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FRACTIONATION OF SOl L PHOSPHORUSS. C. CHANGANDM. L. JACKSON

University oi Wisconsin1Received for publication October 25, 1956

Inorganic phosphates in the soil can be classified into four main groups:calcium phosphate, aluminum phosphate, iron phosphate, and the reductant-soluble phosphate extractable after removal of the first three forms. Calciumphosphate exists mainly as apatite, but dicalcium, monocalcium, and octacal-cium phosphate also exist in small amounts or as transitional forms. Iron, alu-minum, and calcium phosphates also include adsorbed and surface-precipitatedphosphates associated with the respective types of soil particles. The availabilityof soil phosphorus to plants depends apparently on the extensity of the phosphatesurface of various chemical species (10, 11); conventional methods for the de-termination of available phosphorus appear to extract a portion of all chemicalforms having either higher solubility or high specific surface. In contrast, thesystem presented herein for fractionation of inorganic soil phosphorus into thetotal amount of each discrete chemical form permits determination of the chemi-cal status of native soil phosphorus and of the fate of applied phosphate fertilizerwith or without the effect of cropping. The development of a system of fraction-ation of soil phosphorus should thus be helpful in the fields of soil chemistry,

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134 CHANG AND JACKSONMATERIALS

For control tests of the new system of soil inorganic phosphate fractionation,synthetic iron phosphate, synthetic aluminum phosphate, and apatite of knowncomposition were employed. The methods were then tested with soil samples.The various materiaIs used in these experiments are briefly described as follows:

I ron phosphate. Precipitated by digestion of 10 m!. of M FeCh with 30 m!. ofM NaH2P04 in a total volume of 600 m!. of solution on a steam plate for 2days, having a suggested formula as FeP04.2H20, and containing 17.2 percent P. .,

Aluminum phosphate. Precipitated by digestion of 30 ml. of M AlCh with90 m!. of M NaH2P04 in a total volume of 600 ml. of solution on a steam plateovernight, having a suggested formula as AlP04.2H20, and containing 20.6 percent P.

Apatite from Florida. Ground to pass a 100-mesh sieve, and containing 17.2per cent P.Catalina. A2 horizon, 12-36 inches, a latosol (probablya low humic latosol)from Puerto Rico, with 18.2 per cent extractable Fe20a, 0.045 per cent total P,and a pH of 6.2.Wahiawa clay loam. Lower A horizon, 10-18 inches, a dark reddish-brown lowhumic latosol from Hawaii, with 10.1 per cent extractable Fe20a, and a pHof 6.7.

Miami silt loam. Ap horizon, -6 inches, a gray brown podzolic soil fromKirwin farm near Madison, Wisconsin, with 0.83 per cent extractable Fe20a,

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FRACTIONATION OF SOIL PHOSPHORUS 135P. Fifty mI. of this solution is diluted to 500 mI. to make a solution of 5 ppm. Pin concentration.0.8 M boric acid. 50 g. HaB03 dissolved in water with heating and diluted tolliter. .

Chlororrwlybdic acid solution. 15.0 g. c.p. (NH4)&Mo724.4H20dissolved inabout 300 mI. of distilled water and warmed to about 50C. (solution filtered ifcloudy). Solution is cooled and 350 mI. of 10N HCI added slowly with stirring.Solution is again cooled, diluted with water to lliter, and stored in a brown bottle.Chlorostannous reductant A. 10 g. SnCI2.2H20, dissolved in 25 mI. of concen-trated-'.HCI. This stock solution is stored in a brown bottle. 3 mI. of this stocksolution are freshly diluted to 1 liter not longer than 4 hours before use.Chlorostannous reductant B. 25 g. SnCI2.2H20, dissolved in 100 mI. concen-trated HCI, diluted to lliter, and stored in a brown bottle with a siphon undera lO-mm. layer of white mineral oil.Sulfomolybdic acid solution. 25 g. (NH4)6Mo024.4H20 dissolved in 200 mI. of

water heated to 60C. and filtered. A 275-mI. volume of arsenic-free andphosphorus-free concentrated H2S04 is diluted to 800 mI. After both solutionshave cooled, the ammonium molybdate solution is added to the sulfuric acid so-lution slowly with shaking, and the solution is made up to Iliter when cooled.30 per cent phosphorus-free H202, reagent grade. Superoxol shaken with kao-linite at the ratio of 1 g. to 10 mI. and the suspension filtered and centrifuged.Three additional treatments in this way decreases the phosphorus concentra-tion to approximately 0.1 ppm. (7).

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136 CHANG AND JACKSONadded to make up the volume and the solution is well mixed. The color is measured on anphotoelectrocolorimeter at 660 ml' within 5-20 minutes. The extract may have a light colordue to organic matter, but it does not interfere with reading at 660 IDi'. If the color is strong,an aliquot may be prepared without the reductant and used as a blank.

Extraction and determination of phosphorus in iron phosphateThe soil sample saved after the extraction of aluminum phosphate is washed twice with25-ml. portions of saturated NaCI solution. It is then extracted with 50 ml. of 0.1 N NaOHon a shaking machine for 17hours. The soil suspension is centrifuged for 15minutes at 2400rpm. and recentrifuged, if necessary, to obtain a clear solution which is decanted into an-

other centrifuge tube. The soil sa'mple is saved for extraction of calcium phosphate. Forsurface soils the decanted extract is usually highly colored with considerable organic matter,in which case 2 ml. of 2 N H2SO. are added to the solution and then one or a few drops ofconcentrated H2SO. until the organic colloids begin to fiocculate. The suspension is thencentrifuged and the clear solution collected.To determine the phosphorus in the clear solution, an aliquot (usually 2 ml.) is placedin a 50-ml. volumetric fiask. Water is added to give about 20ml. The solution is adjusted toabout pH 3 by addition of 2N NaOH until 2, 6 dinitrophenol indicator color turns to yellowand then is brought back to colorless by addition of 2 N H2S04. Then 2.0 ml. ofsulfomolybdic acid solution is added and water is added to make 48 ml. and the solutionmixed. Three drops of chlorostannous reductant B are added to develop the coloroWateris added to volume and the solution mixed. The color is measured within 5-10 minutes at660 ml'.

,

liI'

I,,,IExtraction and determination of phosphorus in calcium phosphate

The soil sample saved after extraction of iron phosphate is washed twice with 25-ml.portions of saturated NaCI solution. It is then extracted with 50 ml. of 0.5 N H2SO. for 1hour on a shaking machine. The suspension is centrifuged to get the solution and the sampleis saved for extraction of reductant soluble iron phosphate.

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FRACTIONATION OF SOIL PHOSPHORUS 137

.,

and the solution gently heated over a burner; vigorous splashing must not occur. Theburner is moved under or away from the flask as needed. One drop of 0.5M FeCIs (or 10ml.of 100ppm. Fe solution) is added to moderate the oxidation. The cessation of foaming (gasevolution) and beginning of ordinary boiling indicate the completion of oxidation. Com-plete drying must be avoided before the oxidation is complete, otherwise the very concen-trated HsOs and high temperature wiIl ignite the organic matter, leaving some carbonparticles. SmaIl amounts of distilled water are added as necessary. After completion ofoxidation, the solution is boiled for an additional1 or 2 minutes and then dried on a steamplate. About 10ml. of 2 N NaOH is added. The solution is boiled for 1-2 minutes and di-gested on a steam plate for 5 minutes. The suspension is poured into a 15-ml. centrifugetube -nd centrifuged to throw down the iron oxide precipitate; the supernatant liquid isthen decanted into a 5O-ml.volumetric flask. The original 1SO-ml.flask is washed with 10ml. of water into the same 15-ml. centrifuge tube and centrifuged. The supernatant solutionis poured into the same SO-ml.volumetric flask. The flask washing is repeated oncemore andthe supernatant solution is placed in the same volumetric flask. The combined supernatantsolutions are made to volume and the phosphorus determined by the same method as foriron phosphate.

Extraction and determination of phosphorus in occluded aluminum phosphateFor soils high in iron oxide, the residue is extraced with neutral NaF to remove oc-cluded aluminum phosphate. Alternatively, the residue is extracted with 0.1 N NaOH toremove occluded aluminum-iron phosphate (barrandite-like). The phosphorus in the solu-tion is determined in the same way as aluminum phosphate or iron phosphate, respectively.In a complete system of fractionation of soil phosphorus, total phosphorus and organicphosphorus are determined on two separate samples.

SOLUBILITY OF PURE PHOSPHATES

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-q,II

138 CHANG AND JACKSONTABLE 1

Phosphorus fractionation into discrete chemical forms

II

,'II

Ili,

'.., * Figures in this column represent references at end of this paper.

MethodPhosphorusFraction Extractant Formso PhosphateExtractable AdaptedFrom.1. Al-phosphate Neutral 0.5 N NH.F Al-phosphate completely (3)

Fe-phosphate slightly2. Fe-phosphate 0.1 li. NaOH Al-phosphate (13)

Fe-phosphateOrganic phosphorus.3. Ca-phosphate 0.5 N H2SO. Ca-phosphate completely (6)Al- and Fe-phosphate con-siderably4. Reductant soluble Fe- Na2S20.-citrate Fe-phosphate completely (1)phosphate (iron ox- Al-phosphate negligibly

ide occluded)5. Occluded Al-phos- Neutral 0.5 N NH.F Al-phosphate completely -phate6. Occluded Al- Fe- 0.1 N NaOH (Alternative or addition to -phosphate 5)Al- and Fe-phosphate com-pletely

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FRACTIONATION OF SOIL PHOSPHORUS 139TABLE 2

Solubility of phosphatesTotal P

Amount of Phosphates Taken Taken Dissolvedmg./50 ml. mg. %

Extractant = neutral 0.5 N NH.FAl-phosphate: _' 4.1 57.20. . . . ... .. . .. . . . . . .. . . . . . . . .. . . . . . . . . . 2.1 88.40. . . . . .. . . . . .. . . . . . . .. . . . . . . . .. . . .. . . . 1.0 100.5.................................... .Fe-phosphate: 3.4 2.20.................................... . 1.7 2.80.. . . . . . . . .. . . . . . . . . . . . . . . . . . .. . . . . . . . 0.9 3.1..................................... 0.4 4.12.5. .. . . . . . . . . . . . . . . . . . . . . ... . . . . . . . . . .. 0.1 7.5.5*................................. .Apatite: 3.4 0.90................................... .. 1.7 1.00.................................... . 0.9 0.95.................................... .

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140 CHANG AND JACKSONTABLE 3

Amount of soil phosphorus extracted by successive extractions with each extractant

with NaOH in both Catalina latosol and Miami silt loam. Phosphorus in syn-thetic iron phosphate (equivalent to 3400 ppm. in 1 g. soi!) can be completelydissolved in one extraction of NaOH (table 2), yet successive NaOH extractionsof soils which contain much less P continuously dissolve a portion of iron phos-phate in the second and third extractions (table 3). Where does the P in thesecond and third extractions come from? The Catalina latosol and the Miamisilt loam contain about 320 ppm. and 150ppm. P, respectively, in reductant soluble

Cata1ina Latosol A. Horizon Mia.miSilt Loamc. Horizon.36-42in.Extractants ExtractionsSuccessive One Successive Oneextractions extraction extractions extraction

ppm. ppm. ppm. ppm.Neutral 0.5 N NH.F 1st 2.5 O 72 772nd O 12

:Jd O 100.1 N NaOH 1st 37 42 94 1282nd 11 10

3rd 6.2 100.5 N H2SO. 1st 7.8 6.3 113 -2nd 6.5 3.5

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FRACTIONATION OF SOIL PHOSPHORUS 141TABLE 4

Reductant 80luble phosphorus, and NaOH or NH.F soluble phosphorus afterreductant extraction

aluminwn phosphate dissolved. The soil phosphorus dissolved by this treatmentmay properly be termed, therefore, reductant soluble iron phosphate. An ironoxide precipitate apparently is formed on the surface of iron phosphate and onthe surface of aluminwn-iron phosphate (barrandite-like) in the course of chem-ical weathering in soils by hydrolysis of iron phosphate and other iron salts. Therelative insolubility of iron oxide in the NH4F, NaOH, or H2S04 extraction must

DISCRETEPHOSPHATESDISCRETEPARTICLES IPRECIPITATEDONUR- lcCLUDEDPHOSPHATES(somewhal a.aUable) IFACES,lNClUDINGHEMI- (1i"lo a.ailablolSORBEi> P04 (AI(masl a.ailable ICAlCIUf,1 PHOSPHA TE I CALCIUM PHOSPHATE I CALCIUM PHOSPHATE(apol lto diealeium I PRECIPITATEDON 10CCLUDEDNCALC.IUM

Wabiawa Catalina MiamiSilt LoamTreatmentsofReductionChelation Latosol Latosol Ap:0-6in. C"36-42in.ppm. ppm. ppm. ppm.

One treament....... . . . . . . .. . .. .. 504 320 150 158Total of two treatments. . . . . . . . . - 325 138 150NaOH after"one treatment... . . .. 119 48 8 9NH.F after one treatment. . . . . . . 97 40 6 -

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---142 CHANG AND JACKSONaccount for the fact that the phosphate thus covered can only be dissolved afterthe removal of the iron oxide coating.The Wahiawa latosol, the Catalina latosol, and a Miami silt loam after re-moval of aluminum, iron, and calcium phosphate by successive extraction withNH4F, NaOH, and H2S04 were treated once with sodium dithionite-citrate andyielded 504, 320, and 150 ppm. of reductant soluble phosphorus, respectively,(table 4). Since two such treatments with dithionite applied to a second sample ofthe latter two soils did.Jlot extract materially more phosphorus, one treatmentwould appear to be sufIicient to complete extraction of the reductant solubleiron phosphate.After one dithionite-citrate treatment of the soils, further extraction witheither NaOH or NH4F dissolved appreciable amounts of phosphorus. Since onlyaluminum phosphate is soluble in either NaOH or neutral NH4F, most of theresidual phosphate must be aluminum phosphate. The somewhat higher amountextracted by NaOH than by NH4F (table 4) indicates that there is also someiron phosphate left after the reduction-chelation, most likely in aluminum-ironphosphate (barrandite-like) since any pure iron phosphate would have dissolvedin the dithionite-citrate extraction. The physical distribution of the variousdiscrete chemical forms of soil inorganic phosphates is summarized in figure 1.

uble fonabundarcalciumphosphaThe ratsoil to ::1tion of ]as a fUIThelsiltloaIJence bewithin

TABLE 5Fractionation of soil phosphorus in replicated samples

A prcrete c:reduct:iron p]tractaIammo!pletelyextrac'hydro:but aIphospJtheref.these

FRACTIONATION OF PHOSPHORUSThe phosphorus of soils belonging to the latosol, gray-brown podzolic, andchernozem groups were fractionated (table 5). The results of the replicated

samples are in good agrement. Iron phosphate, particularly the reductant sol-

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FRACTIONATION OF SOIL PHOSPHORUS 143uble form, dominates higWy-weathered soils, but these forms are also the mostabundant in the gray-brown podzolic (Miami) soil. Aluminum phosphate andcalcium phosphate occur in significant amounts in the latter soils, while calciumphosphate is dominant in the calcareous subsoil of the Chernozem (Barnes) soil.The ratio of aluminum phosphate to iron phosphate varies from 0.2 in Miamisoil to 2 in the little-weathered Barnes subsoil, and in this connection fractiona-tion of phosphorus in several soil profiles (4) was found to vary even more widelyas a function of the degree of chemical weathering.The fusion analysis of the residual samples of both Catalina latosol and Miamisilt loam after ali extractions yielded 15 and 4 ppm. of P respectively. The differ-ence between the added total and determined total amount of P in the soils iswithin the cumulative experimental error.

SUMMARYA procedure was developed for fractionation of soil phosphorus into the dis-crete chemical forms, calcium phosphate, aluminum phosphate, iron phosphate,reductant soluble (iron oxide coated) iron phosphate, and occluded aluminum-iron phosphate, based on the selective solubility of phosphates in various ex-tractants. Strengite, variscite, and apatite were used for control study. Neutralammonium fluoride in a single extraction dissolves aluminum phosphate com-pletely, iron phosphate slightly, and apatite negligibly, when the phosphate-

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144 CHANG AND JACKSON

(5) CHANG,S. C., ANDJACKSON,M. L. 1956 Removalof phosphorus from hydrogenperoxide by kaolinte. Seienee 124:1209.(6) DEAN,L. A. 1938 An attempted fractionation of soil phosphorus. J. Agri. Sei. 28:234-246.(7) FISHER, R., ANDTHOMAS,R. P. 1935 The determination of forros of inorganicphosphorus in soils. J. Amer. Soe. Agron. 27: 863-873.(8) FRAPS,G. S. 1906 Availability of phosphoric acid of the soil. J. Amer. Soe. Agron.28: 823-834.(9) GHANI,M. O. 1943 Fractionation of soil phosphorus: L lndia J. Agri. Sei. 13:29-45.(10) KI'I'TRICK,. A., ANDJACKSON,M. L. 1956 Electron microscope observations of thereaction of phosphate-"with mineraIs, Ieading to a unified theory of phosphatefixation in soils. J. Soil Sei. 7: 81-89.(11) OLSEN,S. R., ANDWATONABE,. S. 1957 A method to determine a phosphorusadsorption maximum of soils as measured by the Langmuir isotherm. Soil. Sei.Soe. Amer. Proe. 21: 144-149.(12) TURNER,R. C., ANDRlcE, H. M. 1954 Role of fiuoride ion in release of phosphate

adsorbed by AI and Fe hydroxide. Soil Sei. 74: 141-148.(13) WILLIAMS,C. H. 1950 Studies on soil phosphorus: L J. Agri. Sei. 40: 233-242.(14) WILLIAMS,R. 1937 The soIubility of soil phosphorus and other phosphorus com-'t>ounds in sodium hydroxide soIution. J. Agri. Sei. 27: 260-270.