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Agricultural Phosphorusand EutrophicationUnited States
Department ofAgriculture
AgriculturalResearchService
ARS–149
July 1999
A.N. Sharpley, T. Daniel, T. Sims, J. Lemunyon, R. Stevens, and R. Parry
United StatesDepartment ofAgriculture
AgriculturalResearchService
ARS–149
July 1999
Agricultural Phosphorusand Eutrophication
Sharpley is a soil scientist with the USDA–ARS, Pasture Systems and WatershedManagement Research Laboratory, University Park, PA; Daniel is a professor with theDepartment of Agronomy, University of Arkansas, Fayetteville, AR; Sims is a profes-sor with the Department of Plant Science, University of Delaware, Newark, DE;Lemunyon is an agronomist with the USDA–NRCS, Grazing Lands TechnologyInstitute, South National Technical Center, Fort Worth, TX; Stevens is an extensionsoil scientist with Research and Extension, Washington State University, Prosser,WA;and Parry is a national program manager with the U.S. Environmental ProtectionAgency, Washington, DC.
Abstract
Sharpley, A.N., T. Daniel, T. Sims,J. Lemunyon, R. Stevens, and R.Parry. 1999. Agricultural Phospho-rus and Eutrophication. U.S. Depart-ment of Agriculture, AgriculturalResearch Service, ARS–149, 42 pp.
Inputs of phosphorus (P) are essen-tial for profitable crop and animalagriculture. However, P export inwatershed runoff can accelerate theeutrophication of receiving freshwaters. The rapid growth andintensification of crop and animalfarming in many areas has createdregional imbalances in P inputs infeed and fertilizer and P output infarm produce. In many of theseareas, soil P has built up to levels inexcess of crop needs and now hasthe potential to enrich surface runoffwith P.
The overall goal of efforts to reduceP losses from agriculture to watershould be to increase P use effi-ciency, balance P inputs in feed andfertilizer into a watershed with Poutput in crop and animal produce,and manage the level of P in thesoil. Reducing P loss in agriculturalrunoff may be brought about bysource and transport controlstrategies. This includes refiningfeed rations, using feed additives toincrease P absorption by animals,moving manure from surplus todeficit areas, finding alternative usesfor manure, and targeting conserva-tion practices, such as reducedtillage, buffer strips, and covercrops, to critical areas of P exportfrom a watershed. In these criticalareas high P soils coincide withparts of the landscape where surfacerunoff and erosion potential arehigh.
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Keywords: eutrophication, fertilizer,phosphorus, P input, P output,runoff
While supplies last, copies of thispublication may be obtained at nocost from USDA–ARS, PastureSystems & Watershed ManagementResearch Laboratory, Curtin Road,University Park, PA 16802–3702.
Copies of this publication may bepurchased from the National Tech-nical Information Service, 5285 PortRoyal Road, Springfield, VA 22161;telephone (703) 605–6000.
iii
Issued July 1999
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 ................................................................... 22
Targeting Remediation ................................................................... 23
Summary ............................................................................................ 29
References ......................................................................................... 34
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Contents
1
AgriculturalPhosphorus andEutrophication
Introduction
Eutrophication
Phosphorus (P) is an essentialelement for plant and animal growthand its input has long been recog-nized as necessary to maintainprofitable crop and animalproduction. Phosphorus inputs canalso increase the biologicalproductivity of surface waters byaccelerating eutrophication.Eutrophication is the natural agingof lakes or streams brought on bynutrient enrichment. This processcan be greatly accelerated by humanactivities that increase nutrientloading rates to water.
Eutrophication has been identifiedas the main cause of impairedsurface water quality (U.S. Environ-mental Protection Agency 1996).Eutrophication restricts water use
for fisheries, recreation, industry,and drinking due to the increasedgrowth of undesirable algae andaquatic weeds and to 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 et al.1992).
2
ances in P inputs and outputs. Onaverage, only 30 percent of thefertilizer and feed P input to farmingsystems is output in crop and animalproduce. Therefore, when averagedover the total utilizable agriculturalland area in the United States, anannual P surplus of 30 lb/acre exists(National Research Council 1993).
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 became
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 aquaticproductivity. However, inDelaware’s inland bays (coastalestuaries), nitrate-N leaching haselevated N concentrations to thepoint where P is now the limitingfactor in eutrophication.
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 0.3ppm), 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 rapid growthand intensification of crop andanimal farming in many areas hascreated regional and local imbal-
3
less expensive), they could spatiallyseparate grain and animal produc-tion. By 1995, the major animal-producing states imported over 80percent of their grain for feed(Lanyon and Thompson 1996). Infact, less than a third of the grainproduced on farms today is fed onthe farm where it is grown (U.S.Department of Agriculture 1989).
The potential for P surplus at thefarm scale can increase whenfarming systems change fromcropping to intensive animalproduction, since P inputs becomedominated by feed rather thanfertilizer. With a greater reliance onimported feeds, only 27 percent ofthe 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
nutrients to a poultry farm and,therefore the primary source of anyon-farm nutrient excess, is in animal
feed. Annual P surpluses of 80 to110 lb/acre/yr were estimated bySims (1997) for a typical poultry
Table 1. Farming system and P balance
Farming system
P Crop* Dairy † Poultry ‡ Hogs§
Input - - - - - - - - - - - - - - - lb P/acre/yr - - - - - - - - - - - - - -
Fertilizer 20 10 — —
Feed — 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 growing corn and alfalfa.† 100-acre dairy farm with 65 dairy holsteins averaging 14,500 lb
milk/cow/yr, 5 dry cows, and 35 heifers. Crops were corn for silageand grain, alfalfa, and rye for forage.
‡ 30-acre poultry farm with 74,000 layers; output includes 335 lbP/acre/yr in eggs, 20 lb P/acre/yr sold in crops (corn and alfalfa),and 10 lb P/acre/yr manure exported from the farm.
§ 75-acre farm with 1,280 hogs, output includes 40 lb P/acre/yrmanure exported from the farm.
4
impacts, which in some cases occurmany miles from P sources. By thetime these water-quality impacts aremanifest, remedial strategies aredifficult and expensive toimplement; 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. A better understanding ofwhere P is coming from is needed,how much P in soil and water is toomuch, and how and where theseinputs and losses can be reduced inorder to develop agricultural re-source management systems that
sustain production, environmentalquality, and farming communities.
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 soildepth containing 2 million poundsof soil, the conversion factor forppm to lbs P/acre is 2.0. For moredetailed information on the methodsused for soil P testing, how theywere developed, and why they varyamong regions, see Fixen and Grove(1990), Sharpley et al. (1994, 1996),and Sims (1998).
grain farm in Delaware. Thisscenario is consistent with otherconcentrated animal productionindustries, 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 highP soils may be major contributingfactors to surface water eutrophica-tion. Agricultural runoff is all waterdraining from an area (field orwatershed) including surfacerunoff, subsurface flow, leaching,and tile drainage processes. Phos-phorus loss in agricultural runoff isnot of economic importance tofarmers because it amounts to only1 or 2 percent of the P applied.However, P loss can lead tosignificant off-site economic
5
Soil Phosphorus
Soil P exists in organic and inor-ganic forms, but these are notdiscrete entities with integradesamong forms occurring (fig. 1).Organic P consists of unde-composed residues, microbes, andorganic matter in the soil. InorganicP is usually associated with Al, Fe,and Ca (aluminum, iron, andcalcium, respectively) compoundsof varying solubility and availabilityto plants. Phosphorus has to beadded to most soils so that there areadequate levels for optimum cropgrowth and yield. However, P canbe rapidly fixed in forms andtherefore 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. 1). As a result, soil P tests were
Crop harvest
Manure P Fertilizer P
Labile Labile and fixed
Solution P
StableSoil test P
Stable
Organic P Inorganic P
Figure 1.The phosphorus cycle in soil
6
• 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
40
developed 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 due to sorption ofadded P, greater biological activity,cycling of P from roots toaboveground plant biomass, andmore organic material in surfacelayers (fig. 2). In reduced tillagesystems, fertilizers and manures arenot 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.
Figure 2. 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
Continual long-term application offertilizer or manure at levels ex-ceeding crop needs, will increasesoil P levels (fig. 3). In many areasof intensive, confined animalproduction, manures are normallyapplied at rates designed to meet
Figure 3. Increase in soil test P from applying more Pthan a crop needs each year (as Bray–I P). (Adaptedfrom a 25-year study by Barber 1979.)
Figure 4. Applying manure to meet crop Nneeds (about 200 lb available N/acre) addsmuch more P than corn needs.
crop N requirements but to avoidground water quality problemscreated by leaching of excess N.This often results in a buildup ofsoil test P above amounts sufficientfor optimal crop yields. As illus-trated in figure 4, the amount of P
added in average applications ofdairy manure (8 to 10 tons/acre and0.5 percent P) and poultry litter (4tons/acre and 1.5 percent P) areconsiderably greater than what isremoved in harvested crops; theresult is an accumulation of soil P.
So
il te
st P
(p
pm
)
Annual P surplus (lb/acre/yr)
Initial soiltest P is 18 ppm
80
60
40
20
0-20 0 20 40
P removed (lb/acre)
P added(lb/acre)Dairy manure
Poultry litter
P added in manure or removed by crop0 50 100 150
Corn
8
In 1997, 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 5 that high soilP levels are a regional problem,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. 5).
Figure 5. A survey of agricultural soils analyzed by state soiltest laboratories in 1997, shows a regional buildup of soiltest P near P-sensitive waters (Fixen 1998).
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
9
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
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. 6). From 1992 to1996 in Sussex County, Delaware,with its high concentration ofpoultry operations, 87 percent offields 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).
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 will
Figure 6. Elevated soil test P levels (as Mehlich–1 P) areusually localized in areas of confined animal operations.
10
P includes P associated with soilparticles and organic materialeroded during flow events andconstitutes 60 to 90 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.This dissolved form comes fromthe release of P from soil and plantmaterial (fig. 7). 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 immediatelyavailable for biological uptake.Sediment P is not readily available,but it can be a long-term source of Pfor aquatic biota (Sharpley 1993,Ekholm 1994).
remain above crop sufficiency levelsis of economic and environmentalimportance to farmers who mustintegrate manure P into sustainablenutrient management systems. Forexample, if a field has a highpotential to enrich agriculturalrunoff with P due to excessive soilP, how long will it be before cropuptake will lower soil P levels sothat manure can be applied againwithout increasing the potential forP loss? McCollum (1991) estimatedthat without further P additions, 16to 18 years of corn (Zea mays L.)and soybean [Glycine max (L.)Merr.] production would be neededto deplete soil test P (Mehlich–3 P)(Mehlich 1984) in a Portsmouth finesandy loam from 100 ppm to theagronomic threshold level of 20ppm.
The Loss of Phosphorus inAgricultural Runoff
The 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. 7). Sediment
11
In most watersheds, P export occursmainly in surface runoff, rather thansubsurface flow. However, in someregions, notably the Coastal Plains
and Florida, as well as fields withsubsurface drains, P can be trans-ported in drainage waters. Gener-ally, the concentration of P in water
percolating through the soil profileis low due to 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 (Sharpley andSyers 1979, Bengston et al. 1992).
Irrigation, especially furrow irriga-tion, can significantly increase thepotential for soil and water contactand therefore, can increase P loss byboth surface runoff and erosion inreturn 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.
Figure 7. Phosphorus can be released from soil and plant material tosurface and subsurface runoff water or lost by erosion.
Subsurfaceflow
Tile flow
P leachingis small
Erosion ofparticulate P
Release of soil and plantP to surface runoff
Total runoff P
12
Surface runoff Subsurface tile drainage
Arkansasy = 0.32 + 0.0031x
r2 = 0.76
New Zealandy = 0.02 + 0.0033x
r2 = 0.85
Oklahomay = 0.01 + 0.0031x
r2 = 0.88Run
off-
diss
olve
d P
(m
g/L)
Mehlich-3 soil P, ppm Olsen soil P, ppm
Change point60 ppm
2
1.5
1
0.5
00 100 200 300 400 500
3
2
1
00 30 60 90 120
The Dependence ofAgricultural Runoff Pon Soil P
Many studies report that the loss ofdissolved P in surface runoff isdependent on the P content ofsurface soil (fig. 8). In a review ofseveral studies, Sharpley et al.(1996) found that the relationshipbetween surface runoff P and soil Pvaries with management. Relation-ship slopes were flatter for grass(4.1 to 7.0, mean 6.0) than forcultivated land (8.3 to 12.5, mean10.5), but the slopes were toovariable to allow use of a single oraverage relationship to recommendP amendments based on water-quality criteria. Clearly, several soiland land management factorsinfluence the relationship betweendissolved P in surface runoff andsoil P.
All in all, the loss of P in subsurfaceflow, as well as surface runoff, islinked 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. 8). They postulated thatthis level (60 ppm), which is well
above that needed by major crops foroptimum yield, is a critical pointabove which the potential for Pmovement in land drains greatlyincreases (Ministry of Agriculture,Food and Fisheries 1994). Similarstudies suggest that this change pointcan vary threefold as a function ofsite hydrology, relative drainagevolumes, and soil P release character-istics (Sharpley and Syers 1979).
Figure 8. Effect of soil P on the dissolved P concentration of surface runoff fromseveral pasture watersheds (adapted from Sharpley et al. 1996) and subsurface tiledrainage from Broadbalk fields. (Adapted from Heckrath et al. 1995.)
13
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 subsurfacerunoff water. Second, althoughdissolved P is an important water-quality parameter, it only representsthe 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. Itis the surface inch or two in directcontact with runoff that are impor-tant when using soil testing toestimate P loss. Consequently,different sampling procedures maybe necessary when using a soil testto estimate the potential for P loss.To overcome these concerns,approaches are being developed thatprovide a more theoretically soundestimate, than traditional agronomicchemical extractants do, of theamount of P in soil that can bereleased to runoff water and theamount of algal-available P inrunoff (Sharpley 1993).
An approach, developed in theNetherlands by Breeuwsma andSilva (1992) to assess P leachingpotential, is to determine soil Psaturation (percent saturation =available P/P fixation maximum).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
High P source High transport
Critical source area
Remediation
The overall goal to reduce P lossesfrom 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 controlstrategies. 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
Figure 9. Critical source areas for P loss from a watershed occurwhere areas of high soil P and transport potential coincide.
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. 9).
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. In some areas of the UnitedStates and Europe, reductions inconcentrate P are being imple-mented to reduce the amounts of Pexcreted. Therefore, there is a clearindication that amounts of excretedP can be reduced by carefullymatching dietary P inputs to animalrequirements.
16
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 animalmanures and litters. Enzyme addi-tives for animal feed are beingtested to increase the efficiency of Puptake from grain during digestion.Development of such enzymes thatwould be cost-effective in terms ofanimal weight gain may reduce theP content of manure. One method isto use phytase, an enzyme thatenhances the efficiency of Precovery from phytin in grains fedto poultry and hogs. Anotherpromising method is to developgrain varieties that are lower inphytin.
A third method is to increase thequantity of P in corn that is avail-able to animals by reducing the
amount 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 thatphytate-P concentrations in low-phytic-acid corn grain were as muchas 51 percent less than in wild-type
grain (table 2). Subsequently, Pconcentrations in the litter of poultryfed the low-phytic-acid grain were23 percent less than those in thelitter of poultry fed the wild-typecorn grain. Therefore, the use oflow-phytate corn in poultry feedcan increase the availability of Pand other phytate-bound mineralsand proteins.
Table 2. Total P and phytate-P concentrations in wild-type and low-phytic-acid corn and subsequent P concen-trations in the litter of poultry fed both types of grain
Corn type Grain Poultry litter
Total P Phytate-P Total P
- - - - - - - - - - - - - - -lb P/ton- - - - - - - - - - - - - - -
Wild type 7.6 6.4 5.6
Low phytic acid 7.8 2.6 4.4
SOURCE: Adapted from Ertl et al. (1998).
17
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 soilP 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 of
commercial 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-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 isthe identification of a threshold soiltest P level to estimate when soil Pbecomes high enough to result inunacceptable P enrichment ofagricultural runoff. Table 3 gives
examples from several states, alongwith agronomic threshold concen-trations for comparison.
Environmental threshold levelsrange from 2 times (Michigan) to4 times (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 therefore ispotentially polluting. What will be
18
Table 3. Threshold soil test P values and P management recommendations
Threshold values, ppm
Soil test P Management recommendationsState Agronomic* Environmental method to protect water quality
Arkansas 50 150 Mehlich-3 At or above 150 ppm soil P:Apply no more P, provide buffers next to streams,overseed pastures with legumes to aid P removal, andprovide constant soil cover to minimize erosion.
Delaware 25 50 Mehlich-3 Above 50 ppm soil P:Apply no more P until soil P is significantly reduced.
Idaho 12 50 Olsen Sandy soils, above 50 ppm soil P:100 Silt loam soils, above 100 ppm soil P:
Apply no more P until soil P is significantly reduced.
Ohio 40 150 Bray-1 Above 150 ppm soil P:Reduce erosion and reduce or eliminate P additions.
Oklahoma 30 130 Mehlich-3 30 to 130 ppm soil P:Halve P rate on >8% slopes.130 to 200 lb/acre soil P:Halve P rate and reduce surface runoff and erosion.Above 200 lb/acre soil P:Amount of P applied should not exceed that taken upby the crop and removed as harvested produce.
19
Table 3. Threshold soil test P values and P management recommendations (continued)
Threshold values, ppm
Soil test P Management recommendationsState Agronomic* Environmental method to protect water quality
Michigan 40 75 Bray-1 Below 75 ppm soil P:P application not to exceed crop removal.Above 75 ppm soil P:Apply no P from any source.
Texas 44 200 Texas Above 200 ppm soil P:A&M Amount of P applied should not exceed that taken up by
the crop and removed as harvested produce.
Wisconsin 20 75 Bray-1 Below 75 ppm soil P:Rotate to P-demanding crops and reduce P additions.Above 75 ppm soil P:Discontinue P applications.
SOURCE: Adapted from Sharpley et al. (1996).
* Agronomic threshold concentrations are average values for nonvegetable crops; actual values varywith soil and crop type. Vegetables have higher agronomic P requirements.
20
Soil test P
Cro
p yi
eld
P lo
ss in
sur
face
run
off
Criticalvalue
for yield
Criticalvalue
for P loss
Low Optimum High
MediumLow High
crucial 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. 10).
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 in
Figure 10. As soil P increases, so does crop yield and the potential for Ploss in surface runoff. The interval between the critical soil P value foryield and runoff P will be important for P management.
21
environmental 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 in
need 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 improvedanimal 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 manure
amendments 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 themoment, 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 that
22
the 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 subsequenttransport 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 source
area 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 manure
will 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, contourtillage, cover crops, and impound-ments (settling basins). Basically,these practices reduce the impact ofrainfall on the soil surface, reducerunoff volume and velocity, andincrease soil resistance to erosion.None of these measures, however,should be relied on as the sole orprimary practice to reduce P lossesin agricultural runoff.
Most of these practices are generallymore efficient at reducing sedimentP than dissolved P. Several re-
23
searchers report little decrease inlake productivity with reduced Pinputs following implementation ofconservation measures (Young andDePinto 1982, Gray and Kirkland1986). Many times the impact ofremedial measures used to helpimprove poor water quality will beslow because lake and streamsediments can be a long-term sourceof P in waters even after inputsfrom agriculture are reduced.Therefore, immediate action may beneeded to reduce future 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 toosimplistic to use threshold soil P
levels as the sole criterion to guide Pmanagement 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, threshold
soil 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. 11).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 not
24
Soil test P >100 ppm
Area of high surface runoff potential
Figure 11. The principle of source-areamanagement to more effectively targetmeasures to reduce P export in surfacerunoff from watersheds.
Areas most vulnerable to P loss
Integrated P and N management
25
produce the desired results and mayprove to be an inefficient and poorcost-effective approach to theproblem if this source-area perspec-tive to target application of Pfertility, surface runoff, and erosioncontrol technology is not used.
A simple P index has been devel-oped by the U.S. Department ofAgriculture, Agricultural ResearchService, in cooperation with severalresearch scientists, as a screeningtool for use by field staffs, water-shed planners, and farmers to rankthe vulnerability of fields as sourcesof P loss in surface runoff(Lemunyon and Gilbert 1993). Theindex accounts for and rankstransport and source factors control-ling P loss in surface runoff andsites where the risk of P movementis expected to be higher than that ofothers (table 4). Each site character-istic affecting P loss is weighted
according to the assumption thatcertain characteristics have arelatively greater effect on potentialP loss than others. Each user mustestablish a range of P loss potentialvalues for different geographicareas. An assessment of site vulner-ability to P loss in surface runoff ismade by selecting the rating valuefor each site characteristic from theP index. Each rating is multipliedby the appropriate weighting factor.The P index value for the site is thesum of weighted values of all sitecharacteristics, which can then beused to categorize the site’s vulner-ability to P loss (table 5).
The index is a tool for field person-nel to identify agricultural areas orpractices that have the greatestpotential to accelerate eutrophica-tion. It can be used to identifymanagement options available toland users and will allow them
flexibility in developing remedialstrategies. The first step is todetermine 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-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.
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-
26
Table 4. The P index
Site characteristic Loss rating (value)(Weighting factor)
None (0) Low (1) Medium (2) High (4) Very high (8)
Transparent Factors
Soil erosion (1.5) N/A < 5 tons/acre 5 to 10 tons/acre 10 to 15 tons/acre > 15 tons/acre
Irrigation erosion (1.5) N/A Infrequent irrigation Moderate irrigation Frequent irrigation Frequent irrigationon well-drained on soils with on soils with on soils withsoils slopes < 5% slopes of 2 to 5% slopes > 5%
Soil runoff class (0.5) N/A Very low or low Medium Optimum Excessive
Distance from > 1,000 ft 1,000 to 500 ft 500 to 200 ft 200 to 30 ft < 30 ftwatercourse (1.0)
27
Table 4. The P index (continued)
Site characteristic Loss rating (value)(Weighting factor)
None (0) Low (1) Medium (2) High (4) Very high (8)
Source Factors
Soil test P (1.0) N/A Low Medium Optimum Excessive
P fertilizer application None < 15 16 to 40 41 to 65 > 65rate, lb P/acre (0.75) applied
P fertilizer application None Placed with Incorporated Incorporated > 3 Surface applied tomethod (0.5) applied planter deeper immediately months before crop pasture or applied
than 2 inches before crop or surface applied > 3 months< 3 months before cropbefore crop
Organic P source None < 15 16 to 40 41 to 65 > 65application rate, lb appliedP/acre (1.0)
Organic P source None Injected deeper Incorporated Incorporated > 3 Surface applied toapplication method (0.5) applied than 2 inches immediately months before crop pasture or applied
before planting or surface applied > 3 months< 3 months before cropbefore crop
28
Table 5. Generalized interpretation of the P index
P index General vulnerability to P loss
< 8 Low potential for P loss. If current farming practices aremaintained, there is a low probability of adverse impacts onsurface waters.
8 to 14 Medium potential for P loss. The chance for adverse impactson surface waters exists, and some remediation should be takento minimize the probability of P loss.
15 to 32 High potential for P loss and adverse impacts on surface waters.Soil and water conservation measures and P management plansare needed to minimize the probability of P loss.
> 32 Very high potential for P loss and adverse impacts on surfacewaters. All necessary soil and water conservation measures and aP management plan must be implemented to minimize the P loss.
ways through soils and watersheds,these narrowly targeted strategiesoften conflict and lead to compro-mised water quality. For example,manure application based on crop Nrequirements to minimize nitrateleaching to ground water 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 zones hasa neutral or beneficial impact onnitrate loss. Some exceptions arepractices that promote water infiltra-tion, which tend to increase leach-
29
ing, and tillage practices that do notincorporate P fertilizers and ma-nures into the soil. No-till is com-monly recommended as a conserva-tion measure for eroding cropland.Conversion to no-till is followed bydecreased soil and total N and P lossin surface runoff and increasednitrate leaching and algal-availableP transport (Sharpley and Smith1994).
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 that in-cludes critical sources of N and Pexport from agricultural watershedsso that optimal strategies at farmand watershed scales can be imple-mented to best manage N and P.
Summary
The 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 Pcontent is important in determiningthe concentration of P in agriculturalrunoff, surface runoff and erosionpotential often override soil levels indetermining 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 have
30
Table 6. Management options to minimize nonpoint source pollutionof surface waters by soil P
Phosphorus index Management options
< 8 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.
8 to 14 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.
31
Table 6. Management options to minimize nonpoint source pollutionof surface waters by soil P (continued)
Phosphorus index Management options
15 to 32 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.
> 32 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.
32
the 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 are implemented tominimize 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 shouldbalance P inputs and outputs atfarm and watershed scales;
• source and transport controlsshould target and identify criticalsource areas of P export fromwatersheds; and
• farmers should link threshold soilP levels that guide manureapplications with site vulnerabilityto P loss.
Consideration of all these factorswill be needed to develop extensionand demonstration projects thateducate farmers, the animalindustry, and the general publicabout what is actually involved inensuring clean water. It is hoped thiswill help overcome the commonmisconception that diffuse or
nonpoint sources are too difficult,costly, or variable to control ortarget substantial reductions (fig.12).
Efforts to implement defensibleremedial strategies that minimize Ploss from agricultural land willrequire interdisciplinary researchinvolving soil scientists,hydrologists, agronomists,limnologists, and animal scientists.Development of guidelines toimplement such strategies will alsorequire consideration of thesocioeconomic and political impactsof any management changes onrural and urban communities and ofthe mechanisms by which changecan be achieved in a diverse anddispersed community of land users.
33
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.
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, high percolation potential, or high soiltest 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.
Figure 12. 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 sitevulnerability to P loss and proximity of P-sensitive waters.
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
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