Maataloustieteellinen A ikakauskirja Vol. 59: 141—145, 1987 RESEARCH NOTE Release of phosphorus, aluminium and iron in fractionation of inorganic soil phosphorus RAINA NISKANEN University of Helsinki, Department of Agricultural Chemistry, SF-00710 HELSINKI, Finland Abstract. Release of phosphorus, aluminium and iron by a modified Chang and Jackson procedure was studied in five mineral soils. Quantities of aluminium and iron released during the procedure and extracted by acid ammonium oxalate were compared. The extractability of P, Al and Fe by 1 M NH 4 CI and that of A 1 and Fe by alkaline 0.5 M NFt 4F was poor. Pro- portions of P extracted by 0.5 M NH 4F (0.2—10.4 mmol/kg soil) and 0.1 M NaOH (0.1— 9.8 mmol/kg soil) were related to the molar ratio of oxalate-extractable iron and aluminium. P extracted by 0.25 M H2 S04 amounted to 2.1 —12.2 mmol/kg soil. Al extracted by 0.1 M NaOH (7 —174 mmol/kg soil) and 0.25 M H 2 S04 (17 —112 mmol/kg soil) amounted to 55—94 % and 16—245 °7o of oxalate-extractable Al, respectively. Fe released by 0.1 M NaOH (I —lO mmol/kg soil) and 0.25 M H2 S04 (30 —196 mmol/kg soil) amounted to I—l 3 % and 62—272 % of oxalate-extractable Fe, respectively. In total, 91—309 % of oxalate-extractable Al and 70—285 % of oxalate-extractable Fe were released by NaOH and H 2 S04 . Index words: phosphorus fractions, extractable aluminium and iron Introduction The fractionation procedure developed by Chang and Jackson (1957) is frequently used in estimation of inorganic soil phosphorus (e.g. Kaila 1964, Hartikainen 1979). Phos- phorus fractions bound by aluminium and iron oxides and phosphorus of calcium phos- phates such as apatite are considered to be extracted successively in the procedure. The reagents used are known partly to extract soil aluminium and iron, but the amounts released besides phosphorus are infrequently deter- mined. The aim of this study was to examine the simultaneous release of phosphorus, alu- minium and iron in fractionation and to com- pare the extractability of aluminium and iron with their extractability by acid ammonium oxalate. Material and methods The material consisted of five mineral soil samples from the Viikki Experimental Farm, 141 JOURNAL OF AGRICULTURAL SCIENCE IN FINLAND https://www.c-info.fi/en/info/?token=nryvzKgAfTE2CuMY.e1n082W7y-tFc7LbrVbOzw.sDtNQkFeyK2h5ZCdOEvs2q6yAtsE3q_0XZyHFPj_iVWndWFJ0X1G63jJXmsLcEOrSPWoNY1bNZLhe0Ttz7E3JMPejNrFTNh4VdKXZLDupyo8MVuvvmiYQbcDZxZ1wEcQDwOkqDCiMTZzAnRu6h5j4qOnQJ3aXYSqIUQOZw Table I. Characteristics of experimental soils. Soil sample 1 2 3 4 5 Sampling depth, cm o—2o 20—40 20—40 o—2o0—20 20—40 pH(CaCl 2 ) 5.1 4.6 4.8 5.3 5.0 Org. C, % 3.6 0.8 2.6 4.4 1.0 Particle-size distribution, % <0.002 mm 13 2 47 10 26 0.002—0.02 mm 20 I 30 7 2 0.02—0.06 mm 27 7 18 15 23 0.06—0.20 mm 31 35 5 61 42 0.20—2.00 mm 9 56 0 7 6 Oxalate-soluble Al mmol/kg 186 104 76 23 11 Oxalate-soluble Fe mmol/kg 53 32 224 140 11 Oxalate-soluble Fe/Al 0.3 0.3 2.9 6.1 1.0 University of Helsinki, (Nos. 2—4) and South Karelia (Imatra) (No. 1): Nos. 1 and 4 from plough layer (0—20 cm) and Nos. 3 and 5 from deeper layer (20 —40 cm) of cultivated soils, No. 2 from deeper layer of virgin soil (Table 1). The samples were air-dried and ground to pass a 2-mm sieve. Soil pH was measured in soil-0.01 M CaCl 2 suspension (1:2.5 v/v) (Ryti 1965). The organic carbon content was determined by a modified (Gra- ham 1948) Alten wet combustion method. The particle-size distribution of the inorganic matter of soil was determined by the pipette method (Elonen 1971). The amorphous alu- minium and iron were extracted by acid am- monium oxalate (0.18 M ammonium oxalate, 0.10 M oxalic acid, pH 3.3, 1:20 w/v) (Tamm 1922) and determined by atomic absorption spectrophotometry. The soils were extracted by a modified Chang and Jackson (1957) fractionation pro- cedure using a slightly alkaline NH 4F (pH 8.5) as recommended by Fife (1959). The extracts were analysed for phosphorus by a molybdenum blue method modified by Kai- la (1955) and for aluminium and iron by atomic absorption spectrophotometry. Frac- tionation was carried out in triplicate. Results and discussion In the fractionation procedure, the extract- ability of phosphorus, aluminium and iron by 1 M NH 4CI was poor (Table 2). Accord- ing to Kaila (1964, 1965) and Hartikainen (1979), the extractability of phosphorus by 1 M NH4CI is generally low in Finnish mineral soils. As an anion of a strong acid, CF can- not participate in ligand exchange reactions with phosphate complexed by aluminium and iron oxides. The content of phosphate ex- tractable by NH 4CI is worth mentioning only when the sorption capacity of soil is covered with phosphate. In the experimental soils, however, the ratio of fractionated phospho- rus to oxalate-soluble aluminium and iron was low. According to Kaila (1964), the occurrence of phosphorus in the forms soluble in NH 4F and NaOH is to some extent regulated by the molar ratio of active aluminium and iron con- tents in Finnish soils. More phosphorus was extracted by 0.1 M NaOH than by 0.5 M NH 4F from soils No. 3 and 4 which con- tained more oxalate-extractable iron (mmol/ kg soil) than aluminium (Table 2). In the other soils, the NH4F-soluble fraction was greater than the NaOH-soluble one. No aluminium was found in NH 4F extracts and iron was poorly soluble (Table 2). The fluoride-soluble iron in soils No. 1 and 2 amounted to 6—7 %, in the other soils to less than 2 % of the oxalate-soluble iron. Fluoride does not measurably complex ferric iron at a pH above 8.0 (Fife 1959). 142 Table 2. Soil P, Al and Fe (mmol/kg soil) extracted successively by the Chang and Jackson procedure. Soil sample 1 2 3 4 5 P extracted by 1 M NH 4CI 0.0 0.0 0.0 0.0 0.0 P extracted by 0.5 M NH4F 10.4 2.3 0.8 0.7 0.2 P extracted by 0.1 M NaOH 3.9 0.9 9.8 6.0 0.1 P extracted by 0.25 M H 2S0 4 2.1 4.8 5.1 6.6 12.2 NaOH-P/NH4F-P 0.4 0.4 12.3 8.6 0.5 Al extracted by I M NH 4 CI 0 0 0 0 0 A 1 extracted by 0.5 M NF1 4F 0 0 0 0 0 Al extracted by 0.1 M NaOFl 174 78 42 21 7 Al extracted by 0.25 M H 2 S04 85 17 112 22 27 Fe extracted by 1 M NF14 CI 0 0 0 0 2 Fe extracted by 0.5 M NFI 4F 3 2 0 3 0 Fe extracted by 0.1 M NaOH 2 2 3 10 I Fe extracted by 0.25 M H 2S0 4 69 31 196 87 30 NaOH is frequently used in extraction of humic matter and oxides ofaluminiumand sil- ica (Jackson 1965). In the experimental soils, 0.1 M NaOH-soluble aluminium amounted to over 50 % (55 —94 °7o) of oxalate-extractable aluminium, NaOH-extractable iron only to 1 —l3 °7o of oxalate-extractable iron. Accord- ing to Aleksandrova (1960), the solubility of iron in 0.1 M NaOH is low. Because humic matter is extracted by NaOH and alkaline NH 4F, it is possible that the iron released by these reagents is derived from humic com- plexes. The proportion of H 2S0 4-soluble phos- phorus was high in soils No. 2, 4 and 5 which were predominantly coarse (Table 2). About half of the fractionated phosphorus in soils No. 2 and 4 was extracted by H 2S0 4 . In soil No. 5 of low oxalate-soluble aluminium and iron content, phosphorus was mainly H2S0 4 - soluble. In soils No. 1 and 3 of high alumin- ium and iron content, H2SG 4 extracted 13 and 33 % of the fractionated phosphorus, respectively. Amply of aluminium and iron was extracted by 0.25 M H 2S0 4 (Table 2) which is an effec- tive extractant of iron oxides (Hsu 1964). H 2S0 4-soluble aluminium in soils No. 1 and 2 amounted to 46 and 16 %, respectively, and in soils No. 3—5 to 96—245 % of oxalate- extractable aluminium. In the fractionation procedure, iron was mainly released by H2S0 4, 62 % in soil No. 4 and 97—272 °7o of oxalate-extractable iron in the other soils. In the course of the fractionation proce- dure, large amounts of aluminium and iron were released besides phosphorus by 0.1 M NaOH and 0.25 M H 2S0 4, release of the lat- ter being particularly drastic. In total, these two reagents extracted aluminium and iron in amounts corresponding to 91—309 °7o and 70 —285 % of oxalate-extractable aluminium and iron, respectively. Chang and Jackson (1957) developed their procedure for fractionation of soil phospho- rus into discrete chemical forms using alumin- ium, iron and calcium phosphate minerals variscite, strengite and apatite as controls. However, variscite and strengite are not likely to occur in normal agricultural soil. The solubility product of variscite controls the phosphorus concentration in solution only when the pH of the equilibrium solution is be- low 3.1 (Bache 1963). At higher pH values, variscite dissolves incongruently, whereby a more basic solid phase of aluminium hydroxy- phosphate is formed (Taylor and Gurney 1962 a, b, 1964). Strengite is never likely to be in equilibrium with any soil solution (Bache 1963). According to the modern concept, adsorbed phosphate is more important in soil than dis- 143 Crete phosphate compounds. In acid soil, phosphate is largely adsorbed through ligand exchange onto surfaces of aluminium and iron oxides. From this viewpoint, the soil phos- phorus available is best extracted by solutions which release phosphate through ligand exchange without dissolution ofaluminium or iron from the oxide surface. In the fractiona- tion procedure the alkaline ammonium fluo- ride was the extractant best fulfilling these pre- sumptions. The oxides of aluminium were largely dissolved by NaOH and H 2S0 4, the latter dissolving effectively also iron oxides. According to Khanna and Ulrich (1967), in acid soils, the H2S0 4-soluble phosphates can- not be designated solely as calcium phos- phates. They may also include acid-soluble portions of occluded phosphates. Although the selectivity of the extractants for different forms of phosphate is limited (Bromfield 1967 a, b, Vahtras and Wiklan- der 1970) and it varies in the original phos- phate fraction during extraction (Bromfield 1970, Rajendranand Sutton 1970), the frac- tionation scheme of Chang and Jackson does, however, provide information on the general trends of phosphate transformation reactions. References Aleksandrova, L. N. 1960. The use of sodium pyro- phosphate for isolating free humic substances and their organic-mineral compounds from the soil. Soviet Soil Sci. 2: 190—197. Bache, B. W. 1963. Aluminium and iron phosphate studies relating to soils. I. Solution and hydrolysis of variscite and strengite. J. Soil Sci. 14: 113—123. Bromfield, S. M. 1967 a. Phosphate sorbing sites in acid soils. An examination of ammonium fluoride as a selective extractant for aluminum-bound phosphate in phosphated soils. Aust. J. Soil Res. 5: 93—102. 1967 b. An examination of the use of ammonium fluo- ride as a selective extractant for aluminum-bound phosphate in partially phosphated systems. Aust. J. Soil Res. 5: 225—234. 1970. The inadequacy of corrections for resorption of phosphate during the extraction of aluminum-bound soil phosphate. Soil Sci. 109; 388—390. Chang, S. C. & Jackson, M. L. 1957. Fractionation of soil phosphorus. Soil Sci. 84: 133—144. Elonen, P, 1971. Particle-size analysis of soil. Acta Agr. Fenn. 122: 1 122. Fife, C. V. 1959. An evaluation of ammonium fluoride as a selective extractant for aluminium-bound soil phosphate: II Preliminary studies on soils. Soil Sci. 87; 83—88. Graham, E. R. 1948. Determination of soil organic mat- ter by means of a photoelectric colorimeter. Soil Sci. 65: 181 183. Hartikainen, H. 1979. Phosphorus and its reactions in terrestrial soils and lake sediments. J. Scient. Agric. Soc. Finl. 51: 537—624. Hsu, P. H. 1964. Adsorption of phosphate by aluminum and iron in soils. Soil Sci. Soc. Am, Proc. 28: 474—478, Jackson, M, L. 1965. Free oxides, hydroxides, and amorphous alumino-silicates. Methods of soil analy- sis. Part I. Agronomy 9: 578—603. Kaila, A. 1955. Studies on the colorimetric determina- tion of phosphorus in soil extracts. Acta Agr. Fenn. 83: 25—47. 1964. Fractions of inorganic phosphorus in Finnish mineral soils. J. Scient. Agric. Soc. Finl. 36: I —l3. 1965. Some phosphorus test values and fractions of inorganic phosphorus in soils. J. Scient. Agric. Soc. Finl. 37: 175—185. Khanna, P, K. & Ulrich, B. 1967. Phosphatfraktio- nierung im Boden und isotopisch austauschbares Phos- phat verschiedener Phosphatfraktionen. Z. Pflanzener- nähr., Diing., Bodenk. 117: 53—65. Rajenoran, N. & Sutton, C. D. 1970. Re-sorption of soil phosphate during fractionation. J. Soil Sci. 21: 199—202. Ryti, R. 1965. On the determination of soil pH. J. Scient. Agric. Soc. Finl. 37; 51—60. Tamm, O. 1922. Eine Methode zur Bestimmung der anorganischen Komponente des Gelkomplexes im Boden. Statens Skogsförsöksanstalt. Medd. 19: 387—404. Stockholm. Taylor, A. W, & Gurney, E. L. 1962 a. Solubility of amorphous aluminum phosphate. Soil Sci. 93: 241—245. & Gurney, E. L. 1962 b. Phosphate equilibria in acid soil. J. Soil Sci. 13: 187—197. & Gurney, E. L. 1964. Solubility of variscite. Soil Sci. 98: 9—13. Vahtras, K. & Wiklander, L. 1970. Phosphate studies in soils: With special reference toChang and Jackson’s fractionation procedure. Lantbrukshögsk. Ann. 36: 115—134. Ms received March 2, 1987 144 SELOSTUS Fosforin, aluminiumin ja raudan uultuminen fraktioitaessa maan epäorgaanista fosforia Raina Niskanen Helsingin yliopisto, Maanviljelyskeinian laitos, 00710 Helsinki Fosforin, aluminiumin ja raudan uultumista mukail- lussa Changin ja Jacksonin fraktioinnissa tutkittiin vii- dellä kivennäismaalla. Fraktioinnissa uuttuneiden alumi- niumin ja raudan pitoisuuksia verrattiin (tappamalla am- moniumoksalaalilla uuttuvan aluminiumin ja raudan pi- toisuuksiin. Fosforin, aluminiumin ja raudan uuttumi- nen 1 M NH 4Cl:lla sekä aluminiumin ja raudan uuttu- minen emäksisellä 0.5 M NH 4F:lla oli vähäistä. Fluori- diuuttoisen fosforin((0.10.42—10.4 mmol/kg maata) ja 0.1 M NaOHdla uuttuvan fosforin (0,1 —9.8 mmol/kg maata) osuudet fraktioinnissa uupuvasta fosforista riippuivat oksalaattiuuttoisen raudan ja aluminiumin moolisuhtees- ta. H 2S04 :lla uuttui fosforia 2.1 —12.2 mmol/kg maata. Aluminiumin uuttuminen 0.1 M NaOH:lla (7 —174 mmol/kg maata) ja 0.25 M H2 S0 4:lla (17 —112 mmol/ kg maata) vastasi 55—94 % ja 16—245 % oksalaattiuut- toisesta aluminiumista. NaOH-uuttoinen rauta (1 —lO mmol/kg maata) ja H 2 S04-uuttoinen rauta (30 —196 mmol/kg maata) vastasivat 1 —l3 % ja 62—272 % oksalaattiuuttoisesta raudasta. NaOHdla ja H 2S0 4:lla uuttui yhteensä 91 —309 % oksalaattiuuttoisesta alumi- niumista ja 70—285 % oksalaattiuuttoisesta raudasta. 145