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Agricultural Phosphorus and Eutrophication Second Edition United States Department of Agriculture Agricultural Research Service ARS–149 September 2003

Agricultural Phosphorus and Eutrophication Phosphorus and Eutrophication Second Edition ... surface blooms of cyanobacteria ... economic importance to farmers

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Agricultural Phosphorusand EutrophicationSecond Edition

United StatesDepartment ofAgriculture

AgriculturalResearchService

ARS–149

September 2003

A.N. Sharpley, T. Daniel, T. Sims, J. Lemunyon, R. Stevens, and R. Parry

United StatesDepartment ofAgriculture

AgriculturalResearchService

ARS–149

September 2003

Agricultural Phosphorusand EutrophicationSecond Edition

Sharpley is a soil scientist with the USDA–ARS, Pasture Systems and WatershedManagement Research Unit, University Park, PA; Daniel is a professor with theDepartment of Agronomy, University of Arkansas, Fayetteville, AR; Sims is aprofessor with the Department of Plant Science, University of Delaware, Newark, DE;Lemunyon is an agronomist with the USDA–NRCS, Resource Assessment Division,Fort Worth, TX; Stevens is an extension soil scientist with Research and Extension,Washington State University, Prosser, WA; and Parry is a national program managerwith the U.S. Environmental Protection Agency, Washington, DC.

Abstract

Sharpley, A.N., T. Daniel, T. Sims, J.Lemunyon, R. Stevens, and R. Parry.2003. Agricultural Phosphorus andEutrophication, 2nd ed. U.S. Departmentof Agriculture, Agricultural ResearchService, ARS–149, 44 pp.

Inputs of phosphorus (P) are essentialfor profitable crop and livestockagriculture. However, P export inwatershed runoff can accelerate theeutrophication of receiving fresh waters.The rapid growth and intensification ofcrop and livestock farming in manyareas has created regional imbalances inP inputs in feed and fertilizer and Poutput in farm produce. In many ofthese areas, soil P has built up to levelsin excess of crop needs and now has thepotential to enrich surface runoff with P.

The overall goal of our efforts to reduceP losses from agriculture to watershould be to increase P use-efficiency,balance P inputs in feed and fertilizer

into a watershed with P output in cropand animal produce, and manage thelevel of P in the soil. Reducing P loss inagricultural runoff may be broughtabout by source and transport controlstrategies. This includes refining feedrations, using feed additives to increaseP absorption by animals, movingmanure from surplus to deficit areas,finding alternative uses for manure, andtargeting conservation practices, such asreduced tillage, buffer strips, and covercrops, to critical areas of P export froma watershed. In these critical areas, highP soils coincide with parts of thelandscape where surface runoff anderosion potential are high.

Keywords: eutrophication, fertilizer,phosphorus, P input, P output, runoff

While supplies last, copies of thispublication may be obtained at no costfrom USDA–ARS, Pasture Systems &Watershed Management Research Unit,Curtin Road, University Park, PA16802–3702.

ii

The U.S. Department of Agriculture(USDA) prohibits discrimination in allits programs and activities on the basisof race, color, national origin, sex,religion, age, disability, politicalbeliefs, sexual orientation, or marital orfamily status. (Not all prohibited basesapply to all programs.) Persons withdisabilities who require alternativemeans for communication of programinformation (Braille, large print,audiotape, etc.) should contact USDA’sTARGET Center at 202–720–2600(voice and TDD).

To file a complaint of discrimination,write USDA, Office of Civil Rights,Room 326–W, Whitten Building, 1400Independence Avenue, SW, Washing-ton, DC 20250–9410 or call 202–720–5964 (voice and TDD). USDA is anequal opportunity provider and em-ployer.

An Adobe Acrobat pdf of this publica-tion is available at www.ars.usda.gov/np/index.html.

Photocopies or microfiche copies of thispublication may also be purchased fromthe National Technical InformationService, 5285 Port Royal Road,Springfield, VA 22161; phone (703)605-6000 or 1-800-533-6847 and on theWeb at www.ntis.gov. NTIS is requiredby law to maintain archival copies of allFederal technical publications and makethem available for sale on a cost-recovery basis.

iii

Issued July 1999,Revised September 2003

Introduction ......................................................................................... 1

Eutrophication .................................................................................. 1

Agricultural Production .................................................................... 2

Soil Phosphorus ................................................................................... 5

The Loss of Phosphorus in Agricultural Runoff ............................... 10

Forms and Processes ....................................................................... 10

The Dependence of Agricultural Runoff P on Soil P ..................... 12

Remediation ....................................................................................... 14

Source Management ....................................................................... 15

Transport Management ................................................................... 21

Targeting Remediation ................................................................... 22

Making Management Decisions.......................................................28

Summary ............................................................................................ 31

References ......................................................................................... 34

v

Contents

1

AgriculturalPhosphorus andEutrophicationIntroduction

Eutrophication

Phosphorus (P) is an essentialelement for plant and animal growthand its input has long been recog-nized as necessary to maintainprofitable crop and animal produc-tion. Phosphorus inputs can alsoincrease the biological productivityof surface waters by acceleratingeutrophication. Eutrophication is thenatural aging of lakes or streamsbrought on by nutrient enrichment.This process can be greatly acceler-ated by human activities thatincrease nutrient loading rates towater.

Eutrophication has been identifiedas the main cause of impaired

surface water quality (U.S. Environ-mental Protection Agency 1996).Eutrophication restricts water usefor fisheries, recreation, industry,and drinking because of increasedgrowth of undesirable algae andaquatic weeds and the oxygenshortages caused by their death anddecomposition. Associated periodicsurface blooms of cyanobacteria(blue-green algae) occur in drinkingwater supplies and may pose aserious health hazard to animals andhumans. Recent outbreaks of thedinoflagellate Pfiesteria piscicida inthe eastern United States, andChesapeake Bay tributaries inparticular, have been linked toexcess nutrients in affected waters.Neurological damage in peopleexposed to the highly toxic, volatilechemical produced by these algaehas dramatically increased publicawareness of eutrophication and theneed for solutions (Burkholder andGlasgow 1997).

Eutrophication of most fresh wateraround the world is accelerated by Pinputs (Schindler 1977, Sharpley etal. 1994). Although nitrogen (N)and carbon (C) are also essential tothe growth of aquatic biota, mostattention has focused on P inputsbecause of the difficulty in control-ling the exchange of N and Cbetween the atmosphere and waterand the fixation of atmospheric Nby some blue-green algae. There-fore, P is often the limiting element,and its control is of prime impor-tance in reducing the acceleratedeutrophication of fresh waters.When salinity increases, as inestuaries, N generally becomes theelement controlling aquatic produc-tivity. However, in Delaware’sinland bays (coastal estuaries),nitrate-N leaching has elevated Nconcentrations to the point where Pis now the limiting factor ineutrophication.

2

Lake water concentrations of Pabove 0.02 ppm generally accelerateeutrophication. These values are anorder of magnitude lower than Pconcentrations in soil solutioncritical for plant growth (0.2 to 0.3

ppm), emphasizing the disparitybetween critical lake and soil Pconcentrations and the importanceof controlling P losses to limiteutrophication.

Agricultural Production

Confined animal operations are nowa major source of agriculturalincome in several states. Animalmanure can be a valuable resourcefor improving soil structure andincreasing vegetative cover, therebyreducing surface runoff and erosionpotential. However, the rapidgrowth and intensification of cropand animal farming in many areashas created regional and localimbalances in P inputs and outputs.On average, only 30 percent of thefertilizer and feed P input to farmingsystems is output in crops andanimal produce. Therefore, whenaveraged over the total usableagricultural land area in the UnitedStates, an annual P surplus of 30 lb/acre exists (National ResearchCouncil 1993). This has led to Papplications in excess of cropremoval, soil P accumulations, andan increased risk of P loss in runoff(Kellogg and Lander 1999) (fig. 1).

Figure 1. Watersheds with a high potential for soil and water degradation frommanure P (Adapted from Kellogg and Lander 1999).

3

Before World War II, farmingcommunities tended to be self-sufficient in that enough feed wasproduced locally and recycled tomeet animal requirements. AfterWorld War II, increased fertilizeruse in crop production fragmentedfarming systems, creating special-ized crop and animal operations thatefficiently coexist in differentregions within and among countries.Since farmers did not need to relyon manures as nutrient sources (theprimary source until fertilizerproduction and distribution becameless expensive), they could spatiallyseparate grain and animal produc-tion. Today, less than a third of thegrain produced is fed on farmswhere it is grown (Lanyon 2000)resulting in a major one-waytransfer of P from grain-producingto animal-producing areas.

The potential for P surplus at thefarm scale can increase when

farming systems change fromcropping to intensive animal pro-duction, since P inputs becomedominated by feed rather thanfertilizer. With a greater reliance onimported feeds, only 27 percent of

the P in purchased feed for a74,000-layer operation on a 30-acrefarm in Pennsylvania could beaccounted for in farm outputs (table1). This nutrient budget clearlyshows that the largest input of

Table 1. Farming system and P balanceFarming system

P Crop* Dairy † Poultry ‡ Hogs§

Input - - - - - - - - - - - - - - - lb P/acre/yr - - - - - - - - - - - - - -

Fertilizer 20 10 0 0

Feed 0 20 1,375 95

Output –18 –13 – 365 -60

Balance +2 +17 +1,010 +35 SOURCE: Lanyon and Thompson (1996) and Bacon et al. (1990).

* 75-acre cash crop farm growingcorn and alfalfa.

† 100-acre dairy farm with 65 dairyholsteins averaging 14,500 lbmilk/cow/yr, 5 dry cows, and 35heifers. Crops were corn forsilage and grain, alfalfa, and ryefor forage.

‡ 30-acre poultry farm with 74,000layers; output includes 335 lb P/acre/yr in eggs, 20 lb P/acre/yrsold in crops (corn and alfalfa),and 10 lb P/acre/yr manureexported from the farm.

§ 75-acre farm with 1,280 hogs,output includes 40 lb P/acre/yrmanure exported from the farm.

4

nutrients to a poultry farm and,therefore, the primary source of anyon-farm nutrient excess, is in animalfeed. Annual P surpluses of 80 to110 lb/acre/yr were estimated bySims (1997) for a typical poultrygrain farm in Delaware. Thisscenario is consistent with otherconcentrated animal productionoperations, including dairy andhogs.

Phosphorus accumulation on farmshas built up soil P to levels thatoften exceed crop needs. Today,there are serious concerns thatagricultural runoff (surface andsubsurface) and erosion from high Psoils may be major contributingfactors to surface water eutrophica-tion. Agricultural runoff is all waterdraining from an area (field orwatershed) including surface runoff,subsurface flow, leaching, and tiledrainage processes. Phosphorus loss

in agricultural runoff is not ofeconomic importance to farmersbecause it generally amounts to only1 or 2 percent of the P applied.However, P loss can lead to signifi-cant off-site economic impacts,which in some cases occur manymiles from P sources. By the timethese water-quality impacts aremanifest, remedial strategies aredifficult and expensive to imple-ment; they cross political andregional boundaries; and because ofP loading, improvement in waterquality will take a long time.

Nitrogen-based management hasbeen practiced and advocated byfarm advisers for many years.Farmers are only now becomingaware of P issues. Many are con-fused and feel that science hasmisled them or let them down bynot emphasizing the P managementissues. Therefore, the research

community must do a better job oftransferring and translating itsfindings to the agricultural commu-nity as a whole. For example, wemust be able to show where P iscoming from, how much P in soiland water is too much, and how andwhere these inputs and losses can bereduced in order to develop agricul-tural resource management systemsthat sustain production, environmen-tal quality, and farming communi-ties.

In this publication, P is in itselemental form, rather than as P

2O

5,

which is commonly used in fertilizeranalysis. The conversion factor fromP to P

2O

5 is 2.29. When discussing

plant available forms of soil P, asdetermined by soil testing laborato-ries, we will refer to them as “soiltest P” (ppm or mg/kg) and identifyin each case the specific method ofanalysis used. Based on a 6-inch soil

5

depth containing 2 million poundsof soil, the conversion factor forppm to lb P/acre is 2. For moredetailed information on the methodsused for soil P testing, how theywere developed, and why they varyamong regions see Fixen and Grove(1990), Pierzynski (2000), Sharpleyet al. (1994, 1996), and Sims (1998).

Soil PhosphorusSoil P exists in organic and inor-ganic forms, but these are notdiscrete entities with indistinctforms occurring (fig. 2). Organic Pconsists of undecomposed residues,microbes, and organic matter in thesoil. Inorganic P is usually associ-ated with Al, Fe, and Ca (aluminum,iron, and calcium, respectively)compounds of varying solubility andavailability to plants. Phosphorushas to be added to most soils so thatthere are adequate levels for opti- Figure 2. The phosphorus cycle in soil

StableStable

Crop harvest

Manure P Fertilizer P

Labile Labile and fixed

Solution P

Soil test POrganic P Inorganic P

6

mum crop growth and yield. How-ever, P can be rapidly fixed inrelatively insoluble forms andtherefore be unavailable to plants,depending on soil pH and type (Al,Fe, and Ca content). Convertingstable forms of soil P to labile oravailable forms usually occurs tooslowly to meet crop P requirements(fig. 2). As a result, soil P tests weredeveloped to determine the amountof plant-available P in soil and fromthis how much P as fertilizer ormanure should be added to meetdesired crop yield goals.

In most soils, the P content ofsurface horizons is greater than thatof the subsoil because of sorption ofadded P, greater biological activity,cycling of P from roots toaboveground plant biomass, andmore organic material in surfacelayers (fig. 3). In reduced tillagesystems, fertilizers and manures are

• No manure• 40 lb P/acre/yr• 90 lb P/acre/yr• 110 lb P/acre/yr

Soil test P (ppm)0

050

So

il d

epth

(in

ches

)

100 150 200

10

20

30

40Figure 3. Soil test P (as Mehlich–3 P) accumulatesat the surface with repeated application of P for 10years. Note that typical fertilizer P applications for acorn crop in Oklahoma with a medium soil test P(20 to 40 ppm Mehlich–3 P) is about 20 lb P/acre.(Adapted from Sharpley et al. 1984.)

7

not incorporated or they are incor-porated only to shallow depths,thereby exacerbating P buildup inthe top 2 to 5 inches of soil. In somesituations, P can easily movethrough the soil, as we will discusslater.

Continual long-term application offertilizer or manure at levels exceed-ing crop needs will increase soil Plevels (fig. 4). In many areas ofintensive, confined animal produc-tion, manures are normally appliedat rates designed to meet crop N

requirements but to avoid ground-water quality problems created byleaching of excess N. This oftenresults in a buildup of soil test Pabove amounts sufficient foroptimal crop yields. As illustrated infigure 5, the amount of P added in

P removed (lb/acre)

P added(lb/acre)Dairy manure

Poultry litter

P added in manure or removed by crop0 50 100 150

CornS

oil

test

P (

pp

m)

Annual P surplus (lb/acre/yr)

Initial soiltest P is 18 ppm

80

60

40

20

0-20 0 20 40

Figure 4. Increase in soil test P from applying more Pthan a crop needs each year (as Bray–I P). Anegative surplus indicates crop and soil removal.(Adapted from a 25-year study by Barber 1979.)

Figure 5. Applying manure to meet crop N needs(about 200 lb available N/acre) adds much more Pthan corn crop needs.

8

average applications of dairymanure (8 to 10 tons/acre and 0.5percent P) and poultry litter (4 tons/acre and 1.5 percent P) are consider-ably greater than what is removed in

harvested crops; the result is anaccumulation of soil P.

In 2000, several state soil testlaboratories reported that themajority of agricultural soils ana-

lyzed had soil test P levels in thehigh or above categories, whichrequire little or no P fertilization. Itis clear from figure 6 that high soilP levels are a regional problem,

Figure 6. A survey of agricultural soils analyzed by state soil test laboratories in 1997 and 2000 showsa regional buildup of soil test P near P-sensitive waters (Fixen 1998, Fixen and Roberts 2000).

Percent of samples testing high or above

<25%

25 - 50%

>50%

79

7070

56

65

66

52

77

6457

65

74

63 51

5857

5874

56

53

53

7053

71

6774

23

4243

41

27

29

39

44

50

43

49

4840 25 40

46

1997

Percent of samples testing high or above

<25%

25 - 50%

>50%

60

5948

53

42

54

41

76

5831

63

74

55 47

5340

6669

54

49

62

5530

79

7882

22

4243

43

22

31

41

40

42

53

61

4240 21 39

64

38

84

8575

North American average of 47% soils testing medium or below for P

2000

9

because the majority of agriculturalsoils in several states still testmedium or low. For example, mostGreat Plains soils still require P foroptimum crop yields. Unfortunately,problems associated with high soil Pare aggravated by the fact that manyof these agricultural soils are locatedin states with sensitive water bodies,such as the Great Lakes, LakeChamplain, the Chesapeake andDelaware Bays, Lake Okeechobee,the Everglades, and other freshwater bodies (fig. 6).

Distinct areas of general P deficitand surplus exist within states andregions. For example, soil testsummaries for Delaware reveal themagnitude and localization of highsoil test P levels that can occur inareas dominated by intensive animalproduction (fig. 7). From 1992 to1996 in Sussex County, Delaware,with its high concentration ofpoultry operations, 87 percent of

Figure 7. Elevated soil test P levels (as Mehlich–1 P) areusually localized in areas of confined animal operations.

Percent in each soil test P category

New Castle Co. DELow livestock density

Sussex Co. DEHigh livestockdensity

Soil test P (ppm)

Low and medium: <25

Delaware

75

50

25

0

75

50

25

0

Optimum: 25-50

High: >50

fields tested had optimum (25 to 50ppm) or excessive soil test P (>50ppm), as determined by Mehlich–1;whereas, in New Castle County,with only limited animal production,

72 percent of fields tested wererated as low (<13 ppm) or medium(13 to 25 ppm).

10

Though rapidly built up by applica-tions of P, available soil P decreasesslowly once further applications arestopped. Therefore, the determina-tion of how long soil test P willremain above crop sufficiencylevels is of economic and environ-mental importance to farmers whomust integrate manure P intosustainable nutrient managementsystems. For example, if a field hasa high potential to enrich agricul-tural runoff with P because ofexcessive soil P, how long will it bebefore crop uptake will lower soil Plevels so that manure can be appliedagain without increasing the poten-tial for P loss? McCollum (1991)estimated that without further Padditions, 16 to 18 years of corn(Zea mays L.) and soybean [Glycinemax (L.) Merr.] production wouldbe needed to deplete soil test P(Mehlich–3 P) (Mehlich 1984) in a

Portsmouth fine sandy loam from100 ppm to the agronomic thresholdlevel of 20 ppm.

The Loss of Phosphorusin Agricultural RunoffThe term “agricultural runoff”encompasses two processes thatoccur in the field—surface runoffand subsurface flow. In reality thesecan be vague terms for describingvery dynamic processes. Forexample, surface or overland flowcan infiltrate into a soil duringmovement down a slope, movelaterally as interflow, and reappearas surface flow. In this publication,agricultural runoff refers to the totalloss of water from a watershed byall surface and subsurface pathways.

Forms and Processes

The loss of P in agricultural runoffoccurs in sediment-bound anddissolved forms (fig. 8). SedimentP includes P associated with soilparticles and organic materialeroded during flow events andconstitutes about 80 percent of Ptransported in surface runoff frommost cultivated land (Sharpley et al.1992). Surface runoff from grass,forest, or noncultivated soils carrieslittle sediment and is, therefore,generally dominated by dissolved P(about 80 percent of P loss). Thisdissolved form comes from therelease of P from soil and plantmaterial (fig. 8). This release occurswhen rainfall or irrigation waterinteracts with a thin layer of surfacesoil (1 to 2 inches) and plant mate-rial before leaving the field assurface runoff (Sharpley 1985).Most dissolved P is immediately

11

available for biological uptake.Sediment P is not readily available,but it can be a long-term source of P

for aquatic biota (Ekholm 1994,Sharpley 1993).

In most watersheds, P export occursmainly in surface runoff, rather thansubsurface flow. However, in someregions, notably the Coastal Plainsand Florida, as well as fields withsubsurface drains, P can be trans-ported in drainage waters. Gener-ally, the concentration of P in waterpercolating through the soil profileis low because of P fixation by P-deficient subsoils. Exceptions occurin sandy, acid organic, or peaty soilswith low P fixation or holdingcapacities and in soils where thepreferential flow of water can occurrapidly through macropores andearthworm holes (Bengston et al.1992, Sharpley and Syers 1979,Sims et al. 1998).

Irrigation, especially furrow irriga-tion, can significantly increase thepotential for soil and water contactand therefore can increase P loss byboth surface runoff and erosion in

Subsurfaceflow

Tile flow

P leachingis small

Erosion ofparticulate P

Release of soil and plantP to surface runoff

Total runoff P

Figure 8. Phosphorus can be released from soil and plant material tosurface and subsurface runoff water or lost by erosion.

12

return flows. Furrow irrigationexposes unprotected surface soil tothe erosive effect of water move-ment. The process of irrigation alsohas the potential to greatly increasethe land area that can serve as apotential source for P movement, afact that is especially important inthe western United States.

The Dependence of AgriculturalRunoff P on Soil P

Many studies report that the loss ofdissolved P in surface runoffdepends on the P content of surfacesoil (fig. 9). In a review of severalstudies, Sharpley et al. (1996) foundthat the relationship between surfacerunoff P and soil P varies withmanagement. Relationship slopeswere flatter for grass (4.1 to 7.0,mean 6.0) than for cultivated land(8.3 to 12.5, mean 10.5), but theslopes were too variable to allow

use of a single or average relation-ship to recommend P amendmentsbased on water-quality criteria.Clearly, several soil and landmanagement factors influence therelationship between dissolved P insurface runoff and soil P.

All in all, the loss of P in subsurfaceflow, as well as surface runoff, is

linked to soil P concentration.Heckrath et al. (1995) found thatsoil test P (Olsen P) greater than 60ppm in the plow layer of a silt loamcaused the dissolved P concentra-tion in tile drainage water to in-crease dramatically (0.15 to 2.75mg/L) (fig. 9). They postulated thatthis level (60 ppm), which is well

0

3

2

1

030 60 90 1205000 100 200 300 400

1.5

0.5

2

1

0

Ru

no

ff d

isso

lved

P (

mg

/L)

Arkansasy = 0.32 + 0.0031x

r2 = 0.76

Mehlich-3 soil P, ppm

Pennsylvaniay = 0.02 + 0.0033x

r2 = 0.85

Oklahomay = 0.01 + 0.0031x

r2 = 0.88

Olsen soil P, ppm

Change point60 ppm

Surface runoff Subsurface tile drainage

Figure 9. Effect of soil P on the dissolved P concentration of surface runoff fromseveral pasture watersheds (adapted from Sharpley et al. 2001) and subsurface tiledrainage from Broadbalk fields. (Adapted from Heckrath et al. 1995.)

13

above that needed by major cropsfor optimum yield, is a critical pointabove which the potential for Pmovement through the soil profilegreatly increases (Ministry ofAgriculture, Food and Fisheries1994). Similar studies suggest thatthis change point can vary threefoldas a function of site hydrology,relative drainage volumes, and soilP release characteristics (Sharpleyand Syers 1979).

These and similar studies comparedagricultural runoff P to soil P usingtraditional soil test methods thatestimate plant availability of soil P.While these studies show promise indescribing the relationship betweenthe level of soil P and surface runoffP, they are limited for severalreasons. First, soil test extractionmethods were developed to estimatethe plant availability of soil P andmay not accurately reflect soil Prelease to surface or subsurface

runoff water. Second, althoughdissolved P is an important water-quality variable, it represents onlythe dissolved portion of P readilyavailable for aquatic plant growth. Itdoes not reflect fixed soil P that canbecome available with changingchemistry in anaerobic conditions.

The final concern is with samplingdepth. It is generally recommendedthat soil samples be collected toplow depth, usually 6 to 8 inches forroutine evaluation of soil fertility.However, it is the surface inch ortwo in direct contact with runoffthat is important when using soiltesting to estimate P loss. Conse-quently, different sampling proce-dures may be necessary when usinga soil test to estimate the potentialfor P loss. To overcome theseconcerns, approaches are beingdeveloped that provide a moretheoretically sound estimate, thantraditional agronomic chemical

extractants do, of the amount of P insoil that can be released to runoffwater and the amount of algal-available P in runoff (Pierzynski2000, Sharpley 1993).

An approach, developed in theNetherlands by Breeuwsma andSilva (1992) to assess P leachingpotential, is to determine soil Psaturation (percent saturation =available P/maximum P fixation).This approach is based on the factthat, as P saturation or the amountof fixed P increases, more P isreleased from soil to surface runoffor leaching water. This method isused to limit the loss of P in surfaceand ground waters. A critical Psaturation of 25 percent has beenestablished for Dutch soils as thethreshold value above which thepotential for P movement in surfaceand ground waters becomes unac-ceptable.

14

Remediation

The overall goal to reduce P lossfrom agriculture to water shouldincrease the efficiency of P use bybalancing P inputs in feed andfertilizer into a watershed with Poutputs in crop and animal produceand managing the level of P in thesoil. Reducing P loss in agriculturalrunoff may be brought about bysource and transport control strate-gies. The transport of P fromagricultural land in surface runoffand erosion has been reduced;however, much less attention hasbeen directed toward source man-agement.

When looking at management tominimize the environmental impactof P, there are several importantfactors that must be considered. Tocause an environmental problem,there must be a source of P (that is,

high soil levels, manure or fertilizerapplications, etc.) and it must betransported to a sensitive location(that is, for leaching, runoff, ero-sion, etc.). Problems occur wherethese two come together. A high Psource with little opportunity fortransport may not constitute an

environmental threat. Likewise, asituation where there is a highpotential for transport but no sourceof P to move is also of little threat.Management should focus on theareas where these two conditionsintersect. These areas are called“critical source areas” (fig. 10).

High P source High transport

Critical source area

Figure 10. Critical source areas for P loss from a watershed occurwhere areas of high soil P and transport potential coincide.

15

Source Management

Reducing off-farm inputs of P infeed

Manipulation of dietary P intake byanimals may help balance farm Pinput and output in animal opera-tions because feed inputs are oftenthe major cause of P surplus (table1). Morse et al. (1992) recorded a17-percent reduction in P excretionwhen the daily P intake of dairycows was reduced from 82 to 60 g/day. The U.S. National ResearchCouncil (2001) recommends dietaryP levels for animal production anddairy cows that range between 0.32and 0.38 percent P, depending onmilk yields (table 2). Dietary P inexcess of these recommendationscan be decreased without harmingproduction or animal health. In fact,Wu et al. (2001) found essentiallyall P fed in excess of 0.32 percentwas excreted by high-producingdairy cows (table 2).

The potential effect of overfeedingP to dairy cows and land whenapplying manure on runoff P wasdemonstrated by Ebeling et al.(2002). When cows had 0.31 and0.47 percent P in their diets and themanure (0.48 and 1.28 percent P,respectively) was applied to siltloams covered with corn residues in

Wisconsin, runoff P increaseddramatically from 7 to 79 g/ha(table 2).

Clearly, amounts of excreted P canbe reduced by carefully matchingdietary P inputs to animal require-ments. As P requirements canchange during an animal’s lifecycle, including lactation in dairy

Dietary P Milk P excreted1 Runoff dissolved P2

level production1

% kg/day g P/day ppm g/ha

0.31 42.4 43 0.30 7

0.39 38.7 66 N.D.3 N.D.

0.47 39.4 88 2.84 79

SOURCE: Adapted from Wu et al. (2001).SOURCE: Adapted from Ebeling et al. (2002). The high P diet in this study was 0.49% P.N.D. No data available from this study.

Table 2. Dairy cattle feed recommendations, milk production, fecal Pexcretion, and losses of P in surface runoff after land application ofmanure

16

cows for example, further gains indecreasing P excretion may be madeby periodically changing dietary Plevels.

It is common to supplement poultryand hog feed with mineral forms ofP because of the low digestibility ofphytin, the major P compound ingrain. This supplementation contrib-utes to P enrichment of manures andlitters. Enzyme additives for animalfeed are being tested to increase theefficiency of P uptake from grainduring digestion. Development ofsuch enzymes that would be cost-effective in terms of animal weightgain may reduce the P content ofmanure. One method is to usephytase, an enzyme that enhancesthe efficiency of P recovery fromphytin in grains fed to poultry andhogs. Another promising method isto develop grain varieties that arelower in phytin.

A third method is to increase thequantity of P in corn that is avail-able to animals by reducing theamount of phytate produced bycorn. This would decrease phytate-P, which contributes as much as 85percent of P in corn grain, andincrease inorganic P concentrationsin grain. Ertl et al. (1998) manipu-lated the genes controlling phytateformation in corn and showed thatthe use of low-phytate corn inpoultry feed can increase theavailability of P and other phytate-bound minerals and proteins andreduce P excretion.

Soil P management and estimat-ing threshold levels for environ-mental risk assessment

The long-term use of commercialfertilizers has increased the P statusof many agricultural soils to opti-mum or excessive levels. The goalof P fertilization was to remove soil

P supply as a limitation to agricul-tural productivity; however, formany years actions taken to achievethis goal did not consider theenvironmental consequences of Ploss from soil to water. The con-straint on P buildup in soils fromcommercial fertilizer use wasusually economic, with mostfarmers recognizing that soil testsfor P accurately indicated when tostop applying fertilizer P. Some“insurance” fertilization has alwaysoccurred, particularly in high-valuecrops, such as vegetables, tobacco,and sugarcane. However, the use ofcommercial fertilizers alone wouldnot be expected to grossly overfer-tilize soils because farmers wouldcease applying fertilizer P when itbecame unprofitable. Today’sdilemma is caused by the realizationthat soils considered optimum insoil test P (or perhaps only slightlyoverfertilized) from a crop produc-

17

tion perspective may still provideenvironmentally significant quanti-ties of soluble and sediment P insurface runoff and erosion.

Environmental concern has forcedmany states to consider developingrecommendations for P applicationsand watershed management basedon the potential for P loss in agricul-tural runoff. A major difficulty is

and Michigan) to 4 times (Pennsyl-vania and Texas) the agronomicthresholds.

Soil test results for environmentalpurposes must be interpretedcarefully. The comments given onsoil test reports—low, medium,optimum, high, and so forth—wereestablished based on the expectedresponse of a crop to P. However,one cannot assume a direct relation-ship between the soil test calibrationfor crop response to P and runoffenrichment potential. In otherwords, one cannot accurately projectthat a soil test level above anexpected crop response levelexceeds crop needs and is thereforepotentially polluting. What will becrucial in terms of managing Pbased in part on soil test levels willbe the interval between the criticalor threshold soil P value for cropyield and runoff P (fig. 11).

Figure 11.As soil P in-creases, sodoes cropyield and thepotential for Ploss in surfacerunoff. Theinterval be-tween thecritical soil Pvalue for yieldand runoff Pwill be impor-tant for Pmanagement.

the identification of a threshold soiltest P level to estimate when soil Pbecomes high enough to result inunacceptable P enrichment ofagricultural runoff. Table 3 givesexamples from several states, alongwith agronomic threshold concen-trations for comparison.

Environmental threshold levelsrange from less than 2 times (Maine

Soil test P

Cro

p y

ield

P lo

ss in

su

rfac

e ru

no

ff

Criticalvalue

for yield

Criticalvalue

for P loss

Low Optimum High

MediumLow High

18

Threshold values, ppmSoil test P Management recommendations

State Agronomic Environmental method to protect water quality

Table 3. Threshold soil test P values and P management recommendations

Arkansas 50 150 Mehlich-3 Above 150 ppm P: add no P, provide buffers next to streams,overseed pastures with legumes to aid P removal, and provideconstant soil cover to minimize erosion.

Colorado 15* 20 Olsen Above 20 ppm P: use P index.

Delaware 50 150 Mehlich-3 Above 150 ppm P: develop P-based nutrient managementplan (for example, P addition not to exceed crop removal) or

use P index.

Idaho 40 40 Olsen Above 40 ppm P: addition not to exceed crop removal andplan required to decrease soil test P to < 40 ppm andminimize transport potential.

Kansas 50 200 Mehlich-3 Above 200 ppm P: no P addition regardless of P indexrating.

Maine 20 20 Morgan Row crops > 20 ppm soil P: addition not to exceed cropremoval for highly erodible soils or soil in P sensitivewatershed.Sod crop: P addition not to exceed crop removal if soil testP is > 5 times crop removal.

Maryland 25 75 Mehlich-1 Use P index > 75 ppm P: soils with high index must reduceor eliminate P additions.

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Threshold values, ppmSoil test P Management recommendations

State Agronomic Environmental method to protect water quality

Table 3. Threshold soil test P values and P management recommendations (continued)

Michigan 40 75 and 150 Bray-1 75 to 150 ppm P: P addition not to exceed crop removal.Above 150 ppm P: apply no P until soil text P is < 150 ppm P.

Ohio 40 150 Bray-1 Above 40 ppm P: no fertilizer P addition.Above 150 ppm P: apply no P until soil test P is < 150 ppm P.

Oklahoma 30 130 and 200 Mehlich-3 Non-nutrient limited watershed 130 to 200 ppm P - half P rateand adopt measures to decrease runoff and erosion;> 200 ppm P - P addition not to exceed crop removal.Nutrient limited watershed 60 to 130 ppm P - half P rate;> 130 ppm P - add no P.Slope – 8 to 15% halve P rate: > 15% add no P.

Pennsylvania 50 200 Mehlich-3 Above 200 ppm P and < 150 ft from stream: use P index.

Texas 44 200 Texas A&M Above 200 ppm P: addition not to exceed crop removal.

Wisconsin 30 100 Bray-1 50 to 100 ppm P: P addition not to exceed crop removal.Above 100 ppm P: P additions must be < crop removal or useP index to determine if P additions are restricted.

In Your Area

SOURCE: Adapted from Lory and Scharf 2000, Sharpley et al. 1996.*AB-DTPA is ammonium bicarbonate – diethylelenetriaminepentaacetic acid (Pierzynski 2000).

20

There is reluctance on the part ofmost soil testing programs toestablish upper threshold limits forsoil test P. Reasons range from thefact that soil tests were not origi-nally designed or calibrated forenvironmental purposes to anunjustified reliance upon soil test Palone by environmental regulatoryagencies. Refusing to participate inthe debate on the appropriateness ofcritical limits for soil test P isextremely shortsighted and mayforce others with less expertise toset the limits that are so important tothe soil testing and agriculturalcommunity. A foresighted stanceacknowledges that agronomicallybased soil tests can play a role inenvironmental management of soilP but are only a first step in a morecomprehensive approach. Thisawareness will enhance the credibil-ity of soil testing programs andimprove the contribution they maketo the agricultural community.

Manure management

Farm advisers and resource plannersare recommending that P content ofmanure and soil be determined bysoil test laboratories before landapplication of manure. This isimportant because without suchdeterminations, farmers and theiradvisers tend to underestimate thenutritive value of manure. Soil testresults can also demonstrate thepositive and negative long-termeffects of manure use and the timerequired to build up or deplete soilnutrients. For instance, soil tests canhelp a farmer identify the soils inneed of P fertilization, those wheremoderate manure applications maybe made, and fields where nomanure applications need to bemade for crop yield response.

Commercially available manureamendments, such as slaked lime oralum, can reduce ammonia (NH

3)

volatilization, leading to improved

animal health and weight gains;reduce the solubility of P in poultrylitter by several orders of magni-tude; and decrease dissolved P,metal, and hormone concentrationsin surface runoff (Moore and Miller1994, Moore et al. 1995, Nichols etal. 1997). Also, the dissolved Pconcentration (11 mg/L) of surfacerunoff from fescue treated withalum-amended litter was muchlower than that from fescue treatedwith unamended litter (83 mg/L)(Shreve et al. 1995). Perhaps themost important benefit of manureamendments for air and waterquality would be an increase in theN:P ratio of manure via reduced Nloss because of NH

3 volatilization.

An increased N:P ratio of manurewould more closely match crop Nand P requirements.

A mechanism should be establishedto facilitate movement of manuresfrom surplus to deficit areas. At the

21

moment, manures are rarely trans-ported more than 10 miles fromwhere they are produced. However,mandatory transport of manure fromfarms with surplus nutrients toneighboring farms where nutrientsare needed faces several significantobstacles. First, it must be shownthat manure-rich farms are unsuit-able for manure application basedon soil properties, crop nutrientrequirements, hydrology, actual Pmovement, and sensitive waterbodies. Then, it must be shown thatthe recipient farms are more suitablefor manure application. The greatestsuccess with redistribution ofmanure nutrients is likely to occurwhen the general goals of nutrientmanagement set by a national (orstate) government are supported byconsumers, local governments, thefarm community, and the animalindustry. This may initially requireincentives to facilitate subsequent

transport of manures from one areato another. Again, this may be ashort-term alternative if N-basedmanagement is used to apply thetransported manures. If this hap-pens, soil P in areas receivingmanures may become excessive in 3to 5 years.

Innovative methods are being usedby some farmers to transportmanure. For example, grain or feedtrucks and railcars are transportingdry manure back to the grain sourcearea instead of returning empty(Collins et al. 1988). In Delaware, alocal poultry trade organization hasestablished a manure bank networkthat puts manure-needy farmers incontact with manure-rich poultrygrowers. Even so, large-scaletransportation of manure fromproducing to non-manure-producingareas is not occurring.

Composting, another potential tool,may also be considered a manage-ment tool to improve manuredistribution. Although it tends toincrease the P concentration ofmanures, composting reduces thevolume of manures and thereforetransportation costs. Additionalmarkets are also available forcomposted materials. As the valueof clean water and the cost ofsustainable manure management isrealized, it is expected that alterna-tive entrepreneurial uses for manurewill be developed, become morecost-effective, and create expandingmarkets.

Transport Management

Phosphorus loss via surface runoffand erosion may be reduced byconservation tillage and cropresidue management, buffer strips,riparian zones, terracing, contour

22

farming tillage, cover crops, andimpoundments (settling basins).Basically, these practices reduce theimpact of rainfall on the soil sur-face, reduce runoff volume andvelocity, reduce sediment transport,and increase soil resistance toerosion. None of these measures,however, should be relied on as thesole or primary practice to reduce Plosses in agricultural runoff.

Most of these practices are generallymore efficient at reducing sedimentP than dissolved P. Several re-searchers report little decrease inlake productivity with reduced Pinputs following implementation ofconservation measures (Gray andKirkland 1986, Knuuttila et al.1994, McDowell et al. 2002). Manytimes, the impact of remedialmeasures used to help improve poorwater quality will be slow becauselake and stream sediments can be along-term source of P in waters

even after inputs from agricultureare reduced. Therefore, immediateaction may be needed to reducefuture problems.

Targeting Remediation

Threshold soil P levels are beingproposed to guide P managementrecommendations. In most cases,agencies that seek these levels hopeto uniformly apply a threshold valueto areas and states under theirdomain. However, it is too simplis-tic to use threshold soil P levels asthe sole criterion to guide P man-agement and P applications. Forexample, adjacent fields havingsimilar soil test P levels but differ-ing susceptibilities to surface runoffand erosion, due to contrastingtopography and management,should not have similar P manage-ment recommendations. Also, it hasbeen shown that in some agricul-

tural watersheds, 90 percent ofannual algal-available P export fromwatersheds comes from only 10percent of the land area during a fewrelatively large storms (Pionke et al.1997). For example, more than 75percent of annual water dischargefrom watersheds in Ohio (Edwardsand Owens 1991) and Oklahoma(Smith et al. 1991) occurred duringone or two severe storms. Theseevents contributed over 90 percentof annual total P export (0.2 and 5.6lb/acre/yr in Ohio and Oklahoma,respectively). Therefore, thresholdsoil P values will have little mean-ing unless they are used in conjunc-tion with an estimate of a site’spotential for surface runoff anderosion.

A sounder approach advocated byresearchers and an increasingnumber of advisers is to link areasof surface runoff and high soil Pcontent in a watershed (fig. 12).

23

Soil test P >100 ppm

Area of high transport potential

Figure 12. Identifying P loss vulnerability(high soil test P and transport potential)to more effectively target measures toreduce P export in surface runoff fromwatersheds.

Areas most vulnerable to P loss

Integrated P and N management

24

Preventing P loss is now taking onthe added dimension of defining,targeting, and remediating sourceareas of P where high soil P levelscoincide with high surface runoffand erosion potentials. This ap-proach addresses P management atmultifield or watershed scales.Furthermore, a comprehensive Pmanagement strategy must addressdown-gradient water-quality im-pacts, such as the proximity of P-sensitive waters. Conventionallyapplied remediations may notproduce the desired results and mayprove to be an inefficient and costlyapproach to the problem if thissource-area perspective to targetapplication of P fertility, surfacerunoff, and erosion control technol-ogy is not used.

The concept of a simple P index hasbeen developed by a group ofresearch scientists with diverseexpertise as a screening tool for use

by field staffs, watershed planners,and farmers to rank the vulnerabilityof fields as sources of P loss insurface runoff (Lemunyon andGilbert 1993). The index accountsfor and ranks transport and sourcefactors controlling P loss in surfaceand subsurface runoff, delineatingsites where the risk of P movementis expected to be higher than that ofothers (table 4).

Site vulnerability to P loss insurface runoff is assessed byselecting rating values for a varietyof source and transport factors.Source factors of the P index arebased on soil test P and fertilizerand manure rate, method, andtiming of application. The correc-tion factor of 0.2 for soil test P isbased on field data that showed afive-fold greater concentration ofdissolved P in surface runoff withan increase in mineral fertilizer or

manure, compared to an equivalentincrease in Mehlich-3 P (Sharpleyand Tunney 2000).

To calculate transport potential foreach site, erosion, surface runoff,leaching potential, and connectivityvalues were first summed. Arelative transport potential wasdetermined by dividing this summedvalue by 22, which is the valuecorresponding to high transportpotential (erosion is 7, surfacerunoff is 8, leaching potential is 0,and connectivity is 8). This normal-ization process assumes that when asite’s full transport potential isrealized, the transport factor is 1 orgreater. Transport factors less than1 represent a fraction of the maxi-mum potential.

A P index value, representingcumulative site vulnerability to Ploss, is obtained by multiplying thesummed transport and source

25

0.7 1.0 1.1Modified connectivity Riparian buffer- applies Grassed waterway Direct connection-applies

to distance < 150 ft. or none to distance > 150 ft.

Transport factor = Modified connectivity x (Transport sum/22)

Phosphorus index value = 2 x Source factor x Transport factor

Table 4. The P indexing approach using Pennsylvania's index version from July 2001Transport Factors Your field

Erosion Soil loss (ton/A/yr)

0 2 4 6 8Runoff potential Very low Low Medium High Very high

0 1 2*Sub-surface drainage None Some Patterned

0 2 4 6 8Contributing distance > 500 ft 500 to 350 ft 350 to 250 ft 250 to 150 ft < 150 ft

Transport sum = Erosion + Runoff potential + Sub-surface drainage + Contributing distance

*As an example, indices for other states can be found on the National Phosphorus Research Project'shome page at http://pswmru.arsup.psu.edu/†Or rapid permeability soil near a stream.

(cont.)–

26

Manure 0.5 0.8 1.0P availability Treated manure/Biosolids Dairy Poultry/Swine

Manure rating = Rate x Method x Availability

Table 4. The P indexing approach using Pennsylvania's index version from July 2001 (cont.)Source Factors Your field

Soil test Soil test P (ppm P)

Soil test rating = 0.20* Soil test P (ppm P)

0.2 0.4 0.6 0.8 1.0Fertilizer Placed or injected Incorporated Incorporated > 1 Incorporated > 1 Surface appliedapplication 2" or more deep < 1 week week or not week or not to frozen ormethod incorporated incorporated snow-covered

April – October Nov. – March soil

Fertilizer rating = Rate x Method

Manure P rate Manure P (lb P2O

5/acre)

0.2 0.4 0.6 0.8 1.0Manure Placed or injected Incorporated Incorporated > 1 Incorporated > 1 Surface appliedapplication 2" or more deep < 1 week week or not week or not to frozen ormethod incorporated incorporated snow-covered

April – October Nov. – March soil

Fertilizer P rate Fertilizer P (lb P2O

5/acre)

27

factors. Index values are normal-ized so that the break between highand very high categories is 100. Inmost indices, this simply requiresmultiplying the index value by 2.The P index value for a site can thenbe used to categorize the site’svulnerability to P loss (table 5).

The index is a tool for field person-nel to identify agricultural areas ormanagement practices that have thegreatest potential to accelerateeutrophication. It can be used toidentify management optionsavailable to land users and willallow them flexibility in developingremedial strategies. The first step isto determine the P index for soilsadjacent to sensitive waters andprioritize the efforts needed toreduce P losses. Then, managementoptions appropriate for soils withdifferent P index ratings can beimplemented. General recommenda-

P index Rating General interpretation

< 60 Low Low potential for P loss. If current farming practicesare maintained, there is a low risk of adverseimpacts on surface waters.

60 to 80 Medium Medium potential for P loss. The chance for adverseimpacts on surface waters exists, and someremediation should be taken to minimize therisk of P loss.

80 to 100 High High potential for P loss and adverse impacts onsurface waters. Soil and water conservation measuresand P management plans are needed to minimize therisk of P loss.

> 100 Very high Very high potential for P loss and adverse impactson surface waters. All necessary soil and waterconservation measures and a P management planmust be implemented to minimize the P loss.

Table 5. General interpretation of the P index

Your Field ➔

28

tions are given in table 6; however,P management is very site specificand requires a well-planned, coordi-nated effort among farmers, exten-sion agronomists, and soil conserva-tion specialists.

Making ManagementDecisions

Farm N inputs can usually be moreeasily balanced with plant uptakethan P inputs can, particularly whereconfined animal operations exist. Inthe past, separate strategies foreither N or P were developed andimplemented at farm or watershedscales. Because N and P havedifferent chemistry and flow path-ways through soils and watersheds,these narrowly targeted strategiesoften conflict and lead to compro-mised water quality. For example,manure application based on crop N

requirements to minimize nitrateleaching to groundwater oftenresults in excess soil P and enhancespotential P losses in surface runoff.In contrast, reducing surface runofflosses of P via conservation tillagecan increase water infiltration intothe soil profile and enhance nitrateleaching.

For P, a primary strategy is tominimize surface runoff and par-ticulate transport. In most cases,decreasing P loss by plant cover,crop residues, tillage and plantingalong contours, and buffer zonesalso decreases nitrate loss. Someexceptions are practices that pro-mote water infiltration, which tendto increase leaching, and tillagepractices that do not incorporate Pfertilizers and manures into the soil.No-till is commonly recommendedas a conservation measure forcropland that is eroding. Conversionto no-till is followed by loss of soil

and total N and P in surface runoffand increased nitrate leaching andalgal-available P transport (Sharpleyand Smith 1994).

Nitrogen losses can occur from anylocation in a watershed, so remedialstrategies for N can be applied to thewhole watershed. Phosphorus lossesusually occur from areas prone tosurface runoff; therefore, the mosteffective P strategy would be to (1)avoid excessive soil P buildup in thewhole watershed and thereby limitlosses in subsurface flow and (2)use more stringent measures for themost vulnerable sites to minimizeloss of P in surface runoff.

Development of sound remedialmeasures should consider theseconflicting impacts of conservationpractices on resultant water quality.Clearly, a technically sound frame-work must be developed thatincludes critical sources of N and P

29

Table 6. Management options to minimize nonpoint - source pollution of surface waters by soil P Phosphorus index Management options

< 60 Soil testing: Test soils for P at least every 3 years to monitor buildup or decline in soil P. (Low)

Soil conservation: Follow good soil conservation practices. Consider effects of changes in tillage practicesor land use on potential for increased transport of P from site.

Nutrient management: Consider effects of any major changes in agricultural practices on P loss beforeimplementing them on the farm. Examples include increasing the number of animal units on a farm orchanging to crops with a high demand for fertilizer P.

60 to 80 Soil testing: Test soils for P at least every 3 years to monitor buildup or decline in soil P. Conduct a more (Medium) comprehensive soil testing program in areas identified by the P index as most sensitive to P loss by surface

runoff, subsurface flow, and erosion.

Soil conservation: Implement practices to reduce P loss by surface runoff, subsurface flow, and erosion inthe most sensitive fields (that is, reduced tillage, field borders, grassed waterways, and improved irrigationand drainage management).

Nutrient management: Any changes in agricultural practices may affect P loss; carefully consider thesensitivity of fields to P loss before implementing any activity that will increase soil P. Avoid broadcastapplications of P fertilizers and apply manures only to fields with low P index values.

(cont.)–

30

Table 6. Management options to minimize nonpoint - source pollution of surface waters by soil P (cont.) Phosphorus index Management options

80 to 100 Soil testing: A comprehensive soil testing program should be conducted on the entire farm to determine fields (High) that are most suitable for further additions of P. For fields that are excessive in P, estimates of the time required

to deplete soil P to optimum levels should be made for use in long-range planning.

Soil conservation: Implement practices to reduce P loss by surface runoff, subsurface flow, and erosion in themost sensitive fields (that is, reduced tillage, field borders, grassed waterways, and improved irrigation anddrainage management). Consider using crops with high P removal capacities in fields with high P index values.

Nutrient management: In most situations involving fertilizer P, only a small amount used in starter fertilizersis needed. Manure may be in excess on the farm and should only be applied to fields with lower P indexvalues. A long-term P management plan should be considered.

> 100 Soil testing: For fields that are excessive in P, estimate the time required to deplete soil P to optimum levels (Very high) for use in long-range planning. Consider using new soil testing methods that provide more information on

environmental impact of soil P.

Soil conservation: Implement practices to reduce P loss by surface runoff, subsurface flow, and erosion in themost sensitive fields (that is, reduced tillage, field borders, grassed waterways, and improved irrigation anddrainage management). Consider using crops with high P removal capacities in fields with high P index values.

Nutrient management: Fertilizer and manure P should not be applied for 3 years or more. A comprehensive,long-term P management plan must be developed and implemented for an entire crop rotation.

31

export from agricultural watershedsso that optimal strategies at farmand watershed scales can be imple-mented to best manage N and P.

SummaryThe overall goal to reduce P lossesfrom agriculture should be tobalance off-farm inputs of P in feedand fertilizer with outputs in prod-ucts and to manage soils in waysthat retain crop nutrient resources.Source and transport control strate-gies can provide the basis forincreasing P-use efficiency inagricultural systems.

Future advisory programs shouldreinforce the fact that all fields donot contribute equally to P exportfrom watersheds. Most P exportcomes from only a small portion ofthe watershed as a result of rela-tively few storms. Although soil P

content is important in determiningthe concentration of P in agriculturalrunoff, surface runoff and erosionpotential often override soil levelsin determining P export. If water orsoil do not move from a field orbelow the root zone, then P will notmove. Clearly, management systemswill be most effective if targeted tothe hydrologically active sourceareas in a watershed that operateduring a few major storms.

Manure management recommenda-tions will have to account for sitevulnerability to surface runoff anderosion, as well as soil P content,because not all soils and fields havethe same potential to transfer P tosurface runoff and leaching. As aresult, threshold soil P levels shouldbe indexed against P transportpotential, with lower values for Psource areas than for areas notcontributing to water export.

Phosphorus applications at recom-mended rates can reduce P loss inagricultural runoff via increasedcrop uptake and cover. It is of vitalimportance that managementpractices be implemented to mini-mize soil P buildup in excess ofcrop requirements, reduce surfacerunoff and erosion, and improvecapability to identify fields that aremajor sources of P loss to surfacewaters.

Overall—• management systems should

balance P inputs and outputs atfarm and watershed scales;

• source and transport controlsshould target and identifycritical source areas of Pexport from watersheds; and

• farmers should link thresholdsoil P levels that guide manureapplications with sitevulnerability to P loss.

32

Consideration of all these factorswill be needed to develop extensionand demonstration projects thateducate farmers, the animal indus-try, and the general public aboutwhat is actually involved in ensur-ing clean water. It is hoped this willhelp overcome the common miscon-ception that diffuse or nonpointsources are too difficult, costly, orvariable to control or target substan-tial reductions (fig. 13).

Efforts to implement defensibleremedial strategies that minimize Ploss from agricultural land willrequire interdisciplinary researchinvolving soil scientists, hydrolo-gists, agronomists, limnologists, andanimal scientists. Development ofguidelines to implement suchstrategies will also require consider-ation of the socioeconomic andpolitical impacts of any manage-ment changes on rural and urban

communities and of the mechanismsby which change can be achieved ina diverse and dispersed communityof land users.

33

Phosphorus does not move through thesoil. While most P losses occur with

surface runoff, P may move through soilswith combinations of low P-fixing capaci-ties, with preferential flow (or subsurfacedrains), or high soil test P contents.

Erosion control will stop P losses inrunoff. Erosion control is not the sole

answer; reduction of dissolved P loss inrunoff can only be achieved by minimizingP loss at the source and implementingpractices that reduce total P in runoff.

By controlling point sources we cansolve water quality problems. Although

point source inputs have been reduced inmany areas, nonpoint source inputs nowcontribute to a greater share of water qualityproblems.

Most management practices arepermanent solutions. In most cases the

only permanent solution to reducing P lossesis balancing farm and watershed P inputsand outputs.

Soils are infinite sinks for P. Researchshows that soils cannot indefinitely fix

applied P. Continued applications of Pbeyond crop requirements, a commonscenario where organic wastes have beenheavily used in agriculture, are a majorcause of soil P saturation.

Crop N requirements should drivemanure management. Basing manure

management on mature N and crop N needscan lead to undesirably high P applicationsdue to the unfavorable N:P ratios of mostmanures and crop requirements.

Figure 13. Several myths about P still exist:

Phosphorus management strategies canbe universally applied. All fields and

water bodies are not created equal; manage-ment plans for P and best managementpractices must be tailored to site vulnerabil-ity to P loss and proximity of P-sensitivewaters.

We don't know enough about agricul-tural P. We know a lot about how P

reacts with soil and is transferred to runoff,but we have not adequately disseminatedthis information to land users and state andFederal agencies.

34

ReferencesBacon, S.C., L.E. Lanyon, and R.M.Schlauder, Jr. 1990. Plant nutrientflow in the managed pathways of anintensive dairy farm. AgronomyJournal 82:755–761.

Barber, S.A. 1979. Soil phosphorusafter 25 years of cropping with fiverates of phosphorus application.Communications in Soil Scienceand Plant Analysis 10:1459–1468.

Bengston, L., P. Seuna, A. Lepisto,and R.K. Saxena. 1992. Particlemovement of meltwater in asubdrained agricultural basin.Journal of Hydrology 135:383–398.

Breeuwsma, A., and S. Silva. 1992.Phosphorus fertilization and envi-ronmental effects in The Nether-lands and the Po region (Italy).Report No. 57. Agricultural Re-search Department, The Winand

Staring Centre for Integrated Land,Soil and Water Research,Wageningen, The Netherlands.

Burkholder, J.A., and H.B. Glasgow,Jr. 1997. Pfiesteria piscicida andother Pfiesteria–dinoflagellatebehaviors, impacts and environmen-tal controls. Limnology and Ocean-ography 42:1052–1075.

Collins, E.R., Jr., J.M. Halstead,H.V. Roller, et al. 1988. Applicationof poultry manure— logistics andeconomics. In E.C. Naber, ed.,Proceedings of the National PoultryWaste Management Symposium,pp.125–132. Ohio State UniversityPress, Columbus.

Ebeling, A.M., L.G. Bundy, J.M.Powell, and T.W. Andraski. 2002.Dairy diet phosphorus effects onphosphorus losses in runoff fromland-applied manure. Soil ScienceSociety of America Journal 66:284–291.

Edwards, W.M., and L.B. Owens.1991. Large storm effects on totalsoil erosion. Journal of Soil andWater Conservation 46:75–77.

Ekholm, P. 1994. Bioavailability ofphosphorus in agriculturally loadedrivers in southern Finland.Hydrobiologia 287:179–194.

Ertl, D.S., K.A. Young, and V.Raboy. 1998. Plant genetic ap-proaches to phosphorus manage-ment in agricultural production.Journal of Environmental Quality27:299–304.

Fixen, P.E. 1998. Soil test levels inNorth America. Better Crops 82:16-18.

Fixen, P.E., and J.N. Grove. 1990.Testing soils for phosphorus. InR.L. Westerman, ed., Soil Testingand Plant Analysis, 3rd ed., pp.141–180. SSSA Book Series No. 3.Soil Science Society of America,Madison, WI.

35

Fixen, P.E., and T.L. Roberts. 2002.Status of soil nutrients in NorthAmerica. In Plant Nutrient Use inNorth American Agriculture, pp. 9–12. Potash & Phosphate Institute,Potash & Phosphate Institute ofCanada, Foundation for AgronomicResearch Technical Bulletin 2001–1.

Gray, C.B.J., and Kirkland, R.A.1986. Suspended sediment phospho-rus composition in tributaries of theOkanagan Lakes, BC. Water Re-search 20:1193–1196.

Heckrath, G., P.C. Brookes, P.R.Poulton, and K.W.T. Goulding.1995. Phosphorus leaching fromsoils containing different phosphorusconcentrations in the Broadbalkexperiment. Journal of Environmen-tal Quality 24:904–910.

Kellogg, R.L., and C.H. Lander.1999. Trends in the potential ofnutrient loading from confinedlivestock operations. In The State of

North America’s Private Land,USDA-NRCS, U.S. GovernmentPrinting Office, Washington, DC.(http://www.nrcs.usda.gov/techni-cal/land/pubs.ntrend.html).

Knuuttila, S., O.P. Pietilainen, andL. Kauppi. 1994. Nutrient balancesand phytoplankton dynamics in twoagriculturally loaded shallow lakes.Hydrobiologia 276:359–369.

Lanyon, L.E. 2000. Nutrient man-agement: regional issues facing theChesapeake Bay. In A.N. Sharpley,ed., Agriculture and PhosphorusManagement: The Chesapeake Bay,pp. 145–158. CRC Press, BocaRaton, FL.

Lanyon, L.E., and P.B. Thompson.1996. Changing emphasis of farmproduction. In M. Salis and J.Popow, eds., Animal Agricultureand the Environment: Nutrients,Pathogens, and Community Rela-tions, pp. 15–23. Northeast Re-

gional Agricultural EngineeringService, Ithaca, NY.

Lemunyon, J.L., and R.G. Gilbert.1993. Concept and need for aphosphorus assessment tool. Jour-nal of Production Agriculture6:483–486.

Lory, J.A., and P.C. Scharf. 2000.Threshold phosphorus survey.SERA–17 Minimizing PhosphorusLosses from Agriculture. (http://www.ces.soil.ncsu.edu/sera17/).

McCollum, R.E. 1991. Buildup anddecline in soil phosphorus: 30-yeartrends on a Typic Umprabuult.Agronomy Journal 83:77–85.

McDowell, R.W., A.N. Sharpley,and A.T. Chalmers. 2002. Land useand flow regime effects in phospho-rus chemical dynamics in the fluvialsediment of the Winooski River,Vermont. Ecological Engineering18:477–487.

36

Mehlich, A. 1984. Mehlich 3 soiltest extractant: a modification ofMehlich 2 extractant. Communica-tions in Soil Science and PlantAnalysis 15:1409–1416.

Ministry of Agriculture, Food andFisheries. 1994. Fertilizer recom-mendations for agricultural andhorticultural crops. Ministry ofAgriculture, Fisheries and FoodReference Book 209. Her Majesty’sStationery Office, London.

Moore, P.A., Jr., and Miller, D.M.1994. Decreasing phosphorussolubility in poultry litter withaluminum, calcium and iron amend-ments. Journal of EnvironmentalQuality 23:325–330.

Moore, P.A., Jr., T.C. Daniel, D.R.Edwards, and D.M. Miller. 1995.Effect of chemical amendments onammonia volatilization from poultrylitter. Journal of EnvironmentalQuality 24:293–300.

Morse, D., H.H. Head, C.J. Wilcox,et al. 1992. Effects of concentrationof dietary phosphorus on amountand route of excretion. Journal ofDairy Science 75:3039–3045.

National Research Council. 1993.Soil and water quality: an agendafor agriculture. National AcademyPress, Washington, DC.

National Research Council. 2001.Nutrient requirements of dairycattle, 7th rev. ed. National Academyof Sciences, Washington, DC.

Nichols, D.J., T.C. Daniel, P.A.Moore, Jr., et al. 1997. Runoff ofestrogen hormone 17ß-estradiolfrom poultry litter applied topasture. Journal of EnvironmentalQuality 26:1002–1006.

Pierzynski, G.M., ed. 2000. Meth-ods of phosphorus analysis for soils,sediments, residuals, and waters.Southern Cooperative Series

Bulletin 396 Southern Extension–Research Activity–17 (SERA 17).(http://www.soil.ncsu.edu/sera17/publications/sera17-2/pm_cover.htm).

Pionke, H.B., W.J. Gburek, A.N.Sharpley, and J.A. Zollweg. 1997.Hydrologic and chemical controlson phosphorus loss fromcatchments. In H. Tunney, ed.,Phosphorus Loss to Water fromAgriculture, pp. 225–242. CABInternational Press, Cambridge,England.

Schindler, D.W. 1977. Evolution ofphosphorus limitation in lakes.Science 195:260–262.

Sharpley, A.N. 1985. Depth ofsurface soil-runoff interaction asaffected by rainfall, soil slope andmanagement. Soil Science Societyof America Journal 49:1010–1015.

37

Sharpley, A.N. 1993. Assessingphosphorus bioavailability inagricultural soils and runoff. Fertil-izer Research 36:259–272.

Sharpley, A.N., and S.J. Smith.1994. Wheat tillage and waterquality in the Southern Plains. SoilTillage Research 30:33–38.

Sharpley, A.N., and J.K. Syers.1979. Loss of nitrogen and phos-phorus in tile drainage as influencedby urea application and grazinganimals. New Zealand Journal ofAgricultural Research 22:127–131.

Sharpley, A.N., S.J. Smith, B.A.Stewart, and A.C. Mathers. 1984.Forms of phosphorus in soil receiv-ing cattle feedlot waste. Journal ofEnvironmental Quality 13:211–215.

Sharpley, A.N., S.J. Smith, O.R.Jones, et al. 1992. The transport ofbioavailable phosphorus in agricul-tural runoff. Journal of Environmen-tal Quality 21:30–35.

Sharpley, A.N., S.C. Chapra, R.Wedepohl, et al. 1994. Managingagricultural phosphorus for protec-tion of surface waters: issues andoptions. Journal of EnvironmentalQuality 23:437–451.

Sharpley, A.N., T.C. Daniel, J.T.Sims, and D.H. Pote. 1996. Deter-mining environmentally sound soilphosphorus levels. Journal of Soiland Water Conservation 51:160–166.

Sharpley, A.N., P.J.A. Kleinman,R.J. Wright, et al. 2001. The na-tional phosphorus project: address-ing the interface of agriculture andenvironmental phosphorus manage-ment in the USA. Section 2.3.Indicators for EnvironmentalPerformance. In J. Steenvoorden, F.Claessen, and J. Willems, eds.,Agricultural Effects on Ground andSurface Waters: Research at theEdge of Science and Society,

International Association of Hydro-logical Sciences, Publication No.273. IAHS Press, Wallingford,England.

Sharpley, A.N., and H. Tunney.2000. Phosphorus research strate-gies to meet agricultural and envi-ronmental challenges of the 21st

century. Journal of EnvironmentalQuality 29:176–181.

Shreve, B.R., P.A. Moore, Jr., T.C.Daniel, et al. 1995. Reduction ofphosphorus in runoff from field-applied poultry litter using chemicalamendments. Journal of Environ-mental Quality 24:106–111.

Sims, J.T. 1997. Agricultural andenvironmental issues in the manage-ment of poultry wastes: recentinnovations and long-term chal-lenges. In J. Rechcigl and H.C.MacKinnon, eds., Agricultural Usesof By-products and Wastes, pp. 72–

38

90. American Chemical Society,Washington, DC.

Sims, J.T., ed. 1998. Soil testing forphosphorus: environmental uses andimplications. A publication ofSERA–IEG 17, USDA–CSREESRegional Committee. SouthernCooperative Series Bulletin No.398.

Sims, J.T., R.R. Simard, and B.C.Joern. 1998. Phosphorus losses onagricultural drainage: historicalperspectives and current research.Journal of Environmental Quality27:277–293.

Smith, S.J., A.N. Sharpley, J.R.Williams, et al. 1991. Sediment-nutrient transport during severestorms. In S.S. Fan and Y.H. Kuo,eds., Fifth Interagency Sedimenta-tion Conference, pp. 48–55. LasVegas, NV. Federal Energy Regula-tory Commission, Washington, DC.

U.S. Environmental ProtectionAgency. 1996. Environmentalindicators of water quality in theUnited States. EPA 841–R–96–002.

Wu, Z., L.D. Satter, A.J.Blohowiak, et al. 2001. Milkproduction, phosphorus excretion,and bone characteristics of dairycows fed different amounts ofphosphorus for two or more years.Journal of Dairy Science 84:1738–1748.