Maataloustieteellinen A ikakauskirja Vol. 62: 9—15, 1990 Sorption capacity of phosphate in mineral soils II Dependence of sorption capacity on soil properties RAINA NISKANEN University of Helsinki, Department of Agricultural Chemistry, SF-00710 Helsinki, Finland Abstract. The dependenceof the indicator of phosphate sorption capacity on extractable Al and Fe and other soil properties was studied in a material consisting of 102 mineral soil samples. The sum of P adsorbed on soil during two days from a solution containing P 5 mmol/1 and P extracted by 0.02 M EDTA (pH 5.3) as an estimate of the initial P content in the soil was used as the indicator of P sorption capacity. In clay and silt soils (n =51), the Al and Fe extracted by 0.05 M oxalate (pH 2.9) together with the organic C content explained 85 Vo, the Al and Fe extracted by 0.05 M K 4 P2 07 (pH 10) together with the clay content 87 Vo, the Al and Fe extracted by 0.02 M EDTA (pH 5.3) 91 Vo, and the Al extracted by 1 M CH 3COONH4 (pH 4.8) together with the organic C and clay con- tents 78 Vo of the variation of the indicator of phosphate sorption capacity. In coarse soils (n =51), the variation of the indicator was explained well only by oxalate-extractable metals, which together with soil pH and clay content explained 80 Vo of the variation. Extractable Al was generally the most important explainer of variation. The results suggest that forms of ex- tractable Al and Fe explaining the variation of the indicator of P sorption capacity in clay and silt soils are partially different from those in coarse soils. Index words: acetate-extractable AI, EDTA-extractable Al, Fe and P, oxalate-extractable A 1 and Fe, pyrophosphate- extractable A 1 and Fe, soil pH, clay content, organic carbon content Introduction In the previous paper (Niskanen 1990), the phosphate sorption capacity of some mineral soils was estimated by means of sorption isotherms. When sorption properties of large soil material are to be studied, it is, however, laborious to determine complete sorption curves. Bache and Williams (1971) proposed the use of a single value sorption index to characterize the phosphate sorption proper- ties of soil. Sorption determined from only one concentration may, however, be unsatis- factory if the initial phosphate content in soil is high (Barrow 1978). In such cases it may be valid to correct the sorption results by ad- 9 JOURNAL OF AGRICULTURAL SCIENCE IN FINLAND https://www.c-info.fi/en/info/?token=0MuvAoVwtVoLCD80.5Fxtk-0VBBQX6Np1wOEFkg.bNzP9iYKpd7GVChtLHWlRcg3x8FGgFU3_Ki9gHqq5s_my6vcNEjUtf09iU_ndhAHOSGCxUP4jY-wYw--VzF2kGpIkWC89GErPC4K3bGFXe8jCnABIo008LoCaMLgQIpb0-lqJwZbUUYkE7aEaT28viVNKyxlgbjzk4vG5g ding an estimate of the initial P content. The aim of this paper was to study the dependence of the corrected indicator of P sorption ca- pacity, determined by the one-point method, on extractable Al and Fe and other soil properties. Material and methods The material consisted of 51 clay or silt soils and 51 coarse soil samples, which have been presented more thoroughly in a previous paper (Niskanen 1989) (Table 1). The sam- ples were air-dried at room temperature and ground to pass through a 2-mm sieve. The pH of the soil was measured in a soil-0.01 M CaCl2 suspension (1:2.5) (Ryti 1965). The particle-size distribution of inorganic matter was determined by the pipette method (Elo- nen 1971), the organic carbon content by the Alten wet combustion method (Graham 1948). Soil aluminium and iron were extracted by the following methods (Niskanen 1989): 0.05 M oxalate (pH 2.9, ratio 1:20 w/v, shak- ing time 2 h), 0.05 M K 4P207 (pH 10, 1:100 w/v, 3 h) and 0.02 M Na 2-EDTA (pH 5.3, 1:50 w/v, 3 h). Aluminium was additionally extracted by 1 M ammonium acetate (pH 4.8, 1:10 w/v, 2 h) (McLean et al. 1958). The metals contained in filtrated extracts were de- termined by atomic absorption spectropho- tometry. To determine the phosphate sorption, 5 g of soil was treated at + 20°C for two days with 100 ml of solution containing KH 2PG 4 5 mmol/1. The ionic strength of the solution was 0.01, adjusted with KCI 5 mmol/1. To in- hibit microbial activity, the solution contained 0.01 % NaNj. The suspensions were daily shaken for eight hours. At the beginning and the end of the experiment, the pH of the sus- pensions was measured. The phosphorus con- centration in filtrates was determined by a modified molybdenum blue method (Kaila 1955). The amount of retained phosphate was calculated as the difference between the phos- phate quantity present initially and that re- maining in the supernatant. The experiment was carried out in triplicate. Phosphorus extracted by 0.02 M Na2-EDTA and determined by the molybdenum blue method was used as the estimate of the initial phosphorus content in soil. The sum of re- tained and initial phosphate content was used as the indicator of the P sorption capacity of the soil. Results The phosphate concentration (5 mmol/1) used to determine the sorption of phosphate Table I. Soil characteristics. Clay and silt soils Coarse soils (n =51) (n=51) x s range x s range pH (CaCy 5.4 Organic C, Vo 3.7 Clay (<2 pm), Vo 40 Silt (2—20 pm), Vo 33 Coarser fractions (>2O pm), Vo 28 Oxalate-extractable A 1 mmol/kg soil 71 » Fe » » 102» Pyrophosphate-extractableA 1 mmol/kg soil 38 » Fe » » 29» EDTA-extractable A 1 mmol/kg soil 19 » » Fe » » 17 Acetate-extractable AI mmol/kg soil 8,4 0.8 3.9—7.2 2.8 0.8—14.6 14 15—72 13 6—61 14 7—61 41 29—222 42 32—202 50 4—243 22 s—lll 18 4—85 11 2—46 10.5 0.2—48.9 5.2 1.0 3.5—7.3 2.8 2.2 0.6—12.3 8 8 I—2B 12 7 1—29 81 12 51—98 81 55 11—249 53 31 3—144 43 28 4—104 19 15 4—77 16 11 2—61 6 7 1—32 8.66.6 0.9—34.8 10 Table 2. Sorption of phosphate, 0.02 M Na,-EDTA-extractable phosphate and indicator of phosphate sorption capacity (mmol/kg soil). Sorption of P 0.02 M Na2 -EDTA- Indicator of P extractable P sorption capacity n x s range x s range x s range Clay and silt soils: Surface soils 27 28.7 20.2 7.8—81.8 5.4 4.9 0.3—19.4 34.1 18.5 12.7—82.3 Subsoils 24 27.1 19.5 9.9—86.7 2.7 2.3 0.0—7.9 29.8 18.6 12.7—87.2 All 51 28.0 19.7 7.8—86.7 4.2 4.1 0.0—19.4 32.1 18.5 12.7—87.2 Coarse soils: Surface soils 26 16.6 11.3 —3.5—41.8 3.1 3.1 0.2—13.6 19.6 10.5 —0.3—42.6 Subsoils 25 16.5 13.5 —7.2—51.7 0.9 1.2 0.0—5.3 17.2 13.4 —6.4—52.1 All 51 16.3 12.6 —7.2—51.7 2.0 2.6 0.0—13.6 18.3 12.2 —6.4—52.1 All soils: Surface soils 53 22.7 17.5 —3.5—81.8 4.3 4.2 0.2—19.4 27.0 16.7 —0.3—82.3 Subsoils 49 21.6 17.6 —7.2—86.7 1.8 2.0 0.0—7.9 23.3 17.4 —6.4—87.2 on experimental soils was the same as Bache and Williams (1971) used in determination of the P sorption index. The initial pH of the soil suspensions was 4.0—6.6 and after the sorption, lasting two days, thepH was slight- ly higher, the increase being no more than 0.5 pH units. The experimental soils sorbed, on average, a little more than 20 % of the ad- ded P (100 mmol/kg soil) (Table 2). The sorp- tion was higher in clay and silt soils than in coarser soils. The material consisted of 23 samples, mainly clay and silt soils, which ad- sorbed more than 30 % of the added P. In all, 20 samples adsorbed less than 10 % of added P; these were mainly coarser soils. There were two coarser soil samples which released P rather than adsorbed it. The phosphorus extracted by EDTA was used as an estimate of the initial content of the adsorbed P in the soil (Table 2). EDTA extracted, on average, more P from surface soils than from subsoils, and more P from clay and silt soils than from coarser ones. In sur- face soils, the EDTA-extractable P seemed to increase with increasing pH (r= o.6B***, n = 53). In surface layers of clay and silt soils, the correlation coefficient between extractable P and soil pH was o.Bo*** (n = 27). In coarse surface soils, the extractability of P increased with an increasing clay content (r = o.69***, n = 26). EDTA-extractable P is thought to be con- nected with inorganic P fractions in soil which are bound to aluminium and iron (Alexan- der and Robertson 1972) and also to calcium (Ahmed and Islam 1975, Sahrawat 1977, Hartikainen 1979). Several studies (Alexan- der and Robertson 1972, Ahmed and Islam 1975, Olsen 1975, Sahrawat 1977, Onken et al. 1980) verified that acidic EDTA solution extracts P which is available to plants. The indicator of P sorption capacity (Tab- le 2), which includes sorbed and EDTA- extractable P, amounted, on average, to about 25 mmol/kg soil. The mean value of the indi- cator was higher in clay and silt soils than in coarser soils. The dependence of the indicator of P sorp- tion capacity on soil properties was studied by means of linearregression analysis. The vari- ables were as follows: X, = indicator of P sorption capacity (mmol/kg soil) X 2 = extractable aluminium (mmol/kg soil) X 3 = extractable iron (mmol/kg soil) X 4 = organic carbon content (%) X 5 = soil pH X 6 = clay content (%) In clay and silt soils, oxalate-extractable aluminium and iron together with the organic carbon content explained (P = 0.001) 85 % of 11 the variation of the indicatorof P sorption ca- pacity. The regression equation and partial correlation and P coefficients were as follows; X, = -8.13 + 0.293X2 + 0.117X3 + 2.01X4 r 12.34 (3,2.34 = 0-64 r ,3.24 = 0-56*** (3,3.24 = 0.27 r 14.23 = o.46*** (314.23 =0.31 The content of oxalate-extractable Al ex- plained 54 °7o, the content of iron 31 % and the content of organic carbon 21 % of the var- iation in sorption when the effect of the oth- er two independent variables was eliminated. Oxalate-extractable Al and Fe, soil pH and clay content explained (P =0.001) 80 % of the variation in the indicator of P sorption capa- city in coarse soils, the regression equation being: X,= -3.33 + 0.185X2 + 0.094X3 -3.18X5 + 0.32X6 r, 2 .356= o.B7*** p 12356 = 0.84 ■r 13.256= 0.43** p 13256 = 0.24 r 15236=-0.44** p 15236 =-0.25 r 16235 = 0.32* p i6235 = 0.20 The content of oxalate-extractable Al ex- plained 76 °7o, the content ofFe 19%, the soil pH 19 % and the clay content 11 % of the variation when the effect of the other indepen- dent variables was eliminated. In both soil groups, oxalate-extractable Al was the most important explainer of the variation in the in- dicator of P sorption capacity. Pyrophosphate-extractable Al and Fe to- gether with clay content explained (P =0.001) 87 % of the variation in the indicator of P sorption capacity in clay and silt soils accord- ing to the equation: X, = 5.23 + 0.184X2 + 0.417X3 + 0.20X6 r l2 . 36 = o.73*** P 1236 = 0.494 r l3 .26 = o.73*** pi326 =0.488 r l6 . 23 = 0.37** p 1623 = 0.150 Pyrophosphate-extractable Al and Fe were equally important variables. Al explained 53 °/o, Fe 54 % and the clay content 14 % of the variation when the effect of the other two independent variables was eliminated. In coarse soils, pyrophosphate-extractable Al ex- plained (P =0.001) 47 % of the variation in the indicator of P sorption capacity accord- ing to the equation: X, = 5.98 + 0.300X2 . EDTA-extractable Al and Fe explained (P = 0.001) 91 % of the variation in the indi- cator of P sorption capacity in clay and silt soils according to the equation: X, = 8.57+ 0.844X2+ 0.438X3 r 123 = o.92*** Pi2.3 = 0.80 r,3.2 = o.6o*** Pi3. 2 = 0.26 Al was a more important independent vari- able than Fe. Al explained 84 % and Fe 36 % of the variation in the indicator of P sorp- tion capacity. In coarse soils, EDTA-ex- tractable Al explained (P =0.01) only 20 % of the variation according to the equation: X, = 10.18 + 0.508X2 . Acetate-extractable Al and the organic car- bon and clay contents explained (P =0.001) 78 % of the variation in the indicator of P sorption capacity in clay and silt soils accord- ing to the regression equation: X, = 5.50 + 0.859X2 + 0.272X4 + 0.24X6 r 1246 = o.Bl*** P 1246 =0.49 r l4 .26 = o.ss*** P 1426 =0.41 r 1624 = 0.34* P, 624 =0.18 Al explained 65 %, the organic carbon con- tent 30 % and the clay content 12 % of the variation when the effect of the other two independent variables was eliminated. In coarse soils, acetate-extractable Al explained (P = 0.001) 36 % of the variation in the indi- cator according to the equation: X[ = 8.75 + 1.112X2 . Discussion In the present soil material, AI and Fe ex- plained the variation of the indicator of P sorption capacity. Aluminium was generally a more important explainer than iron. In numer- ous papers concerning the dependence of the P sorption on soil properties (e.g. Williams et al. 1958, Kaila 1959, 1963, Bromfield 1964, 1965, Saini and MacLEAN 1965, Ahen- korah 1968, Lopez-Hernandez and Burnham 12 1973, 1974, Hartikainen 1979, Wada and Gunjioake 1979), extractable aluminium has been more important than iron in explaining the retention of P. In sorption studies, the Freundlich constant k, the anion exchange ca- pacity (Piper 1944) and the P sorption index (Bache and Williams 1971) are used as indi- cators of P sorption capacity, and metals are extracted by oxalate, dithionite, HCI, ammo- nium acetate and acetic acid solutions. In the study of Kaila (1963), oxalate-extractable Al and Fe explained nearly 80 % of the variation of the Freundlich constant k in clay soils and about 60 % in coarse soils. The partial corre- lationcoefficient for the relationship between k and Al was o.7B*** (n= 151) in clay soils and o.63*** (n= 109) in coarse soils, and for the relationship between k and Fe o.46*** in clay soils and o.3o*** in coarse soils. Kaila’s results show the same tendency as those ob- tained in the present investigation. In some cases iron has been found to be a more important explainer of P sorption than aluminium (Kaila 1963, Bromfield 1965, Ahenkorah 1968, Lopez-Hernandez and Burnham 1973, 1974). In these studies, however, Al is often extracted by a different method than Fe. According to Lopez-Her- nandez and Burnham (1973), extractable Al and Fe explain the sorption of P the better the more efficient the extraction method of met- als is. The greater significance of soil aluminium than soil iron in the adsorption of phosphate may be related partially to the different way these metals occur in soil. Iron oxides and hydroxides are generally present as discrete mineral particles, even when they are present on surfaces of clay minerals (Deshpande et al. 1964, Greenland et al. 1968). Aluminium hydroxides tend to form films over clay par- ticles (El Swaify and Emerson 1975), offer- ing a large surface area for phosphate sorp- tion. In the present study the significance of a larger sorption surface was manifested by the higher sorption of phosphate in clay and silt soils than in coarse soils. In this study, the organic carbon content ex- plained the variation of the indicator of P sorption capacity in clay and silt soils. In many earlier papers, P sorption has been found to depend on the content of organic matter in soil (Williams et al. 1958, Saini and MacLEAN 1965, Ahenkorah 1968, Lopez- Hernandez and Burnham 1974). This indi- cates that active Al and Fe are closely con- nected with soil organic matter, which retards the crystallization of oxides and thus enhances their activity in sorption (Williams et al. 1958, Schwertmann et al. 1968). In coarse soils of the present material, pH, clay content and oxalate-extractable Fe were about equal- ly important explainers of the variation in the indicator of P sorption capacity. In most of the previously mentioned studies, the depen- dence of the indicator of P sorption on soil pH and clay content is weak or insignificant. The present study showed that the forms of extractable Al and Fe explaining the variation of the indicator of P sorption capacity in clay and silt soils are partially different from those of coarse soils. In clay and silt soils, the vari- ation of the indicator was rather well ex- plained by the metals extracted by all of the methods studied, whereas in coarse soils, the variation in the indicator of P sorption capa- city was explained well only by oxalate-ex- tractable metals. In clay and silt soils, the in- dicator of P sorption capacity, in particular, seemed to be related to the EDTA-extractable metals, whereas in coarse soils this fraction of metals explained the variation in P retention only weakly. It has been mentioned before that particularly in clay soils, Al hydroxides occur as films on clay particles, thus provid- ing large surface for retention of P. It may be that EDTA extracted such hydroxide films, whereas EDTA was able to extract only poorly the more crystalline forms, which may con- stitute a considerable proportion of the P re- taining material in coarse soils. 13 References Ahenkorah, Y. 1968. Phosphorus-retentioncapacities of some cocoa-growing soils of Ghana and their relation- ship with soil properties. Soil Sci. 105: 24—30. Ahmed, B. & Islam, A. 1975. The use of sodium EDTA as an extractant for determining available phosphate in soil. Geoderma 14: 261—265. Alexander, T.G, & Robertson, J.A. 1972. EDTA ex- tractable phosphorus in relation to available and in- organic phosphorus forms in soils. Soil Sci. 114: 69—72. Bache, B.W. & Williams, E.G. 1971. A phosphate sorp- tion index for soils. J. Soil Sci. 22: 289—301. Barrow, N.J. 1978. The description of phosphate ad- sorption curves. J. Soil Sci. 29: 447 —462. Bromfield, S.M. 1964. Relative contribution of iron and aluminium in phosphate sorption by acid surface soils. Nature 201: 321—322. 1965. Studies of the relative importance of iron and aluminium in the sorption of phosphate by some Aus- tralian soils. Aust. J. Soil Res. 3: 31 —44. Deshpande, T.L., Greenland, D.J. & Quirk, J.P. 1964. Role of iron oxides in the binding of soil particles. Nature 201: 107—108. Elonen, P. 1971. Particle-size analysis of soil. Acta Agr. Fenn. 122: 1 122. El Swaify, S.A. & Emerson, W.W. 1975. Changes in physical properties of soil clays due to precipitated aluminium and iron hydroxides: I. Swelling and agg- regate stability after drying. Soil Sci. Soc. Am. Proc, 39: 1056—1063. Graham, E.R. 1948. Determination of soil organic mat- ter by means ofa photoelectric colorimeter. Soil Sci. 65: 181 183. Greenland, D.J., Oades, J.M. & Sherwin, J.W. 1968. Electron microscope observations of iron oxides in some red soils. J. Soil Sci. 19: 116—122. Hartikainen, H. 1979. Phosphorus and its reactions in terrestrial soils and lake sediments. J. Scient. Agric. Soc. Finl. 51: 537—624. Kaila, A. 1955. Studies on the colorimetric determina- tion of phosphorus in soil extracts. Acta Agr. Fenn. 83: 25—47. 1959. Retention of phosphate by peat samples. J. Scient. Agric. Soc. Finl. 31: 215—225. 1963. Dependence of the phosphate sorption capaci- ty on the aluminium and iron in Finnish soils. J. Scient. Agric. Soc. Finl. 35: 165—177. Lopez-Hernandez, I.D. & Burnham, C.P. 1973. Extrac- tion methods for aluminium and iron in relation to phosphate adsorption. Commun. Soil Sci. PI. Anal. 4: 9—16. & Burnham, C.P. 1974. The covariance of phosphate sorption with other soil properties in some British and tropical soils. J. Soil Sci. 25: 196—206. McLean, E.0., Heddleson, M.R., Bartlett, R.J. & Holowayshuk, N. 1958. Aluminum in soils: I. Ex- traction methods and magnitudes in clays and Ohio soils. Soil Sci. Soc. Proc. 22: 382 —387. Niskanen, R. 1989. Extractable aluminium, iron and manganese in mineral soils. 11l Comparison of extrac- tion methods. J. Agric. Sci. Finl. 61: 89—97. 1990. Sorption capacity of phosphate in mineral soils. I Estimation of sorption capacity by means of sorp- tion isotherms. J. Agric. Sci. Finl. 62: I—B.1 —8. Oi sen, R.A. 1975. Rate of dissolution of phosphate from minerals and soils. Soil Sei. Soc. Amer. Proc. 39: 634—639. Onken, A.8., Matheson, R. & Williams, E.J. 1980. Evaluation of EDTA-extractable phosphorus as a soil test procedure. Soil Sei. Soc. Amer. J. 44: 783—786. Piper, C.S. 1944. Soil and plant analysis. 368 p. New York. Ryti, R. 1965. On the determination of soil pH. J. Scient. Agric. Soc. Finl. 37: 51 —60. Sahrawat, K.L. 1977. EDTA extractable phosphorus in soils as related to available and inorganic phospho- rus forms. Commun. Soil Sci. PI. Anal. 8: 281 —287. Saini, G.R. & MbcLean, A.A. 1965. Phosphorus reten- tion capacities of some New Brunswick soils and their relationship with soil properties. Can. J. Soil Sci. 45: 15—18. SCHWERTMANN, U., FISCHER, W.R. & PaPENDORF, H. 1968. The influence of organic compounds on the for- mation of iron oxides. Trans. 9th Int. Cong. Soil Sci. 1: 645—655. Wada, K. & Gunjigake, N. 1979. Active aluminum and iron and phosphateadsorption in Ando soils. Soil Sci. 128; 331—336. Williams, E.G., Scott, N.M. & McDonald, M.J. 1958. Soil properties and phosphate sorption. J. Sci. Food Agric. 9: 551—559, Ms received June 8, 1989 14 SELOSTUS Kivennäismaiden fosfaatin pidätyskapasiteetti Il Pidätyskapasiteetin riippuvuus maan ominaisuuksista Raina Niskanen Maanviljelyskemian laitos, Helsingin yliopisto, 00710 Helsinki Kivennäismaiden (n= 102) fosfaatin pidätyskapasiteetin riippuvuutta uuttuvan alumiinin jaraudan pitoisuuksis- ta ja muista maan ominaisuuksista tutkittiin käyttäen fosfaatin pidätyskapasiteetin indikaattorina kahdessa vuorokaudessa maahan pidättyneen (reaktioliuoksessa P 5 mmol/l) ja maassa ennestään olevan, 0,02 M EDTAdIa (pH 5,3) uuttuvan, fosfaatin summaa. Savi- ja hiesumaissa (n =51) maan ominaisuudet selit- tivät melko hyvin fosfaatin pidätyskapasiteetin indikaat- torin vaihtelua. Selitysaste oli 85 %, kun selittävinä muut- tujina olivat 0,05 M oksalaatilla (pH 2,9) uuttuva alumiini ja rauta sekä orgaanisen hiilen pitoisuus. Kun selittävinä muuttujina olivat 0,05 M kaliumpyrofosfaatilla (pH 10) uuttuva alumiini ja rauta sekä saveksen pitoisuus, seli- tysaste oli 87 %. 0,02 M EDTA:IIa (pH 5,3) uuttava alu- miini ja rauta selittivät 91 % ja 1 M ammoniumasetaa- tilla (pH 4,8) uuttuva alumiini sekä orgaanisen hiilen ja saveksen pitoisuus 78 % fosfaatin pidätyskapasiteetin indikaattorin vaihtelusta. Karkeissa maissa (n =51) ai- noastaan oksalaattiuuttoinen alumiini ja rauta selittivät hyvin pidätyskapasiteetin indikaattorin vaihtelua, yhdessä maan pH:n ja saveksen pitoisuuden kanssa ne selittivät 80 % vaihtelusta. Molemmissa maaryhmissä uuttuva alu- miini oli yleensä tärkeämpi selittäjä kuin uuttuva rauta. Tulokset osoittavat, että fosfaatin sorptiokapasiteetin indikaattorin vaihtelua selittävä alumiini ja rauta olivat savi- ja hiesumaissa uuttuvuudeltaan osittain erilaisia kuin karkeissa maissa. 15