DIVISION S-8-NUTRIENT MANAGEMENT& SOIL & PLANT ANALYSIS

Reaction in Soil of Phosphorus Released from Poultry LitterJ. Stephen Robinson and Andrew N. Sharpley*

ABSTRACTPoultry production generates large amounts of litter or manure,

which can be a valuable source of P for crops. However, litter applica-tion rates are usually based on data for mineral P fertilizer recommen-dations. In order to determine if this is agronomically and environmen-tally sound, the availability, fractions, and sorptivity of P from poultrylitter or Kr^Pd, were determined in six Oklahoma soils followingincubation for up to 28 d. An average 50% more P from KHiPOt-treated (78 mg kg ~') than from leachate-treated soils (52 mg kg ~ *) wasbioavailable, as determined by extraction with Fe-oxide-impregnatedpaper strips, after 28-d incubations. Conversely, more NaHCO3-extractable P was found in leachate-treated (66 mg kg "') than KHjPO4-treated soils (42 mg kg"1). Calculated from Langmuir isotherms, Psorption maxima averaged 548 mg kg ~' for leachate-treated and 304mg kg"1 for KHiPCVtreated soils, while binding energies averaged0.527 and 0.456 L mg"1, respectively. The higher P sorption maximaand binding energies of leachate-treated soils may result from theformation of Ca-P complexes, given the increased Ca content of thesesoils. The different reaction in soil of P added as poultry litter leachateto that added as KH2POj, indicates manure application rates shouldbe based on soil tests that are sensitive to P source-dependent sorptioncharacteristics and/or manure trials, and not just on mineral fertilizertrials.

IN AREAS of intensive poultry production, the largeamounts of litter produced (manure plus bedding mate-

rial) can be a valuable nutrient source for local agricul-tural crops. For example, 14 kg ha""' of dissolved IPwere released in 5- to 50-min simulated rainfall eventsfrom poultry litter equivalent to a 10 Mg ha""1 (4.5 tacre"') poultry litter application (a rate commonly usedby poultry producers in southeastern Oklahoma; Sharpleyet al., 1993; Robinson and Sharpley, 1995). Theseamounts of P released would meet the P uptake require-ments for yields of 3 Mg ha~' wheat (Triticum aestivumL.; 11 kg P ha"1) and 5 Mg ha~' fescue (Festuca arundi-nacea Schreb.; 15 kg P ha~') in the Southern Plains(Johnson et al., 1993). However, the loss of N and Pin runoff from land receiving repeated applications ofpoultry litter can accelerate eutrophication of downstreamwater bodies (Edwards and Daniel, 1993; Sharpley etal., 1994).

Conventionally, animal manure applications are basedon the N requirements of crops. However, the ratio ofN/P uptake by crops is usually much wider than thatprovided in poultry litter. For example, the ratio of

J.S. Robinson, Dep. of Soil Science, Univ. of Reading, Whiteknights,Reading, RG6 2DW, England; and A.N. Sharpley, USDA-ARS, PastureSystems and Watershed Management Research Lab., Curtin Road, Univer-sity Park, PA 16802-3702. Received 23 Jan. 1995. *Corresponding author(ans3@psu.edu).

Published in Soil Sci. Soc. Am. J. 60:1583-1588 (1996).

N/P uptake for crops grown in the Southern Plains is8:1, whereas the mean ratio of N/P in poultry litteris about 3:1 (Edwards and Daniel, 1992). Therefore,repeated litter applications based on soil N and cropremoval of N can lead to accumulation of large amountsof P in the surface soil (Kingery et al., 1993; Sharpleyet al., 1993). Farmers are being encouraged through bestmanagement practices to apply animal manures based oncrop P requirement, as opposed to N, for soils withhigh P contents (Soil Conservation Service, 1994). Thismanagement strategy is an attempt to reduce P accumula-tion in the soil zone, which most influences runoff orerosion.

Current manure application guidelines assume theeffectiveness of manure P, for plant growth, is similarto that of fertilizer P. However, there is also a lack ofcrop response data to manure-P applications. The actualfertilizer value of any P source will be largely influencedby the sorption and precipitation reactions of the P withsoil material. To develop reliable manure managementguidelines that are both agronomically and environmen-tally sound, the fate of manure P should be considered.For example, Edwards and Daniel (1994) found theconcentration of P in runoff water from the first simulatedrainfall-runoff event was greater following poultry litterthan mineral fertilizer application. However, there islittle information comparing the fate in soil of P addedas either manure or mineral fertilizer.

Earlier, we investigated the release of P from poultrylitter during simulated rainfall (Robinson and Sharpley,1995). Our objective in the present study was to deter-mine the fate in soil of P leached from poultry litter incomparison with P from a mineral fertilizer source.This was accomplished by comparing the short-termbioavailability, sorption, and fractionation of P in severalSouthern Plains soils treated with either poultry litterleachate or KH2PO4.

MATERIALS AND METHODSSoils

Six soils, which had not received fertilizer or manure forat least 20 yr, were collected (0-5 cm) from eastern Oklahoma.Poultry litter is commonly applied to these agriculturally impor-tant soils. The soils were air dried and sieved (<2 mm).Selected physical and chemical properties of the soils are shownin Table 1. These include: clay and silt contents, determinedby pipette analysis following dispersion by sodium hexameta-phosphate (Day, 1965); pH in water (pHw) at a soil/water ratioof 1:2.5 (w/v); and Bray-I P extracted from 1 g of soil by 20

Abbreviations: IP, inorganic phosphorus; OP, organic phosphorus; TP,total phosphorus.

1583

1584 SOIL SCI. SOC. AM. J., VOL. 60, SEPTEMBER-OCTOBER 1996

Table 1. Classification and selected physical and chemical proper-ties of the six Oklahoma soils.

Soilt

Baxter siDickson siclNeff fslNewtonia silOkemah siclTiak fsl

Classification

Typic PaleudultsGlossic FragiudultsAquultic HapludalfTypic PaleudollsAquic PaleudollsAquic Paleudults

Clay

—— 127.831.88.0

26.234.014.0

SiltJj ____

62.859.465.464.441.027.0

pH Organic C

5.95.45.45.96.15.6

gkg- '30.416.121.428.948.514.9

Bray I P

mg kg'17.33.05.54.23.65.6

t fsl, si, sil, and sicl represent fine sandy loam, sandy loam, silt loam, andsilty clay loam, respectively.

mL of 0.03 M NH4F + 0.025 M HC1 for 5 min (Bray andKurtz, 1945).

Phosphorus SourcesPoultry litter (bedding material of pine bark shavings, Pinus

sp.) was collected from a broiler house in southeastern Okla-homa and stored at house moisture (9.8% w/w). Simulatedrainfall (at an intensity of 2.54 cm h~' , using a capillarytube-type rain simulator; Munns and Huntington, 1976) wasapplied to 20 g of fresh thoroughly mixed litter, spread uni-formly over a 15 cm (internal diameter) by 10 cm (height)Plexiglas column (Robinson and Sharpley, 1995). This amountof litter corresponds to an application rate of 10 Mg ha"1.Rainfall was applied to the litter for 20 min to collect approxi-mately 200 mL of leachate. The IP content of the leachate,and the TP content following digestion by a semimicro-Kjeldahlprocedure (Bremner and Mulvaney, 1982), were measured bythe colorimetric molybdenum blue method of Murphy andRiley (1962). The OP content was calculated as the differencebetween the TP and IP contents. The leachate had an IP contentof 43 mg L~', approximately 84% of TP, and pH of 7.4.

A stock solution of KH2PO4 was prepared with a P concentra-tion the same as the IP concentration in the leachate (43 mgLr1). The pH of the stock P solution was 6.3. Potassiumorthophosphate was used as the mineral P source because,similar to the poultry litter P source (poultry litter leachate),it is readily soluble. Commonly used fertilizers, such as ammo-nium and calcium phosphates, were not used because of theirdiffering P solubilities, which may influence soil P reaction.

Soil-Phosphorus IncubationsStrip Phosphorus

A 2.5-mL aliquot of poultry litter leachate or KH2PO4 (toprovide approximately 108 u.g IP) and water were mixed with1 g of each of the six soils in 50-mL centrifuge tubes. Alltreatments were incubated in duplicate at 275 K for either 1,5, 14, or 28 d. After incubation, duplicate samples wereextracted with an Fe-oxide strip (strip P; Robinson andSharpley, 1994). The Fe-oxide strip removes soil solution Pplus P that can be readily desorbed and estimates plant-availableP in soil (Menon et al., 1989), and potentially algal-availableP in runoff (Sharpley, 1993). For all strip P extractions, asingle Fe-oxide strip was shaken with the incubation mixturein 37.5 mL of 0.01 M CaCl2 for 16 h. The strip was removed,rinsed free of adhering soil particles, and shaken with 40 mLof 0.5 M NaOH for 1 h to remove P from the strip (Robinsonand Sharpley, 1994).

Phosphorus SorptionThe P sorption procedure used was similar to that developed

by Nair et al. (1984). Phosphorus was added as poultry litterleachate or KH2PO4 to 1 g each of the six soils, to obtain 0

to 860 mg IP kg"1 soil (0, 1, 2, 3, 4, 5, 10, 15, and 20mL of leachate or KH2PO4). All mixtures were prepared induplicate, and made up to 25 mL with distilled water. Followingend-over-end shaking for 24 h, suspensions were centrifugedand filtered (0.45 u,m); solution P was measured by the colori-metric procedure of Murphy and Riley (1962). Sorbed P wascalculated as the difference between P added and P in solution.The slope of the relationship between P sorbed (X) and P insolution (C) provides an estimate of the P buffering capacityof the soil (Rajan and Fox, 1972). Langmuir P sorption iso-therms were constructed and P sorption maxima and bindingenergies determined. Phosphorus sorption maximum was cal-culated as the reciprocal of the slope of the plot C/X and Cand binding energy as slope/intercept of the same plot (Olsenand Watanabe, 1957; Syers et al., 1973).

Phosphorus FractionationA 2.5-mL aliquot of poultry litter leachate or KH2PO4 (to

provide approximately 108 ng IP) or water were mixed induplicate with 1 g of each of thessix soils in 50-mL centrifugetubes. After incubation at 275 K for 1 d, all samples wereextracted sequentially with 20 mL of H2O, 20 mL of 1 MNaCl, 30 mL of 0.5 M NaHCO3, 30 mL of 0.1 M NaOH,and 30 mL of 1 M HC1. The H2O and NaCl extractions wereof 30-min duration; the NaHCO3, NaOH, and HC1 extractionswere of 16-h duration. Residual soil P was determined follow-ing digestion in 30% H2SO4 and ammonium persulfate (Bow-man, 1989). Inorganic P was measured on centrifuged, filtered(0.45 u,m), and neutralized extracts by the colorimetric methodof Murphy and Riley (1962).

The extractants removed IP from pools of decreasing bio-availability. This IP fractionation procedure was a modificationof that developed by Chang and Jackson (1959) and Hedleyet al. (1982). Water-extractable P (solution P) represents themost bioavailable form of soil P (Hedley et al., 1982). TheNaCl step was included to prevent precipitation of Ca-P andCa-organic matter complexes during subsequent alkali extrac-tions by removing exchangeable Ca (Perrott, 1992). Formationof Ca-P compounds have been shown to underestimate alkalifractions and overestimate acid fractions of P (Perrott, 1992).Sodium bicarbonate extracts a portion of soil P from hydrousAl and Fe oxide surfaces that may be desorbed in the plant-soil or runoff environment (Kamprath and Watson, 1980);NaHCOs-IP can be described as long-term bioavailable P.Sodium hydroxide extracts Al- and Fe-P that is not readilydesorbed, and represents IP that is less bioavailable than theprevious extractants (Williams et al., 1980). Extraction withHC1 removes soil P that is relatively stable and biologicallyunavailable Ca bound.

In all cases, the results are presented as means of duplicates.Statistical difference and significance of treatment effects wereevaluated by analysis of variance for paired data and pairedMest.

RESULTSSoil Phosphorus Unavailability

Analysis of variance showed that mean strip P signifi-cantly (P < 0.01) decreased, rapidly at first, followedby a more gradual decline with increasing incubationperiod for both poultry litter leachate- and KH2PO4-treated soils. Decreasing strip P with increasing time isshown for clarity for Baxter, Neff, and Okemah soilsonly (Fig. 1); however, similar statistically significant

100

80

60

~ 40V-

'u>* 20

•~ 0Q.aS 80CO

5 60

40

20

ROBINSON & SHARPLEY: REACTION IN SOIL OF POULTRY LITTER PHOSPHORUS

100

1585

Baxter• Neff

Okemah

Poultry LitterLeadhate

10 20 30Incubation Period (Days)

Fig. 1. Relationship between strip P and incubation period in threesoils incubated with poultry litter leachate or KHiPO4 (108 mginorganic P kg"1).

trends were observed for the other soils (data not pre-sented). After incubation for just 5 d, strip P, averagedfor the two P sources and six soils, had decreased to50% of values after 1 d of incubation.

The strip P content of soil incubated with KH2PO4was consistently greater (P < 0.001, Table 2) than forsoil incubated with the same amount of IP (108 mgkg"1) from litter leachate (Fig. 1). This difference issummarized for strip P values after incubation for 1 d,when strip P in the KFhPCVtreated soils averaged 78mg kg"' and in leachate-treated soils 52 mg kg"1 (Fig.2). After incubation for 28 d, strip P, averaged for thesix soils, was still greater in KHzPCVtreated (28 mgkg""1) than in leachate-treated soils (19 mg kg"1) (P <0.001, Table 2). Apparently, the type of P added tosoil influenced P availability as determined by Fe-oxidestrips, with greater amounts of leachate P sorbed in

Table 2. Statistical analysis of the difference in several P formsbetween soils treated with poultry litter leachate or KH:PO4 asdetermined by the paired f-test.

Parameter!Degrees of Mean of Standard

freedom differences error tStrip P|Strip P§P sorption maximumBinding energyP fractions

H2ONaClNaHCO3NaOHHC1Residual

47111111

111111111111

25.7214.10

2440.071

23.830.17

15.500.120.581.50

1.471.11

18.110.010

4.370.411.371.090.931.76

17.4812.7513.447.32

5.460.41

11.330.110.630.85

0.00000.00000.00000.0000

0.00020.68870.00000.91690.54370.4134

t Units are mg kg"' except L mg"| All incubation times.§ 1-d incubation.

for binding energy.

80

6001

Q.Q.£ 40CO U

oCO

20

P SourceLitter leachateKH2PO4

Baxter NeffDickson

OkemahNewtonia Tlak

Fig. 2. Strip P content of six soils after incubation for 1 d witheither poultry litter leachate or KH2PO4 (108 mg inorganic P kg"l

additions).

forms not extracted by the strips than when KH2PO4 wasadded.

Soil Phosphorus SorptionFor all soils, more P was sorbed from litter leachate

than from KH2PO4 solutions (Table 3). Sorption maximaand binding energies of P were calculated from Langmuirisotherm plots of the data (Table 3). Phosphorus sorptionmaxima and binding energies were consistently greater(P < 0.01) for leachate- than KH2PO4-treated soils (P <0.001, Table 2). Averaged for all soils, P sorption maxi-mum was 548 mg kg~' for leachate-treated and 304 mgkg"1 for KHaPCVtreated soils, while respective bind-ing energies averaged 0.527 and 0.456 L mg"1. Acrossthe range of P additions, relationships between theamount of P sorbed and solution P concentration(C/X vs. C) were significant for all soils and treatments(P < 0.05, Table 3).

Soil Phosphorus FractionsThe amounts of IP in the various fractions differed

among untreated soils, and decreased in the order: H2SO4(97-198 mg kg"1) > NaOH (26-45 mg kg"1) > NaHCO3

Table 3. Soil P sorption maximum and binding energy for Padditions of 0 to 108 mg inorganic P kg"' as poultry litterleachate or KH2PO4. calculated from Langmuir isotherms.

SoU

BaxterDicksonNeffNewtoniaOkemahTiak

PoultrySorption

maximum

mg kg"'512582536578658419

litter leachateBindingenergy

Lmg"'0.2240.5670.8080.5160.6460.403

rt

0.970.970.980.980.960.96

KH2PO4

Sorptionmaximummg kg"'

167328290306491243

Bindingenergy

L mg-'0.1750.5290.6830.4220.6080.316

rt

0.970.990.990.990.980.97

t All r values significant at P < 0.05.

1586 SOIL SCI. SOC. AM. J., VOL. 60, SEPTEMBER-OCTOBER 1996

1£U

^ 100T

I BO

Ii1 60

Q.

OT 40

'cIB

§* 20C

n

<m r^O

[jNaCI

" H NaHCO3

QNaOH

.0HCI

ffl RMldual

-

-

_

J

r$

1

7

^

^

\

/

'/,

\

/

y

/

y/

\

r?

^\N

X1115<s

33*

-63 *

1

0.2

2^

(1-

_

-

-

_

Control KH2P04 LeachateP Source

Fig. 3. Sequentially extracted fractions of P in Baxter soil after incuba-tion for 1 d with either poultry litter leachate or KH2PCXi (108 mginorganic P kg"' additions). Average changes between leachate andKH2PO4-treated soils are given above each bar (* denotes significantdifference between P sources at P < 0.001).

(6-20 mg kg~') or HC1 (6-21 mg kg-') > H2O (0.04-2 mg kg'1) or NaCl (0-0.9 mg kg-'). Addition of eitherleachate or KH2PO4 significantly (P < 0.01) increasedthe concentration of IP removed by all extractants. Therelative magnitude of the increase was influenced by Psource as well as soil type. Due to the large variationin soil P status, the data are presented for one soil(Baxter) only (Fig. 3). Presenting mean P fractionationdata for all soils masks the influence of P source on Pdistribution among the different fractions.

Averaged for the six soils, more P from KH2PO4(37%, P < 0.001, Table 2) accumulated in the H2Ofraction compared with addition as poultry litter leachate

100

O).*en

a.Q.

80

60

40

Leachate PKH2PO4

Strip P = 39.75 +1.14 Water P

20 40 60

Water-Extractable P (mg kg '1)Fig. 4. Relationship between strip P and water-extractable P contents

of six soils following incubation for 1 d with either poultry litterleachate or KH2PO4.

(Fig. 3). The average percentage difference betweenleachate and KH2PO4 for the six soils is shown aboveeach bar (Fig. 3). For leachate-treated soils, more P(33%, P < 0.001, Table 2) was found in the NaHCO3fraction than for the KH2PO4 treatment. Increases inNaOH-, HC1-, and H2SO4-extractable IP were not sig-nificantly different (P > 0.59, Table 2) between P sources(Fig. 3). Overall, therefore, the difference in P bioavail-ability between P sources (Fig. 2) results from a relativelygreater accumulation of P in the H2O fraction of KH2PO4-treated soils and in the less readily extractable NaHCO3fraction with litter leachate-treated soils. Regardless ofP source, solution P correlated well with strip P after1 d of incubation (Fig. 4).

Differences in the amounts of IP removed by neutraland basic extractants (H2O, NaCl, NaHCO3, NaOH)between KH2PO4- and leachate-treated soils are duesolely to P source-soil interaction. This assertion is basedon preliminary work, which showed that H2O, NaCl,NaHCO3, and NaOH extractions and subsequent P mea-surement (acidic molybdenum blue method, Murphy andRiley, 1962) did not hydrolyze OP in poultry-litter leach-ate. Also, for all soils and both P sources in this study,the sum of IP added (108 mg kg"1) is not significantlydifferent (P > 0.05) from the mean of the sum removedby the six extractants (105 + 4.2 mg kg"1; calculatedas the difference between extracted IP in the six treatedand control soils). The balance between IP input andoutput also indicates that OP added in leachate (24 mgkg"1 soil) is extracted and not hydrolyzed by the neutraland basic chemicals.

DISCUSSIONFollowing the addition of P to soil, a variable portion

is sorbed at the reactive surfaces of clay-size soil constit-uents, primarily Al and Fe oxides, silicate clay minerals,or CaCO3 (Holford and Mattingly, 1975; Olsen andKhasawneh, 1980; Sample et al., 1980). Equilibrium israpidly attained between the sorbed and solution P (thefast reaction) (Barrow, 1980). The Fe-oxide strips extractsolution P plus P that is readily desorbed from the reactivesoil surfaces. Therefore, the decrease in strip P withincubation time (Fig. 1) may be attributed to the slowdiffusive penetration of readily desorbed P into the reac-tive surfaces (slow reaction) (Olsen and Khasawneh,1980; Barrow, 1980).

The consistently lower strip P in leachate- than inKH2PO4-treated soils (Fig. 2) is due to a stronger Psorption mechanism and/or a larger number of sorptionsites in soil treated with leachate (Table 2). In acid toneutral soils, typical of those used in this study, a variableproportion of P is adsorbed to the surfaces of Al oxidesby monodentate bonding ("reversible P") (Kamprath andWatson, 1980).

Solution P plus "reversible P" are extracted by NaHCO3(Olsen et al., 1954). Assuming complete removal ofsolution P during extraction with water, P that is subse-quently removed with NaHCO3 represents "reversible"P (Fig. 3). On average, there was 63% less solution P

ROBINSON & SHARPLEY: REACTION IN SOIL OF POULTRY LITTER PHOSPHORUS 1587

in leachate- than in KH2PO4-treated soils, and 57% moreP in the subsequent NaHCO3 extract (Fig. 3). The ab-sence of any effect of P source on NaOH-, HC1-, orresidual-P indicates that fixation of added P in a biologi-cally unavailable form by Al or Fe oxide surfaces or Caprecipitation, for example, is not influenced by differ-ences in P source composition (Fig. 3).

The greater P sorption in leachate- than in KH2PO4-treated soils may be related to the addition to soil of Ca(31 mg L~') that accompanied leachate addition. Largerincreases in P sorption, following addition of leachatecompared with KH2PO4 (Table 2), may be attributed tothe formation of stable Ca-P complexes at soil surfaces.This is consistent with a shift in the dominant form ofsoil P associated with Al and Fe to Ca-P with theapplication of beef, poultry, or swine manure for up to35 yr to several Oklahoma and Texas soils ranging inpH from 5.9 to 7.2 (Sharpley and Smith, 1995).

While strip P after 1 d of incubation correlated wellwith solution P (Fig. 4), there was poor correlation with"reversible" NaHCO3-P (data not presented). The datasuggest that only a small and poorly defined portion of"reversible" P was easily desorbed. Therefore, comparedwith NaHCO3 extraction, the Fe-oxide strips appear tobe more sensitive to P sorption phenomena. On thisbasis, the Fe-oxide strips may provide an indication ofthe short-term (<60 d) bioavailability of P in recentlyfertilized soils.

The lower solution P and strip P, but higher NaHCO3-Pvalues, in leachate- than in KH2PO4-treated soils indicatesthat the former P source may sustain a higher residualP value in the soil. The present data also have waterquality implications. The short-term bioavailability of Pin runoff from soils receiving poultry litter may be lowerthan from soils receiving a similar amount of P as mineralfertilizer, as shown by lower strip P contents followingincubation with leachate than mineral fertilizer (Fig. 2).However, higher "reversible" or NaHCO3-P in leachate-treated soils may increase the long-term potential forbioavailable P loss in runoff than for soils treated withmineral fertilizer. This is consistent with Edwards andDaniel (1994), who added 87 kg P ha"' as poultry litteror mineral fertilizer to a Captina silt loam under fescueand after 7 to 68 d, applied simulated rainfall. The lossof dissolved P in runoff 7 d after application was lowerwith poultry litter (0.75 kg ha~') than mineral fertilizer(1.76 kg ha~'). Sixty-eight days after application, thistrend was reversed with greater P losses in runoff fromthe litter (0.30 kg ha"1) than from mineral fertilizertreatment (0.11 kg ha"1) (Edwards and Daniel, 1994).

Consequently, manure management guidelines that at-tempt to reduce short-term bioavailable P in surface soilshould not be based solely on guidelines that have beendeveloped using mineral fertilizer P. In areas of intensivepoultry production, for example, recommendations forsustainable poultry litter management can be developedif the application rates of poultry litter are based on

poultry litter trials, and on soil tests that are sensitiveto P source-dependent sorption characteristics.

CONCLUSIONSThe reaction of P added as poultry litter leachate was

different from that added as KH2PO4 to six soils. Inleachate-treated soils, the short-term bioavailability ofP (strip P) was consistently lower than in KH2PO4-treatedsoils. Following removal of solution P by extraction withwater, NaHCO3-P in leachate-treated soils was higherthan in soils receiving KH2PO4. From an agronomicstandpoint, the data indicate that poultry litter has ahigher residual P fertilizer value than the KHaPO4 source.From a water quality perspective, runoff from fieldstreated with mineral fertilizer may initially be of greaterbioavailable P concentration than from land applicationof poultry litter. However, in the long term, higherresidual soil P values may support greater bioavailableP concentrations in runoff than from mineral fertilizertreatments. We concluded that manure application ratesfor agronomically and environmentally sound P manage-ment guidelines should not be based solely on data frommineral fertilizer trials.

1588 SOIL SCI. SOC. AM. J., VOL. 60, SEPTEMBER-OCTOBER 1996

ASA, CSSA, and SSSA, Madison, WI.Kingery, W.L., C.W. Wood, D.P. Delaney, J.C. Williams, G.L.

Mullins, and E. van Santen. 1993. Implications of long-term landapplication of poultry litter on tall fescue pastures. J. Prod. Agric.6:390-395.

Menon, R.G., L.L. Hammond, andH.A. Sissingh. 1989. Determina-tion of plant-available phosphorus by the iron hydroxide-impregnated filter paper (Pi) soil test. Soil Sci. Soc. Am. J. 52:110-115.

Munns, J.R., and G.L. Huntington. 1976. A portable rainfall simulatorfor erodibility and infiltration measurements on rugged terrain.Soil Sci. Soc. Am. J. 40:622-644.

Murphy, J., and J.P. Riley. 1962. A modified single solution methodfor determination of phosphate in natural waters. Anal. Chim.Acta 27:31-36.

Nair, P.S., T.J. Logan, A.N. Sharpley, L.E. Sommers, M.A.Tabata-bai, and T.L. Yaun. 1984. Interlaboratory comparison of a stan-dardized phosphorus adsorption isotherm procedure. J. Environ.Qual. 13:591-595.

Olsen, S.R., C.V. Cole, F.S. Watanabe, and L.A. Dean. 1954.Estimation of available phosphorus in soils by extraction withsodium bicarbonate. USDA Circ. 939. U.S. Gov. Print. Office,Washington, DC.

Olsen, S.R., and F.E. Khasawneh. 1980. Use and limitations ofphysical-chemical criteria for assessing the status of phosphorusin soils, p. 361-410. In F.E. Khasawneh et al. (ed.) The role ofphosphorus in agriculture. ASA, CSSA, and SSSA, Madison, WI.

Olsen, S.R., and F.S. Watanabe. 1957. A method to determine aphosphorus adsorption maximum of soils as measured by the Lang-muir isotherm. Soil Sci. Soc. Am. Proc. 21:144-149.

Perrott, K.W. 1992. Effect of exchangeable calcium on fractionationof inorganic and organic soil phosphorus. Commun. Soil Sci. PlantAnal. 23:827-840.

Rajan, S.S.S., and R.L. Fox. 1972. Phosphate adsorption by soils.I. Influence of time and ionic environment on phosphate adsorption.Commun. Soil Sci. Plant Anal. 3:493-504.

Robinson, J.S., and A.N. Sharpley. 1994. Organic phosphorus effectson sink characteristics of iron oxide-impregnated filter paper. SoilSci. Soc. Am. J. 58:758-761.

Robinson, J.S., and A.N. Sharpley. 1995. Release of nitrogen andphosphorus from poultry litter. J. Environ. Qual. 25:62-67.

Sample, B.C., R.J. Soper, and G.J. Racz. 1980. Reactions of phos-phate fertilizers in soils, p. 263-310. In F.E. Khasawneh et al.(ed.) The role of phosphorus in agriculture. ASA, CSSA, andSSSA, Madison, WI.

Sharpley, A.N. 1993. An innovative approach to estimate bioavailablephosphorus in agricultural runoff using iron oxide-impregnatedfilter paper. J. Environ. Qual. 22:597-601.

Sharpley, A.N., S.C. Chapra, R.Wedepohl, J.T. Sims, T.C. Daniel,and K.R. Reddy. 1994. Managing agricultural phosphorus forprotection of surface waters: Issues and options. J. Environ. Qual.23:437-451.

Sharpley, A.N., and S.J. Smith. 1995. Nitrogen and phosphorusforms in soils receiving manure. Soil Sci. 159:253-258.

Sharpley, A.N., S.J. Smith, and R. Bain. 1993. Nitrogen and phospho-rus fate from long-term poultry litter applications to Oklahomasoils. Soil Sci. Soc. Am. J. 57:1131-1137.

Soil Conservation Service. 1994. Waste utilization standard and speci-fications. SCS Tech. Practice Code 633. USDA-SCS, Stillwater,OK.

Syers, J.K, M.G. Browman, G.W. Smillie, and R.B. Corey. 1973.Phosphate sorption by soils evaluated by the Langrnuir adsorptionequation. Soil Sci. Soc. Am. Proc. 37:358-363.

Williams, J.D.H., T. Mayer, and J.O. Nriagu. 1980. Extractabilityof phosphorus from phosphate minerals common in soils and sedi-ments. Soil Sci. Soc. Am. J. 44:462-465.