP or SSP) 20Concentrated (CSP) 46 or triple superphosphate (TSP)Monoammonium phosphate (MAP) 48-55Diammonium phosphate (DAP) 46Ammonium polyphosphates (APP) 40-70

Potassium (K2O) 2

Potassium chloride 60-62Potassium sulfate (SOP) 50Potassium nitrate 44Sulfate of potash-magnesia 22 (Sul-Po-Mag or K-Mag)

Calcium (Ca)Calcitic limestone 32Dolomitic limestone 22Gypsum 22Burned lime 60

Magnesium (Mg)Dolomitic limestone 3-12Magnesium oxide (magnesia) 55-60Magnesium sulfate 9-20Potassium-magnesium sulfate 11

Table 3. Common sources and nutrient contents offertilizers (continued).

Nutrient content ofNutrient and source fertilizer (% by weight)

Sulfur (S) 3

Ammonium sulfate 24Potassium sulfate 18Gypsum 12-18Magnesium sulfate 14

BoronBorax 11Boric acid 17

Chlorine (Cl)Potassium chloride 47

Copper (Cu)Copper sulfate 22Copper ammonium phosphate 30

Iron (Fe)Iron sulfate 19-23Iron chelate 5-14

Manganese (Mn)Manganese sulfate 26-28Manganese chelate 12

Molybdenum (Mo)Ammonium molybdate 54Molybdic acid 47

Nickel (Ni)NA

Zinc (Zn)Zinc sulfate 23-36Zinc chelate 9-14

1 Rock phosphate is the basic material used in all P fertil-izer production. Phosphate for use in fertilizers is eitheracid treated or thermal processed. Acid treatment is themost important and utilizes sulfuric and phosphoric ac-ids.

2 Elemental K is not found in nature due to its chemicalreactivity. Potash (K2O) is the primary source of potas-sium for fertilizer use. Potash is found beneath thesurface in salt beds or in the brine of salt lakes and seas.Many minerals contain potassium, but the most importantare sylvinite (20-30% K2O) and langbeinite (23% K2O).

3 The primary source of S is soil organic matter.


C. Salt Accumulation and Soil LeachingFertilizer is more likely to burn a plant inhot, dry conditions when the plant isstressed. If there is insufficient moisture af-ter fertilizer application, then the salt con-centration can increase, making it evenharder for plants to take up water. Adequatewater will help prevent high salt accumula-tions.Potted plants should be leached every 4 to 6months and garden soils at least once a year.Leach the soil by saturating it with water andletting it drain completely. A rule of thumb isto apply water in an amount that is double thevolume of the pot. For instance, a 6-inch potwill hold about 10 cups of water, so use 20cups of water to leach out accumulated salts.Different plants have different levels of toler-ance for salt accumulation.

XV. Green Manure and Cover CropsBy definition, green manure crops are grown

and incorporated into the soil to improve the soil.Cover crops are grown primarily to reduce soilerosion and nutrient leaching.

Usually, green manure crops are annuals andcover crops are perennials, either legumes orgrasses. Cover crops can be incorporated into thesoil and used as green manure crops.

When managed properly, both green manureand cover crops add nitrogen to the soil for useby the crops planted later. They tend to increasethe level of soil fertility and soil humus. They im-prove the soil’s physical properties of aggrega-tion, porosity, bulk density, and permeability.Their effects are more pronounced in clay soilthan in sandy soil.

A. Green ManureThe benefit of using green manure crops de-pends on the soil, climate, and species ofplants grown. Environmental conditions thataffect microbial growth determine the rateof decay of organic residues. Warm soil,proper aeration, and ample soil moisture in-crease microbial activity, thereby increasingthe rate of decomposition of organic matter.Decomposition releases carbon dioxide andweak acids that help release nutrients. Thechemical composition of the plants affectstheir value as a green manure crop.

XIV. Fertilizer Application and TimingA. Fertilizer Timing

The timing of fertilizer application dependssomewhat on the type of fertilizer and theplant being fertilized. In addition, soil tex-ture determines application frequency andthe amounts to apply. For soils with highsand content, the recommended fertilizerapplication may need to be divided intosmaller quantities applied more frequently.The opposite is true for soils with high claycontent.

As a general rule, the best time to apply anynutrient is close to the time the plant isgrowing the most and needs the nutrients.This prevents environmental losses and thenutrient’s becoming unavailable in the soilbefore the plants can use it. It is best to ap-ply foliar fertilizers when the weather iscool, but not cold, and when there is ad-equate soil moisture.Phosphorus, potassium, and lime can be ap-plied in the fall or as part of a tillage opera-tion. It is important to apply nitrogen earlyin the spring during the active growth pe-riod. If your plants need a lot of nitrogen, itis best not to apply it all at once but rather tosplit the application into smaller amounts attwo or three intervals. Some forms of nitro-gen are very soluble and do not stay in therooting zone very long. As a general rule,apply no more than 50 to 75 lb/acre of nitro-gen in one application. Plants are morelikely to utilize nitrogen applied in smalleramounts more frequently, and our ground-water and streams will be protected fromnitrate pollution.

B. Fertilizer ApplicationThere are several methods for applying fer-tilizers. The most common is to broadcastgranular formulations on the surface or tillthem into the seedbed. Another method isbanding— placing the fertilizer below thesurface in a band below or beside a seed atplanting time. Some fertilizers can be ap-plied through an irrigation system. Fertiliz-ers can also be applied in liquid or powderform. Micronutrients are usually most effec-tively applied as foliar sprays.


Green manure crops have little influence onsoil organic matter content if cultivation iscontinuous. In cooler climates, green ma-nure crops can increase soil organic matterand nitrogen. In warmer climates, cultiva-tion speeds up the rate of decomposition soan increase in soil organic matter content isdifficult to achieve. Using green manurecrops can improve soil structure, which en-hances aggregation, and increase the spacebetween soil particles.Any fast-growing annual crop is a goodchoice for a green manure crop, such asryegrass, wheat, barley, vetch, or field peas.A legume is a great choice because of itsability to fix nitrogen. Green manure cropsshould be seeded immediately following theharvest of the main crop and not allowed togo to seed. Incorporate them at least 2weeks before planting the next crop. Asgrasses and cereal grains reach maturitythey can tie up nitrogen, so they should beincorporated into the soil early in the grow-ing season.

B. Cover CropsUse cover crops for alleyways in orchardsand vineyards to prevent weed growth andin gardens for pathways. Cover crops mayrequire some maintenance such as mowingand fertilization. Cover crops are good forfall gardens in between the rows and in anyother cleared areas.Cover crops provide organic matter andstore nutrients, which helps reduce nutrientlosses from the soil profile by leaching ofnitrogen, potassium, and other nutrients. Le-gumes add nitrogen to the soil and reduceerosion because they are deep rooted.Leaching is more of a problem on soils withhigh sand content.A good ground cover reduces soil erosion byreducing raindrop impact on soil particles.Plant cover increases the water infiltrationrate and minimizes water runoff. Leaves andstems “catch” the rain, and roots create chan-nels for water movement in the soil.Negative aspects of cover crops includecompeting with the main crop for nutrientsand moisture and encroaching on the maincrop. Cover crops may also provide a safe

haven for gophers, mice, insects, and dis-eases that can attack the main crop.

XVI. Organic FertilizersResearch has found no difference in the nutrient

contents of organic food and regularly producedfood. However, organic foods are less likely tohave chemical residues. As far as the plant is con-cerned, it does not matter if nutrients are suppliedby decaying plant material or from commercialfertilizers: nitrogen is nitrogen is nitrogen. Plantsare self-contained biochemical factories, and allthey need are raw materials (nutrients).

Generally, organic fertilizers release nutrientsslowly. Organic fertilizers depend on microor-ganisms to break them down in order to releasetheir nutrients. Therefore, most are effective onlywhen soil moisture and temperature are suitablefor microorganisms to be active. Some examplesof organic fertilizer sources are listed below:• Cottonseed meal—Approximate analysis: 7-3-

2. It can be somewhat acidic so it is often usedto fertilize acid-loving plants.

• Blood meal—Dried, powdered blood collectedfrom slaughter houses is a rich source of nitro-gen and may contribute some trace elements,including iron. Issues associated with animalbyproducts used in food sources may be a con-cern.

• Fish emulsion—A partially decomposed blendof finely pulverized fish, it is high in nitrogen,a source of some trace elements, and has astrong odor.

• Manure—Nutrient content is generally lowand varies with animal source and feed. Ma-nures are best used as soil conditioners ratherthan as sources of nutrients. Fresh manure candamage young plant material due to its highmineral salt content if irrigation is not man-aged properly.

• Sewage sludge—A byproduct of municipalsewage treatment plants, it generally comes intwo forms, activated and composted. Activatedsludge has a higher nutrient content (approxi-mately 6-3-0) than composted sludge (approxi-mately 1-2-0). There is some concern thatlong-term use could lead to the buildup of cer-tain heavy metals. Another concern focuses onits use in a garden around edible plants. Theorigin of the sludge determines its nutrient and


heavy metal contents. A sludge-based nutrientsource should be analyzed for heavy metalsbefore use.

Further Reading and Resources

BooksMinnich, J., and M. Hunt. 1979. Rodale Guide to

Composting. Rodale Press, Emmanus, PA.

Soil Improvement Committee, California FertilizerAssociation. 1997. Western Fertilizer Handbook:Second Horticulture Edition. Prentice Hall.

Havlin, J.L, J.D. Beaton, S.L. Tisdale, and W.L. Nelson.1999. Soil Fertility and Fertilizers: An Introduction toNutrient Management, 6th Ed. Prentice Hall,Englewood Cliffs, NJ.

Booklets and Pamphlets

University of Idaho

CIS 863 Fertilizer Primer

CIS 787 Liming Materials

CIS 815 Northern Idaho Fertilizer Guide: Blueber-ries, Raspberries, & Strawberries

CIS 853 North Idaho Fertilizer Guide: Grass Pastures

CIS 911 Northern Idaho Fertilizer Guide: NorthernIdaho Lawns

BUL 704 Soil Sampling

VideosHow Water Moves Through Soil. University of Arizona.

Available for check-out from the Benewah CountyExtension Office, St. Maries, ID.

Soil Monolith Collecting and Preserving. 1987. Availablefrom the UI Soil and Land Resources Division,College of Agricultural and Life Sciences, Moscow,ID.

Web SitesNatural Resources Conservation Service. Published Soil

Surveys for Idaho. http://soils.usda.gov/survey/printed_surveys/state.asp?state=Idaho&abbr=ID

Idaho State Department of Agriculture http://www.agri.state.id.us

Potash & Phosphate Institute. http://www.ppi-ppic.org

Smithsonian Soils Exhibit. http://www.soils.org/smithsonian/

University of Idaho Master Gardener Program. http://www.ag.uidaho.edu/mg/

University of Idaho Pedology Laboratory. http://soils.ag.uidaho.edu/pedology/

Instructor ResourcesSoil Texture Kits. Nine 2-lb texture samples can bepurchased from the UI Soil Evaluation Team, Moscow,ID, (208) 885-7554. ($75.00)

Suggested Activities for Chapter 4, Soils and Fertilizers.Online at www.ag.uidaho.edu/mg/handbook.htm.

drumford

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Published 1993. Revised 2008.

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Chapter 5 · streams into freshwater lakes or ponds is ... CHAPTER 5 IDAHO MASTER GARDENER PROGRAM HANDBOOK 5 ... A good example of silt is talcum powder. III. Soil Texture