Phosphorus Sorption and Uptake from Sri Lankan AlfisolsR. A. Morris,* R. R. Sattell, and N. W. Christensen

ABSTRACTSoil organic P as well as inorganic P quantity and capacity factors

have been identified as P availability determinants in tropical soils.The effects of these factors on P uptake from 12 Sri Lankan Alfisolswere determined. For the analysis, a P quantity variable (Q, u,g g~'soil) was P extracted by 0.5 M NaHCO, (16-h shaking). The tangentialbuffer capacity inverse (x 103), computed at points on P sorptionisotherms corresponding to Q, was the capacity variable [B, jig so-lution P x 103 g-1 solution/dig sorbed P g~' soil)]. Phosphorus ex-tracted by 0.1 M NaOH (16-h shaking) after removal of labile P wasa moderately labile organic P variable (Op, fig g~' soil). Phosphorusuptake by foxtail millet [Setaria italica (L.)] was determined fromNeubauer pots. A regression equation indicated significant influencesby Q, B, and Of on P uptake. The effect of Op was through an inter-action with Q. Moreover, applied inorganic P increased moderatelylabile organic P by only 11% but labile inorganic P by 255%. Thesignificant interaction was evidence that applied inorganic P increaseduptake from the labile inorganic P pool in soils with high NaOH-extractable organic P relative to uptake from soils with low NaOH-extractable organic P. The interaction between Q and Ov was consis-tent with three alternative hypotheses regarding the contribution oforganic P to soil P status. The first derivative of the regression equa-tion with respect to Q was used to examine a soil x applied P inter-action in the uptake data. Variables from the first derivative explainedabout two- thirds of the soil x applied P interaction of P uptake fromthe Neubauer pots, evidence that P sorption properties as well as Ovdifferences among the 12 soils influenced P availability.

ALFISOLS are the dominant soils of the semiaridtropics and crop yields are often limited by low

native soil P concentrations (El Swaify et al., 1985).Inorganic soil P quantity and buffer capacity are im-portant determinants of P availability (Olsen andKhasawneh, 1980). Recent evidence suggests that or-ganic P is also a significant determinant of P availa-bility in tropical soils (Ayodele and Agboola, 1983;Sharpley et al., 1987).

Soil P sorption by mediterranean Alfisols is the sub-ject of recent studies (Torrent, 1987; Keramidas andPolyzopoulos, 1983; Polyzopoulos et al., 1985) but ithas not been studied extensively in tropical Alfisols.The effects of soil organic P and the quantity andcapacity dimensions of soil inorganic P with respectto P available from tropical Alfisols are needed todevelop P management practices for the semiarid trop-ics.

The objective of this study was to determine theeffects of inorganic P quantities and capacities as wellas organic P on P uptake and on soil P removed bychemical extraction from 12 Sri Lankan Alfisols.

MATERIALS AND METHODSTwelve Sri Lankan Alfisols were sampled and analyzed

for clay content (82-313 g kg-1), pH (4.8-7.0, 0.01 MCaCl2), organic C (4.7-15.9 g kg-1), cation-exchange ca-pacity (3.9-13.4 cmolc kg-1), and inorganic and organic P

Dep. of Crop and Soil Science, Oregon State Univ., Corvallis,OR 97331-7306. Oregon Agric. Exp. Stn. Technical Paper no.9768. Received 8 Aug. 1990. *Corresponding author.

Published in Soil Sci. Soc. Am. J. 56:1516-1520 (1992).

fractions. For each of the 12 soils, P sorption was deter-mined at 12 points corresponding to solution concentrationsfrom near 0 to 13 \j.g P g-1 using Fox and Kamprath'smethod (1970). To account for sorbed native P, anion-ex-changeable P was added to P sorbed from solution. Phos-phorus solutions were selected so that 40% or more of theequilibrated concentrations were < 1 fig P g-1 solution. TheP fractions were obtained from a procedure outlined byHedley et al. (1982). Two fractions in particular were ofrelevance to this study. The first of these, NaHCO3-Pi5 wasextracted by 0.5 M NaHCO3 (0.5 g of soil passing a 0.15-mm sieve, 30 mL of extractant, pH 8.5, shaken for 16 h),and regarded as labile inorganic P. The second fraction,NaOH-P0, was determined by extracting soil recovered fromthe NaHCO3 extraction with 0.1 M NaOH (30 mL of ex-tractant, shaken for 16 h), digesting an aliquot with acidi-fied ammonium persulfate to transform P0 to Pj (U.S.Environmental Protection Agency, 1986) and determiningthe PJ difference between digested and undigested aliquots.NaOH-P0 was regarded as moderately labile organic P(Hedley et al., 1982). Phosphorus extracted by 0.05 M HC1and 0.0125 M H2SO4 (2.5 g of soil and 10 mL of extractant,shaken for 5 min) and designated double-acid P, was alsoof relevance to this study.

A nonlinear regression method was used to fit P sorptiondata to the Langmuir two-surface model:

Ps(C) = k2C) ~l

where PS(C) is P sorbed at C, the concentration of P insolution. Other variables in the model are curve-fitting pa-rameters.

The following formulae, derived from PS(C), were usedto characterize soil P quantity or buffer capacity by a singlevalue.

1. Sorption maxima were estimated as

SM = &! + b2

with bl and b2 from the fitted sorption model.2. Phosphorus retention value [PRV(C)] is the quantity

of P sorbed at a given C and was calculated by sub-stituting the value of C into the sorption model (Juoand Fox, 1977).

3. Buffer capacity was calculated as the first derivativeof PS(C). Tangential buffer capacity (Keramidas andPolyzopoulos, 1983) was calculated by substituting Cinto the first derivative:

TBC(C) = fcA(l + fcjC)-2 + ̂ (l + k2Q~2

where /fcj, k2, blt and b2 were estimated parameters.Maximum buffer capacity, the slope of the sorptioncurve at solution P concentration = 0, was

MBC = klbl + kjy2.

To determine the effects of P sorption by the 12 soils on Puptake, a Neubauer experiment was conducted. One hundredfoxtail millet seeds were planted in Neubauer pots (100 gair-dried soil sandwiched between upper and lower layersof sand). The plants were removed after 25 d and analyzed

Abbreviations: Q, quantity of P; C, concentration in solution; B,P buffering capacity; P?, organic P; Pj, inorganic P; SM, sorptionmaxima; PRV, P retention value; TBC, tangential buffer capacity;MBC, maximum buffer capacity; RMSE, root mean squared er-ror; RSS, root sum of squares; ***, significant at the 0.01 level.

1516

MORRIS ET AL.: PHOSPHORUS SORPTION AND UPTAKE 1517

for total P. The experiment was composed of three repli-cates of factorial combinations of the 12 soils and three Prates (control [PO], medium [PI] and high [P2]). The highP rates corresponded to concentrations of 0.2 jxg P g-1 in0.01 M CaQ2 solution in equilibrium with soil (Fox andKamprath, 1970). The rates were determined from prelim-inary isotherms estimated for a wider sorbed-P range andwith fewer points than used for the isotherms reported here.The intermediate P rates were 75% of the high rate andcorresponded to nominal concentrations of 0.1 n,g P g-1 inthe solution. The P treatments were applied by adding P insolution to each soil as it was mixed with water at the startof a 3-wk soil remoistening period. Soils were sampled forP determinations at the end of the remoistening period, atwhich time millet was planted. Details of the experimentalprocedures and P uptake determinations (designated plantP) were described by Sattell and Morris (1992).

Data were analyzed with SAS (SAS Institute, 1987). Pro-cedure NLIN, a nonlinear regression method, was used tofit P sorption data to PS(C). Procedure CORR was used forcorrelations and procedure REG for regression analyses.

The residual RMSE, a measure of goodness-of-fit for theP sorption curves estimated by nonlinear regression, wasdefined as

RMSE = [RSS(o - \-lll/2

where RSS is the residual sum of squares from the nonlinearregression, o is the number of observations, and p is thenumber of parameters in the model (Kinniburgh, 1986).The RMSE were calculated for each model and the modelswere visually examined as well for systematic over- or un-derprediction in the range of observed PS(C).

500

1 2 3P in solution (jig P g"1 solution)

Fig. 1. Experimental data and Langmuir two-surface sorptionisotherms for four representative soils. Lines are estimatedsurfaces and rectangles are data points.

RESULTS AND DISCUSSIONSorption Characterization

Phosphorus sorption isotherms (Langmuir two sur-face) for four soils, representative of the range amongthe 12 soils, are presented in Fig. 1. In this figure,the P solution range extends to only 3 (xg P g"1 so-lution, whereas data to 13 u,g P g-1 solution were usedto estimate model parameters. The RMSE ranged from2 to 15 |xg P g-1 soil (mean = 6.3 p,g P g-1 soil).Visual examination of models for each soil showedthat systematic over- and underestimation was not crit-ical in either the low or high range.

Ranges and means of estimated sorption maxima,P retentions, and buffer capacities are given in Table1. Phosphorus sorption characterizations tended to behighly intercorrelated. The NaOH- P0 fraction was theonly P fraction correlated with estimated sorptionmaxima, retention values, and buffer capacities (Table1). The negative correlations suggested that P0 com-pounds available to block sorption sites were moreabundant in soils with low buffer capacities, such asSoil 4, than in those with high capacities, such as Soils6 and 11. Anderson et al. (1974) reported that esterphosphates were preferentially sorbed on the same sitesthat sorbed Pj.

Inorganic and Organic Phosphorus Contributionsto Phosphorus Uptake

Sattell and Morris (1992) reported that P uptake bymillet from the soils in this study was related to twofactors: P quantity, represented by NaHCO3-Pi, andmoderately labile organic P, represented by NaOH-P0. Examination of factors contributing to P uptakeby millet was extended by using NaHCO3-Pj andNaOH-P0 as two independent variables (Q and Oe,respectively) as well as B, derived from the Langmuirtwo-surface model. The TBC[C(j2)] was used as anequilibrium buffer-capacity estimate but, in theregression model, its inverse multiplied by 103 wasused. Concentrations (C) at Q were estimated by set-ting the Langmuir two-surface model equal to ex-tracted NaHCO3-Pj and solving for C numerically.Means and ranges of the independent variables aregiven in Table 2.

Models regressing plant P and double-acid P on thevariables were developed by starting with Q as a sin-gle independent variable and introducing variables ac-cording to the additional variance explained (Table 3).Model development was similar for both plant P and

Table 1. Minima, maxima, and means of, and correlations among, buffer capacities (maximum [MBC] and tangential [TBC]),sorption maxima (SM), and P retention values (PRV) estimated from Langmuir two-surface models for 12 Sri Lankan Alfisols;including correlations with NaOH-extractable organic P (NaOH-P0).

Simple correlationsParameterMBC, 11% P sorbed

Mg-' P soln.TBCf, 11% P sorbed

jug-1 P soln.SM, /tgPg-1 soilPRVt, /tgPg-1 soil

Minimum838

70

18044

Maximum12384

382

861214

Mean4413.7168.9

466.3125.1

TBC SM0.22 0.48

0.93*«*

PRV0.68

0.84**»0.96"

NaOH-P0

-0.59*-0.53

-0.66*-0.73**

*,«*,*** Significant at 0.05, 0.01, and 0.001 probability levels, respectively.t TBC and PRV were estimated for solution concentrations of 0.2 (ig P g-1 solution.

1518 SOIL SCI. SOC. AM. J., VOL. 56, SEPTEMBER-OCTOBER 1992

double-acid P. The Q and B regression coefficients inModels 2a and 3a were significant but inclusion ofthe B x Op interaction in Model 4a increased the Bcoefficient while reducing its significance. The inclu-sion of Q x Op decreased the Q coefficient by 80%and nullified its statistical significance as well. Anabrupt coefficient change on inclusion of an additionalvariable is evidence of multicollinearity. Recognizingthat B x Op and Q x Op were functions of otherindependent variables (and, moreover, that Q and Bwere functions of the Langmuir two-surface modelparameters), multicollinearity was not unexpected.Therefore two three-variable models that accountedfor most of the variance explained by the five-variablemodels were selected for further examination (Models7a and 8a for plant P and 7b and 8b for double-acidP; Table 3).

Plant P estimates from Models 7a and 8a were com-pared across the range of Q determined on the PO andP2 treatments for each soil. The same comparisonswere made for double-acid P estimates from Models7b and 8b. Estimates were computed by replacing Qand B with functions of C from the Langmuir two-surface models while holding Op at mean values foreach soil. Plant P estimates from Models 7a and 8aseldom differed by more than 20 ^g P pot-1. Themaximum difference between plant P estimated fromModels 7a and 8a was observed with Soil 8 estimates.For this soil, estimated differences were <2 p,g Ppot"1 at Q = 30 (xg P g-1 soil but diverged graduallyto about 100 |xg P pot-1 at Q = 90 ixg P g-1 soil.

Model 8a, which included only one interaction term,was easier to examine for response behavior. Figure2 shows how plant P responded as NaHCO3-Pj in-creased in four representative soils. The predomi-nantly linear influence that NaHCO3-Pi had on plantP is apparent in the figure. The Op variable, by itsinteraction with Q, altered the slope of the responseto NaHCO3-Pj. The B variable introduced concavecurvature as NaHCO3-Pi increased, but only for soilsthat had low TBC(C), such as Soils 4 and 8. Thesimilarities of double-acid P and plant P responses toNaHCO3-Pi are evident in Fig. 2 and 3.

The Q x Op coefficient in Model 8a indicated that

Table 2. Independent variables in the regressions of P uptakeby millet and P removed by double acid extractant on Pquantity (Q) and capacity (B), and organic P (Of).

Variable DefinitionOf NaHC03-P,fif (TBCCQ,)]-' x

Op§ NaOH-P0

Units Mean/tgPg-'soil 52

103 /igPg-'soln. x 1.8/ig~' P sorbed g soilx 103

H% P g-1 soil 65

Range12-108

0.2-13.3

30-106tlnorganic P (P,) extracted by 0.5 A/NaHC03.tlO3 times the inverse of tangential buffer capacity at a given level of labileinorganic P [TBC(O)]. TBC (O,) was determined by computing the solutionconcentration of P (Q from the Langmuir two-surface model by numericalanalysis at sorbed P (PJ = NaHCO3-extractable inorganic P and substitutingC into the equation for tangential buffer capacity at C [TBC(Q].

§0rganic P (P0) extracted by 0.1 MNaOH.

plant P increased as either Q or Of increased. Thevalue of Q differed among soils and was affected byapplied P. The value of Op also differed among soilsbut, as expected, was only slightly affected by appliedP. Plant P, as a percentage of labile P, increased from13% at 31 M-g NaOH-P0 g-1 soil (Soil 9) to 25% at101 |xg NaOH-P0 g-1 soil (Soil 3). Thus, P uptakeappeared to increase more than proportionally fromsoils with high NaOH-P0 concentrations as NaHCO3-Pj increased (i.e., as applied P increased the concen-tration of labile Pj). Applied P increased mean NaOH-P0 concentration only 11% (mean increase = 7 p,gg-1 soil compared with 46 |xg g"1 soil [255%] forNaHCO3-Pi, Sattell and Morris, 1992). Estimated ef-fects from NaOH-P0 concentration increases derivedfrom applied Pj averaged only 62 jxg pot"1 (range =0-133 (xg P pot-1).

It was obvious that equivalent P uptake could beobtained from different soils if NaHCO3-Pj concen-trations were adjusted (Fig. 2). For example, an es-timated 2500 |xg P pot"1 would be obtained from Soil4 at a concentration of 32 (xg NaHCO3-Pi g-1 soil,whereas the concentration required for an equivalentuptake from Soil 11 would be 82 jig NaHCO3-Pj g-1

soil. Soil P sorption behavior and organic P concen-trations account for the estimated equivalent uptakes.

The Q x Op coefficient was consistent with threerelated explanatory mechanisms. In the first, added Pj

Table 3. Coefficients from regressions of plant P and double-acid-extractable P on inorganic P (P,) quantity (0, P, capacity (B),and organic P (Op) variables.

Modelno.la2a3a4a5a6a7a8aIb2b3b4b5b6b7b8b

DependentvariablePlantPPlant PPlantPPlantPPlant PPlantPPlantPPlantPDouble-acid PDouble-acid PDouble-acid PDouble-acid PDouble-acid PDouble-acid PDouble-acid PDouble-acid P

Independent variablesIntercept

15021312873737

1218120612301220

4.00.8

-12.4-16.3-5.4-2.1-1.6-1.8

<?15.2***14.9***16.5***15.2***2.5NS2.7NS8.7**7.8**0.264***0.258***0.308***0.271***

-0.017NS-0.070NS :

0.070NS0.062NS

B

115.6***88.4***

321.4*384.6**385.5**

82.9**

1.95***1.13**7.83**9.26***9.02***

1.05**

OP

6.20***8.33**

-0.20NS

0.186***0.247***0.054NS

B x 0,

-2.48*-3.21**-3.22**

0.80***

-0.071**-0.088***-0.085***

0.009*

<?* Op

0.23**0.23***0.16***0.17***

0.0052***0.0062***0.0046***0.0046***

R2

0.530.770.820.840.880.880.840.850.460.660.800.840.900.900.830.84

' Significant at 0.05, 0.01, and 0.001 probability levels, respectively. NS = not significant at 0.05 level.

MORRIS ET AL.: PHOSPHORUS SORPTION AND UPTAKE 1519

3500

3000oQ.

^ 2500

0)10 2000S.

Q- 1500

1000

soil 4

soil 11

35

10 20 30 40 50 60 70 80 90 100

NaHCOg-P, (ng g'1 soil)Fig. 2. Phosphorus uptake as a function of labile inorganic P

(NaHCO3-P,) for four representative soils. Estimates weredetermined with labile P, represented by the quantity variable(Q) in Model 8a of Table 3. Tangential buffer capacity (Bin Model Sa) was a function of labile P,. P uptake estimateswere at the mean contents of moderately labile organic P(NaOH-PJ in each soil.

25 -

Q. 2°1 15CO.0) 10 -^3

OQ

soil 4

soil 11

0 10 20 30 40 50 60 70 80 90 100NaHC03-P| (MS g'1 soil)

Fig. 3. Double-acid-extractable P as a function of labile inorganicP (NaHCO3-P,) for four representative soils. Estimates weredetermined with labile P, represented by the quantity variable(Q) in Model 8b of Table 3. Tangential buffer capacity (Bin Model Sb) was a function of labile P,. P uptake estimateswere at the mean contents of moderately labile organic P(NaOH-P0) in each soil.

enhances mineralization of P-containing organic com-pounds. In the second, added P; enhances mineraliza-tion of C compounds, releasing small organic anionsand displacing sorbed Pj in the process. In a thirdmechanism, added Pj increases the fraction of occu-pied P sorption sites, thereby desorbing P0. The sec-ond and third mechanisms increase the potential forP0 mineralization. The positive and significant Q xOp interaction coefficient from the regression analysisis consistent with any combination of these mecha-nisms.

The regression of double-acid P on the Q, B, andOp variables showed that the P removed by the double-acid extraction was explained by the same variablesas those found significant in the plant P regressions.Furthermore, the partial regression coefficients in thedouble-acid P models exhibited patterns of changesimilar to those in the plant P models as variables wereadded or removed. Recognizing that the double-acid-extractable P was significantly correlated with plant Pas well (Sattell and Morris, 1992), this extractant war-rants further evaluation on tropical Alfisols similar tothose in this study.

The Soil X Applied Phosphorus InteractionSattell and Morris (1992) detected a significant soil

x applied P interaction in the analysis of variance ofplant P from the Neubauer experiment. They notedthat Pj applications to the soils were unequal, rangingfrom 22 to 158 |xg Pj g"1 soil, but found no evidencethat applied Pj differences contributed to the signifi-cant soil x applied P interaction. We reexamined theinteraction by using Model 8a to quantify the P sorp-tion behavior of each soil. Plant P responses to Qdiffered among soils, mainly by the slope of the re-sponses (Fig. 2). If slope differences accounted forthe soil x applied P interaction, then, as a first ap-proximation, [d(plant P)/d(<2)] x (£>a - Qb), with Qband <2a representing Q before and after the Pj appli-

cations and with the differential evaluated near Qb,should account for much of the interaction. Plant P inModel 8a was a function of Q, B, and Op. The chainrule can be used to write d(plant P)/dg as [d(plant P)/dC] x (dC/dQ), where C is the concentration of P insolution. Plant P in Model 8a can be expressed interms of C by substituting the Langmuir model for Qand KPfTBQC)]-1 for B. Model 8a differentiated withrespect to C is

d(plantP)/dC = e(dQ/dC) + !Q-3f{d[T:BC(C)]-l/dC}

+ gOp(dQ/dQ

where e, f , and g represent the numeric coefficientsin Model 8a. After differentiating with respect to C,multiplying by dC/dQ (i.e., TBC(C)-1] and takinglimits C -» 0, Q -> 0.

d(plant P)/d0 = e(\) + 2 x

gOp [I]

This equation indicates that, for Q near 0, plant Presponses were functions of sorption parameters andOp. The term in {--•} is d[TBC(C)] - l/dC as C -* 0and the term in [•••] is the MBC.

In soils to which P; had not been applied, NaHCO3-Pj concentrations (i.e., Qb) were small and found in anarrow range (12-29 jjug Pj g-1 soil) (Sattell and Mor-ris, 1992). Furthermore, NaHCO3-Pj increases (i.e.,Ga — Qb) produced by applied P were highly corre-lated with applied P (r = 0.87***, data not shown).Therefore if d(plant P)/dQ differences contributed tothe soil x applied P interaction, then a linear functionof the variables in Eq. [1] times applied P shouldexplain a significant portion of the soil x applied Pinteraction. The relevant variables were ^(applied P),/"(applied P){(*? b, + k\ b2)(kj>i + ^2)-%A:A +

1520 SOIL SCI. SOC. AM. J., VOL. 56, SEPTEMBER-OCTOBER 1992

Table 4. Regression analysis of plant P differences (AP) betweensoils receiving and not receiving inorganic P (P,) for 12 soils,with one zero, two nonzero P, treatments per soil.

Intercept62842

-5922

APt4.27NS6.42**»2.56NS

AP x Fn(*,,6i)t

77.7***58.9**49.0**

A P x Op§

0.112*0.153***

R2

0.140.610.700.68

*,**,*** Significant at 0.05, 0.01, and 0.001 probability levels, respectively.NS = not significant at 0.05 level,

t Applied P,.$ A function of k,, k2, b,, and fr, defined in Eq.[l].§ Op = Organic P extracted by 0.1 M NaOH.

kj)-2\~l, and ̂ (applied P)0p, with the superscriptedp indicating coefficients different from those in Eq.[1], The interaction variable for each soil can be ap-proximately quantified by substituting klf k2, b^ andb2 for each soil and applied P for each P treatmentinto the terms in the preceding sentence.

A regression analysis of the interaction is given inTable 4. The dependent variable, AP, was the differ-ence between plant P from treatments receiving andnot receiving applied P. Applied P was not significantin a single-variable model but it was significant whensorption terms were introduced. Because applied Pwas a factor in all variables, multicollinearity was sub-stantial and applied P was not significant when in-cluded with two independent variables containing it.The final two-variable model in Table 4 accounted formore than two-thirds of the interaction. This analysisimplicated P sorption parameters and organic P, var-iables that characterized the slope of plant P responseto Q [i.e., d (plant P)/d<2], as contributors to the soilx applied P interactions observed among the 12 soils.

CONCLUSIONSPhosphorus sorption by Sri Lankan Alfisols was

described by a Langmuir two-surface model. Sorptionmaxima, buffer capacity, and P retention estimatedfrom the Langmuir two-surface model were intercor-related but, with exceptions, were not correlated withP fractions. Exceptions were negative correlations withthe moderately labile organic P fraction, suggestingthat labile inorganic P and moderately labile organicP competed for sorption sites.

Phosphorus uptake by millet was related to threevariables: inorganic P quantity (labile Pj from a frac-tionation procedure), buffer capacity (THC estimatedfrom a sorption isotherm), and, through interactions,moderately labile organic P (P0). The interactions wereconsistent with three hypotheses on the role of P0: Pjapplications enhance mineralization of P0 compounds,P; promotes generation of small organic C compoundsthat displace sorbed P0, and Pj displaces sorbed P0. Inthe latter two cases, the potential for mineralizationof P0 is increased.

Recognizing that studies relying heavily on corre-lation and regression analysis have limitations, inves-tigations that more explicitly examine mechanismscontributing to P uptake from soil organic P are needed.A better understanding of the P; vs. P0 sorption rela-

tionship is relevant to the P management of tropicalAlfisols for which cycling of tree and crop litter isoften advocated. Because organic P concentration af-fects uptake, cultivation practices that maintain or in-crease moderately labile P0 concentrations as well asPj concentrations are probably desirable.

Soil P removed by the double-acid extractant wascorrelated with P uptake as well as related to the samethree factors to which P uptake was related, suggestingthat the extractant removed P from fractions proportion-ally to that removed by millet roots. The similarity be-tween the P uptake and the double-acid regression modelsare of some pertinence. It suggests that, not only is thedouble-acid extractant valid over the labile Pj and mod-erately labile P0 ranges in the Alfisols examined, but forthe buffer capacity range as well.

ACKNOWLEDGMENTSWe are indebted to the former Deputy Director for Re-

search at the Maha Illuppallama Research Center (Dr. M.Sikurajapathy) for facilities made available for the Sri Lan-kan phase of this study. Partial financial assistance wasprovided by the Department of Crop and Soil Science andthe Office of International Research and Development, Or-egon State University. The program for the numerical es-timation of P solution concentration was graciously providedby Dr. F.T. Lindstrom, Department of Crop and Soil Sci-ence, Oregon State University.