Soil Phosphorus Supplying Capacity Evaluated by Plant Removaland Available Phosphorus Extraction1

B. F. AQUINO AND R. G. HANSON2

ABSTRACTMany Missouri soils test high in extractable P after long periods

of rock phosphate and processed P fertilization. This study wasundertaken to determine the P-supplying capacities of five Missourisoils possessing high levels of extractable P. Initial P levels wereevaluated with selected extractants (Bray I, Bray II, Mehlich II,double acid, dilute SrQ2) and through evaluation of the P bufferingindex. Each soil was cropped under greenhouse conditions for 7consecutive harvests of grain sorghum (Sorghum bicolorL. cv. Warner744R). After each harvest, plant P uptake was calculated and ex-tractable P measured as before. For each soil test, regression anal-yses of plant P uptake were performed. With only one exception,the decline in extractable P in all soils, as measured by all fiveextractants, was significantly correlated with cumulative P uptake.The highest correlations were obtained with Bray I for Sharkey clay(r = 0,95), Bray I and Bray II for Kennebec silty clay loam (r =0.97), Bray II for Mexico silt loam and Broseley loamy fine sand (r= 0.95 and r = 0.90, respectively), and Mehlich II for Tiptonvillesilt loam (r = 0.96). Slopes of the decline in extractable P followingintensive cropping show similarities between Bray I and MehlichII, Bray II and double acid. The quantity of plant available P ex-tracted by Bray II and dilute SrClj was found to be inversely relatedto the P buffer index (PBI). When extrapolating to critical P levelsfor Bray I (11 mg kg~l) and Bray II (24 mg kg'1) only the Bray IIextracted predicted number of harvests in order of soil PBI.

Additional Index Words: extractable P, P buffering index, plantP removal, P soil test, Sorghum bicolor L. Moench.

Aquino, B.F., and R.G. Hanson. 1984. Soil phosphorus supplyingcapacity evaluated by plant removal and available phosphorus ex-traction. Soil Sci. Soc. Am. J. 48:1091-1096.

MANY Missouri soils have received large quan-tities of rock phosphate or commercial phos-

phorus fertilizer and consequently now test high inextractable P. It is important to consider the totalquantity of residual available P in the soil if recom-mendations are to be developed that will insure max-imum economic yield.

It was reported by Mattingly and Widdowson (25),working with an acid soil, and Hagin et al. (14), work-ing with calcareous soils, that freshly applied P is morereadily available than residual P, but there is a de-crease in yield response to freshly applied P when theresidual P level is high. Kamprath (22) found that 685kg P ha"1 supplied adequate P for corn 9 years afterapplication. Arndt and Mclntire (2) observed from asorghum experiment on a Ultisol in Australia that theresidual values of single superphosphate was 50% ofthe initial value and decreased to 8% after 7 years,

1 Contribution from the Missouri Agric. Exp. Stn. Series no. 9366.Univ. of Missouri, Columbia, MO 65211. This work was supportedby Missouri Agric. Exp. Stn. Proj. 387 (Maximizing Plant NutrientUtilization). Received 4 May 1983. Approved 7 May 1984.2 Formerly CAPES (Coordenacao do Aperfeicomento de Pessoalde Nivel Superior) Graduate Student, presently Escola Superior deAgriculture de Mossoro, Caixa Postal 137, Mossoro, RN, Brazil;and Associate Professor of Agronomy, Univ. of Missouri, Colum-bia, MO 65211.

while the residual values of rock phosphate was about60 to 70% of the initial value after 7 years. Initiallythe response to superphosphate was superior to rockphosphate. After a 5-year study, Holford and Gleeson(18) concluded that the unusually high residual effectof previous P application is a result of the low P buffercapacity of soils. This is based on Holford and Mat-tingly's definition (21) that P buffer capacity is theresistance of the soil solution concentration to changewhen P is added or removed from the labile pool.Residual P effect related to rates of initial applicationwas studied by Read et al. (31); after five consecutivecroppings the critical available P level was reachedonly on soils receiving the lowest rate (100 kg P ha"1).Gisken et al. (13), in a greenhouse study with calcar-eous soils previously P fertilized with single super-phosphate in the field, found an inverse relationshipbetween relative P fertilizer efficiency and triple super-phosphate application. Their results suggest a higherefficiency of fertilizer P can be obtained on calcareoussoils when applied in small annual amounts.

The quantification of response to residual P has beenthe object of much study. Barrow and Campbell (5)demonstrated, with a greenhouse study, that P uptakeand/or yield were similar as measures of the residualeffect of a previous P application. Devine et al. (9)and Mattingly (24) related residual available P to an-nual applications of fertilizer materials reporting re-sults as "percentage of superphosphate equivalents,"where uptake or yield from most recently applied sin-gle superphosphate was used as the standard. Fox andKamprath (12) were able to calculate the residual ef-fectiveness of P fertilizer 10 years after application bya measure of the displacement of the adsorption iso-therms, a change in P sorbed from solution as functionof labile P. Fitter (11) used the Olsen (0.5 mol L"1

NaHCO3, pH 8.5) extractant to measure the declinein available P over time after monosodium phosphatewas added and allowed to age up to 180 d in 4 collieryshales. The decline in available P passed through the3 stages of (i) rapid initial drop controlled by pH; (ii)a steady stage period; and (iii) an exponential declinewhose rate was proportional to buffering capacity. Soilswith high residual P were studied under greenhouseconditions by Adepoju et al. (1) finding significant cor-relations between NaHCO3-extracted P and AER resin-extracted P before intensive cropping to the P re-moved by cropping, with NaHCO3-extractable P pro-ducing higher correlation coefficients. Bowman et al.(7) and Novais and Kamprath (29) successfully cor-related cumulative P removed in successive croppingswith P extracted by numerous soil tests. After review-ing greenhouse and field studies of the availability ofresidual P, Barrow (4) concluded that the general trendfor the decline in available P from a previous P fer-tilization is best described by a rapid initial decline,which may last 2 years, followed by a gradual geo-metric decline with time.

1091

1092 SOIL SCI. SOC. AM. J., VOL. 48, 1984

The residual plant-available P in soils can be quan-tified by successive cropping experiments. In such ex-periments, plant-available P is removed until P defi-ciency occurs or a response to added P is measured.This type of study is useful in quantifying the capacityof soils to supply P to plants following P fertilization.

The objectives of the following study were: (i) toassess the P supplying capacities of five diverse, ex-cessively P fertilized Missouri soils by correlating cu-mulative P removal with extractable P by five soil Pextractants; and (ii) to establish the relationship be-tween the rate of decrease in available P, the clay con-tent, and the P buffering index (PBI).

MATERIALS AND METHODSFive Missouri soils (Table 1) with varying physical and

chemical properties, but a common history of high P fertil-ization, were selected. Their classifications are: Broseleyloamy fine sand (loamy, mixed thermic Arenic Hapludalfs);Kennebec silt loam (fine-silty, mixed,, mesic Cumulic Ha-pludolls); Mexico silt loam (fine, montmorillonitic, mesicUdpllic Ochraqualfs); Sharkey clay (very fine, montmoril-lonitic, nonacid, thermic Vertic Haplaquepts); and Tipton-ville silt loam (fine-silty, mixed, thermic typic Argiudolls).Bulk samples were air dried, passed through a 4-mm screen,and mixed in preparation for greenhouse and laboratorystudies. Soil pH, organic matter, exchangeable Ca, Mg, andK, CEC, and available Zn were determined by the methodsemployed by Univ. of Missouri Soil Testing Lab. (8).

Three and one-half kilograms of soil were weighed into3.79-L cans lined with plastic bags. Six pots of each soil wereused as replications. All pots were cropped with seven con-secutive plantings of sorghum (Sorghum bicolor L. [Moench]cv. Warner 744R). With plantings 1 to 4, pots were thinnedto 10 plants, and to increase P depletion they were thinnedto 16 plants for plantings 5 to 7. Thinning was completed 2to 3 d after emergence and plants were permitted to growfor 35 d. The pots were fertilized as follows: 45 mg N kg"1

as (NH4)2SO4 per planting in 3 equal portions (preplant, af-ter 10 d and 20 d), during growth periods 1 to 4; for plantings5 to 7, the N rate was increased to 110 mg N kg"1 in 3increments (40 mg N kg"1 preplant and at 10 d, and 30 mgN kg"1 at 20 d); for plantings 1 to 4, 40 mg K kg"1 wasapplied and increased to 60 mg K kg"1 for plantings 5 to 7.A preplant application of 5 mg Zn kg"1 as ZnSO4-7 H2Owas included before the seventh planting. Plants were cut atthe soil surface, oven dried at 70°C, and ground through a0.841-mm screen for P analysis. The soil samples were airdried, ground through a 2-mm sieve, and stored until ana-lyzed.

Table 1—Initial analysis of the soils.

Analysis

pH(l:lH,O)Organic matter (%)CECmmol( + )kg-'Cammol('/jCa2*)kg-'MgmmoK'/jMg^kg-'Kmmol(K*)kg-Zn(mgkg-'|Clay(%)

Sharkey

6.42.6

27.219.0

1 6.40.85.5

68.2

Mexico

6.42.6

12.79.71.40.6

10.224.6

Kenne-bec6.92.5

15.812.52.20.33.4

28.4

Brose-ley

6.51.36.53.91.20.5

11.67.5

Tipton-ville

6.21.39.16.51.20.42.6

14.1Extractable P (mg kg-')

Bray IBray IIMehlich IIDouble acidDilute SrCl,PBI (mL/g)

8813667570.15

24.4

5731547

2490.16

21.1

528249540.14

20.4

13317110183

0.445.9

58914665

0.1710.6

Plant tissue was analyzed for total P by nitric-perchloricacid digestion as described by Steckel and Flannery (34);Bray I (0.025 mol L"1 HC1, 0.03 mol L"1 NH4F, pH 2.6)and Bray II (0.1 mol L"1 HC1, 0.03 mol L"1 NH^, pH 1.6)according to Brown et al. (8); Mehlich II (0.2 mol L"1

CHjCOCH, 0.15 mol L"1 NH4F, 0.012 mol L"1 HCL, and0.2 mol L"1 H2SO4, pH 2.5) as proposed by Mehlich (26);double acid (0.05 mol L~' HC1 and 0.0125 mol L"1 H2SO4,pH 1.2) as described by Nelson et al. (28); extractable P wasdetermined colorimetrically with a Bausch & Lomb Spec-tronic 20 Spectrophotometer. The dilute SrCl2 (0.001 molL"1 SrCl2) available P was determined using the method asdescribed by Wendt and Corey (36) using a Bausch and LombSpectronic 88 Spectrophotometer.

The P sorption isotherms were determined following astandard procedure developed for soil and sediments3. Thesalient features of the procedures are: soil ratio 1:25, initialP concentrations range of 0, 6.5 X 10"6, 1.6 X 10~5, 3.2 X10~5, 16.1 X 10"5 and 32.2 X 10"5 mol L"1, and 0.01 molL"1 CaQ2; microbial inhibitor of 20 g L"1 choloroform; ex-tracting system with 50 mL tubes to permit 50% head space;extraction time of 24 h on a shaker at 180 oscillations min"1

and temperatures of 25 ± 2°C. The P in the soil extractswas determined colorimetrically by the method of Murphyand Riley (27) using the Bausch and Lomb Spectronic 88Spectrophotometer. Phosphorus sorbed vs. equilibrium Pconcentration was plotted. Because of curvature, the instan-taneous slope depends not only on the nature of the soil,but also on the activity at which the slope is measured.Barrow (3) suggests the slope when measured when AP =0 would be a function of the potential and a characteristicof the soil. This was first referred to as the maximum buffercapacity by Hplford and Mattingly (19) and later as a phos-phorus buffer index by Holford (15). From the isotherms ofP sorption, regression analyses were performed relating Psorbed (Q) to P in equilibrium solution (/). The P bufferindex (PBI) values were calculated by differentiating the fit-ted quadratic equations from the regression analysis and cal-culating the slope values at Q = 0. Initial soil analyses arepresented in Table 1.

Relationships between extractable P and plant P uptakefollowing the seven crops were determined, relating per-centage clay, PBI, and slopes of the regression lines (P de-pletion) of extractable P on P removed by plants. The sta-tistical analyses followed procedures outlined in the SASUser's Guide (32).

RESULTS AND DISCUSSIONBray II soil test was adopted in Missouri when rock

phosphate was the major P source used to increaselevels of soils low in available P. The higher acid con-centration in Bray II vs. Bray I would extract a higherquantity of the rock phosphate P available to plantsand was superior to interpret fertilizer P requirementsfor soils previously fertilized with rock phosphate.Water-soluble P fertilizers are now predominantly usedand consequently Bray I may be a more suitablemethod to measure available P. Research completedbetween 1966 and 1970 on a Baxter silt loam withalfalfa orchardgrass forage mixture by Fisher (10) in-dicated that maximum yield occurred near a criticalsoil test level of 24 mg kg"1 P by Bray II and 11 mgkg"1 P by Bray I. Using these soil test values as criticallevels, the soils in this study were judged to be veryhigh in available P. The Mexico soil was from San-born Field with a history of 93 years of only rock

3 Developed by T. Logan, B. Ellis and D. Nelson. 1981. Personalcommunication, T. Logan.

AQUINO & HANSON: SOIL P SUPPLYING CAPACITY EVALUATED BY PLANT REMOVAL AND P EXTRACTION 1093

Table 2—The linear relationships between change in extractable P in five Missouri soils and cumulative plant P removal.

P extractant

Soil Bray I Bray II Mehlichll Double acid Dilute SrCl1

SharkeyMexicoKennebecBroseleyTiptonville

APt= 90.6-0.54 PRAP = 51.9-0.11 PRAP = 52.9-0.43 PRAP = 136.2-0.82 PRAP = 55.8-0.57 PR

AP = 135.6-0.37 PRAP = 307.5-0.75 PRAP= 82.5-0.48 PRAP = 175.9-1.06 PRAP = 95.5-0.73 PR

AP= 71.1-0.49 PRAP= 46.1-0.18 PRAP = 48.7-0.47 PRAP = 101.2-0.58 PRAP = 46.3-0.60 PR

AP= 59.3-0.39 PRAP = 249.0-0.70 PRAP = 57.6-0.48 PRAP = 92.7-0.58 PRAP= 70.6-0.61 PR

AP = 0.19-0.13 PRAP = 0.17-0.02 PRAP = 0.14-0.04 PRAP = 0.46-0.31 PRAP = 0.19-0.26 PR

f AP = Soil extractable P (mg kg-'); PR = accumulative plant P removed (mg kg-').

phosphate as P treatment. Available records indicatedonly water-soluble P fertilizers were applied to theother 4 soils.

Linear regression equations for change in soil ex-tractable P on cumulative P removal are presented inTable 2 and coefficients of correlations of linear re-lationships in Table 3. These relationships are illus-trated in Fig. 1, 2, 3, 4, and 5 for Bray I, Bray II,Mehlich II, double acid, and dilute SrQ2, respectively.With one exception (dilute SrCl2, Mexico silt loam),all correlations between decline in extractable P andplant P removal are linear and significant at the 1%level. The degree of fit of a linear model is measuredby the correlation coefficient (Table 3) and the simi-larity between extractants by the similarity of theslopes. Comparable slopes were obtained for Bray Iand Mehlich II and Bray II and double acid for theSharkey clay and the Mexico silt loam. With the Ken-nebec silty clay loam, slopes of P decreases were sim-ilar for all except the dilute SrCl2 extractant. WithBroseley loamy fine sand, Mehlich II and double acidhad similar slopes while Bray I and Bray II were dis-tinctly different. The Tiptonville silt loam exhibitedsimilar rates of decline in extractable P with Bray I,Mehlich II, and double acid, and more rapid declinewith Bray II.

The main reaction mechanism of Bray I is forma-tion of stable coordination complexes of the F~ ionwith Fe3"1" and A13+ and the formation of insolubleCaF2 with Ca2+ (27). The F~, ion being very effectivein complexing A13+ ions, will release P from Al-P and

~ 125'ra

~ 100

ucc6

ccm

75

50

25

SharkeyO—o Mexicoa—& BroselevD—a Kennebec

Tiptonville

10 20 30 40 50 60 70 80 90100 110

P REMOVED (mg.kg-1)

Fig. 1— Relationship between Bray I extractable P and P removalthrough seven plantings.

Table 3—Coefficients of correlations of linear relationshipsbetween cumulative P removal and extractable P

for five Missouri soils.

Soil test method

Soil Bray I Bray II Mehlichll D.acid Dilute SrCl,

SharkeyMexicoKennebecBroseleyTiptonville

0.95*0.82*0.97*0.87*0.92*

0.87**0.95**0.97**0.90**0.93**

0.94**0.91**0.96**0.87**0.96**

0.90**0.94**0.95**0.86**0.90**

0.85**0.28 NS0.40**0.82**0.78**

** Significant at the 1% level; NS = not significant.

with the precipitation of Ca2+ as CaF2 extracting Ppresent as CaHPO4 (35). The HC1 provides sufficientH"1" ion activity to dissolve Ca-P and to a lesser extentAl-P and Fe-P. Because Mehlich II has both NH4Fand HC1 plus another acid and complexing agent andpH of 2.5 similar to the Bray I, similarities betweenthe methods can be expected. Similarities in extract-able P exhibited by four of the five soils for these twoextractants in this work has been reported elsewhere(21). The similarities in rate of decline of extractableP with crop removal was exhibited by four of five soilsfor extractants Bray II and double acid. Both extrac-tants of a similar pH would solubilize the Ca-P incommon, with Bray II complexing A13+ and Fe3+ toextract larger quantities of the Al-P and Fe-P. TheKennebec silty clay loam soil exhibited a similar rate

300

"U 250

200-

O 150<o:HX

= 100

ccm

50-

•—• SharkeyO—O MexicoD—D KennebecA—A Broseley

Tiptonville

10 20 30 40 50 60 70 80 90100 110

P REMOVED (mg-kg-1)

Fig. 2—Relationship between Bray II extractable P and P removalthrough seven plantings.

1094 SOIL SCI. SOC. AM. J., VOL. 48, 1984

100 -

_ 90

80 -

70 -

u<CE

Xo_JUJ

60-

50 -

40 -

30-

20 -

10 -

0

•—• SharkeyO—O MexicoO—a KennebecA—A Broseley

Tiptonville

10 20 30 40 50 60 70 80 90 100 110

P REMOVED (mg-kg'1)

Fig. 3—Relationship between Mehlich II extractable P and P re-moval through seven plantings.

of decline in extractable with all extractants exceptdilute SrCl2 suggesting low concentrations of Ca-P andpredominance of Al-P and Fe-P. The dilute SrCl2 so-lution is a neutral salt and the rate of decline in ex-tractable P was not similar to the other four extractionsolutions. The P buffer capacity as defined by Beckettand White (6) and referred to as the P buffer index(FBI) by Holford (17) as the slope of the "quantity-intensity" relationship (AW/AI), this is the relation-ship between the change in sorbed P and the activityof H?PG7. This relationship is similar to an adsorp-tion isotherm. Because of the curvature, the slope wasmeasured at Q = 0 and therefore is a function of thepotential, a constant value, and a characteristic of thesoil. The P buffering index is quite similar for the

0.42 •—• SharkeyO—O Mexicon—n Kennebec

BroseleyTiptonville

10 20 30 40 50 60 70 80 90 100 110

P REMOVED (mg-kg-1)

Fig. 5—Relationship between dilute SrClj extractable P and P re-moval through seven plantings.

250-

S 200-CQ

I-oE 150xUJ

QO< 100

OO 50

0

•—• SharkeyO—O MexicoD—D KennebecA—A Broseley•—• Tiptonville

10 20 30 40 50 60 70 80 90 100 110

P REMOVED (mg-kg-1)

Fig. 4—Relationship between double acid extractable P and P re-moval through seven plantings.

Sharkey, Mexico, and Kennebec soils and low for theBroseley and Tiptonville soils. The slope of the de-cline in extractable P with cropping would be a func-tion of FBI for the P extractant to measure availableP related to the P buffer capacity.

The high level of P extractant by Bray II and doubleacid for the Mexico silt loam is a result of 93 years ofcontinuous rock phosphate application. The lowercorrelation of Bray I extractable P with P removal(r = 0.82) suggests the predominance of hydroxyapa-tite [CA5(PO4)3OH] or carboxy apatite[CAsCPO^COjOHJjF] or both. The dominance of Ca-P forms in Virginia soils receiving rock phosphate hasbeen reported by Martens et al. (23). The lower cor-relation of Bray I with P removal for the Mexico soilcould be because: (i) low acid strength did not extractthe true plant available P; or (ii) the formation of CaFand immobilization of P as reported by Smillie andSyers (33). The higher correlation of the Mehlich IIextractant than Bray II could be a result of a higherconcentration of complexing agents to prevent thereadsorption of P (22).

The similarity of the linear relationship between ex-tractable soil P and plant P removal (regression lineslope) for all soil test methods, and all soils and theirrelationships to soil P buffering index (FBI) and per-cent clay, is presented in a matrix of correlations inTable 4. There was negative correlation (significant atthe 10% level) between the slope of decline in extract-able P by crop removal and FBI for the Bray II anddilute SrCl2 extractants. This suggests that the quan-tity of plant-available P as measured by these two ex-tractants (Bray II, dilute SrQ2) will decrease in an in-verse proportional relationship to the soil P buffercapacity. Pratt and Garber (30) report that increasingclay content decreases the effectiveness of Bray I toextract P by exhaustion of the reagent. No explanationcan be offered for the lack of relationship between FBIand decline in extractable P with Mehlich II and dou-ble acid. The FBI is basically a surface adsorptionparameter; the extractable P levels of the soils in thisexperiment are the results of prolonged applications

AQUINO & HANSON: SOIL P SUPPLYING CAPACITY EVALUATED BY PLANT REMOVAL AND P EXTRACTION 1095

Table 4—Matrix of coefficient of correlation for the linearrelationships between extractable P and plant P removal,

P buffering index and percent clay.Bray Bray Mehlich Double Dilute

I II III acid SrCl,slope slope slope slope slope

Table 5—Calculated consecutive harvests necessary to decreaseextractable soil P to critical Bray I and Bray II levels.t

Based on initial soil P level If at same soil P level}

FBIClay

Bray Islope

Bray IIslope

Mehlich IIslope

Double acidslope

Dilute SrCl,slope

FBI

- 0.37 NS 0.91* 0.33 NS 0.91* 0.71 NS 0.21 NS

0.14 NS 0.72 NS 0.52 NS 0.85*** 0.85***

- - - 0.40 NS 0.86*** 0.61 NS 0.17 NS

- - - - 0.02 NS 0.41 NS 0.73 NS

- 0.85*** 0.35 NS- - - - - - 0.81***

* Significant at the level of 5%.*** Significant at the level of 10%; NS = not significant.

of P fertilizers and appreciable quantities of this Pmay have precipitated as amorphous and crystallinephases whose mechanisms of P release should basi-cally be governed by solubility products. As reportedby Holford and Mattingly (20) and Holford (16,17),with increasing soil P buffer capacity, there is a cor-responding decrease in the proportion of labile P takenup by plants. Holford (17) suggests that a satisfactorysoil test should also extract a proportion of the labileP that decreases with increasing sorptivity. He re-ported that Bray I overcompensated for soil P buffercapacity and Mehlich II extracted large quantities ofnonlabile P in soils with pH above 6.0. Significantcorrelation exists between the slope of Bray I andMehlich II confirming that extracants with similar pHand with complexing agents will react with a similarquantity of soil P. The relationships between extract-able P measured with Bray II and dilute SrCl2 and theFBI and the clay content observed in this study, in-dicates that these two extractants are related to the Psorption capacity of the soil. Additional research isneeded to include a measure of the quantity and in-tensity of soil P as they vary with the buffer capacityto interpret the P fertilizer recommendation necessaryto obtain the extractable soil P level optimal for plantuptake.

At the end of 7 consecutive harvests the extractableP levels (Bray I and Bray II) for some soils approachedcritical levels now used in Missouri. The number ofconsecutive harvests necessary to reach critical soil Plevels, and the number of consecutive harvests nec-essary if all soils were at (i) the initial extractable Plevels and (ii) the same extractable P levels, were cal-culated and are presented in Table 5. These calcula-tions were based on an average P removal per harvestof 15 mg P kg"1, and utilized the linear equations forthe rate of decline in Bray I and Bray II extractable Pby plant removal. When based upon the initial soilextractable P levels, the consecutive harvests neces-sary to deplete the P level for Mexico, Kennebec,Broseley, and Tiptonville soils were similar to Bray Iand Bray II, but Bray II indicated almost double theconsecutive harvests of Bray I with the Sharkey clay.When calculating consecutive plant P removals willall soils assumed to be at the same initial P levels,

Soil Bray I Bray II Bray I Bray II

SharkeyMexicoKennebecBroseleyTiptonville

1025

7116

21 1526 72

9 1910 107 14

4321331522

t Assume 15 mg kg-' plant P removal harvest-', critical soil P levels of:Bray I—11 mg P kg-' and Bray 11—24 mg P kg-'.

J Assume initial Bray I of 130 mg P kg-' and Bray II of mg P kg-' for allsoils.

with Bray II, except for the Tiptonville soil, the orderof soils by number consecutive harvest, highest tolowest, is the same order as the PBI. With Bray I, theorder of highest to lowest in consecutive harvest doesnot follow the order of PBI, in particular, indicatingan overestimation of total plant-available P in theMexico soil and an underestimation of the Sharkeyclay.

CONCLUSIONSThe results form this study indicate the plant re-

moval of residual P from five different soils duringseven successive crops of grain sorghum was well cor-related with estimates of extractable P by the four acid-based extractants. Changs in P in equilibrium withdilute SrCl2 were not as well correlated to crop re-movals. The rate of decline in extractable soil P byplant P removal was significantly related to PBI byonly the Bray II and SrCl2 methods. Without knowingthe previous P fertilizer materials applied, the Bray IImethod would appear to show more promise than BrayI as a measure of labile P as interpreted by the numberof consecutive crop removals.

1096 SOIL SCI. SOC. AM. J., VOL. 48, 1984

soil test for phosphorus and potassium. Miss. Agric. Exp. Stn.Res. Bull. 1007. Univ of Missouri-Columbia.

11. Fitter, A.M. 1974. A relationship between phosphorus require-ment, the immobilization of added phosphate, and the phos-phate buffering capacity of colliery shales. J. Soil Sci. 25:41-50.

12. Fox, R.L., and E.J. Kamprath. 1970. Phosphate sorption iso-therms for evaluating the phosphate requirements of soils. SoilSci. Soc. Am. Proc. 34:902-907.

13. Gisken, M., J. Hagin, and P. Kafkafi. 1972. Crop response tophosphorus fertilization and to residual phosphate. II. Evalu-ation of residual phosphorus "availability" by chemical andplant tests in a greenhouse. Agron. J. 64:591-593.

14. Hagin, J., M. Gisken, and P. Kafkafi. 1972. Crop response tophosphorus fertilization and to residual phosphate levels. II.Greenhouse experiments. Agron. J. 64:593-597.

15. Holford, I.C.R., 1979. Evaluation of soil phosphate bufferingindices. Aust. J. Soil Res. 17:495-504.

16. Holford, I.C.R. 1980a. Greenhouse evaluation of four soil testsin relation to phosphate buffering and labile phosphate in soils.Soil Sci. Soc. Am. J. 44:555-558.

17. Holford, I.C.R. 1980b. Effects of phosphate buffer capacity oncritical levels and relationships between soil tests and labilephosphate wheat growing soils. Aust. J. Soil Res. 18:405-414.

18. Holford, I.C.R., and A.C. Gleeson. 1976. Residual effectivenessof phosphorus on white clover on granitic soils. Aust. J. Agric.Res. 27:509-519.

19. Holford, I.C.R., and G.E.G. Haltingly. 1976a. A model for thebehavior of labile phosphate in soil. Plant Soil 44:219-229.

20. Holford, I.C.R., and G.E.G. Mattingly. 1976b. Phosphate ad-sorption and plant availability of phosphate. Plant Soil 44:377-389.

21. Holford, I.C.R., and G.E.G. Mattingly. 1979. Effects of phos-phate buffering on the extraction of labile phosphate by plantsand by soil tests. Aust. J. Soil Res. 17:511-514.

22. Kamprath, E.J. 1967. Residual effect of large applications ofphosphorus on high phosphorus fixing soils. Agron. J. 59:25-

23. Martens, D.C., J.A. Lutz, and G.D. Jones. 1969. Forms andavailability of phosphorus in selected Virginia soils as relatedto available phosphorus tests. Agron. J. 61:616-621.

24. Mattingly, G.E.G. 1968. Evaluation of phosphate fertilizers. II.Residual of nitrophosphates, Gafsa rock phosphate, basic slagand potassium metaphosphate for potatoes, barley and swedes

grown in rotation, with special references to changs in soil phos-phorus. J. Agric. Sci. 70:139-153.

25. Mattingly, G.E.G., and F.V. Widdowson, 1963. Residual valueof superphosphate and rock phosphate on an acid soil. I. Yieldsand phosphorus uptake in the field. J. Agric. Sci. 60:394-407.

26. Mehlich, A. 1978. New extractant for soil test evaluation ofphosphorus, potassium, magnesium, calcium, sodium, man-ganese, and zinc. Commun. Soil Sci. Plant Anal. 9(6):477-492.

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

28. Nelson, W.L., L.A. Mehlich, and E. Winters. 1953. The devel-opment, evaluation, and use of soil tests for phosphorus avail-ability. In W.H. Pierre and A.G. Norman (ed.) Soil and fertilizerphosphorus. Agronomy 4:153-188.

29. Novais, R., and E.J. Kamprath. 1978. Phosphorus supplyingcapacities of previously heavily fertilized soils. Soil Sci. Soc.Am. J. 42:931-935.

30. Pratt, P.P., and M.J. Garber. 1964. Correlations of phosphorusavailability by chemical tests with inorganic phosphorus frac-tions. Soi Sci. Soc. Am. Proc. 28:23-26.

31. Read, D.W.L., E.D. Spratt, L.D. Bailey, F.G. Warder, and W.S.Ferguspn. 1973. Residual value of phosphatic fertilizer on cher-nozemic soils. Can. J. Soil Sci. 53:389-398.

32. SAS Institute, Inc. 1982. SAS user's guide: statistics. 1982 ed.SAS Inst, Gary, NC.

33. Smillie, G.W., and J.K. Syers. 1972. Calcium fluoride formationduring extraction of calcareous soils with fluoride: II. Impli-cations to the Bray P-l tests. Soil Sci. Soc. Am. Proc. 36:25-30.

34. Steckel, J.E., and R.L. Flannery. 1971. Simultaneous determi-nations of phosphorus, potassium, calcium, and magnesium inwet digestion solutions of plant tissue by auto analyzer, p. 83-96. In L.M. Walsh (ed.) Instrumental methods for analysis ofsoils and plant tissue. Soil Science Society of America. Madison,WI.

35. Thomas, G.W., and D.E. Peaslee. 1973. Testing soils for phos-phorus, p. 115-132. In L.M. Walsh and J.D. Beaton (ed.) Soiltesting and plant analysis, rev. ed. Soil Science Society of Amer-ica, Madison, WI.

36. Wendt, R.C., and R.B. Corey. 1981. Available P determinationby equilibration with dilute SrCl2. Commun. Soil Sci. PlantAnal. 12(6):557-568.