Maataloustieteellinen A ikakauskirja Vol. 59: 41—46, 1987 Sulphate sorption by Finnish mineral soils MARKKU YLI-HALLA Kemira Oy, Espoo Research Centre, Luoteisrinne 2 SF-02270 ESPOO, Finland Abstract. Sulphate sorption by 38 Finnish cultivated mineral soils was determined and its correlation with soil properties was studied. Sulphate sorption was correlated with soil pH (r =—o.46**) and with phosphate sorption (r =o.69***). With increasing soil pH, sulphate sorption decreased in relation to phosphate sorption. Phosphorus status was decisive in ex- plaining the sulphate sorption of the soils. Even if both anions are sorbed by the same soil component (amorphous Al compounds), the sites are not available for sulphate if they are already occupied by phosphate. Sulphate sorption was negligible in soils very rich in easily soluble phosphorus. This was reflected in a close negative correlation between sulphate sorp- tion and acid ammonium acetate (pH 4.65) extractable phosphorus (r =—o.7o***). During the last few decades, phosphorus fertilization has increased the amount of easily soluble phosphorus in Finnish fields, which obviously has decreased the capacity of the soils to retain sulphate. Index words: sulphate, sorption, soil Introduction Sulphate, the plant-available form of sul- phur, enters the soil with wet and dry deposition and fertilizers. Sulphate is also released from soil organic matter. The utilization of these sources of sulphur by plants is partly depend- ent on the ability of the soil to retain sulphate against leaching. The downward movement of seepage water in autumn and spring is likely to remove poorly sorbed sulphate to a con- siderable extent from soil pores. On the other hand, plants growing in soils with a strong sulphate sorption tendency probably require less sulphur fertilization. Recent studies on sulphur sorption have concentrated on coarse forest soils (e.g. Singh 1984, Nodvin et al. 1986). Cultivated soils differ from forest soils e.g. in terms of phosphorus. Cultivated soils are yearly en- riched with easily soluble phosphorus com- pounds. Phosphate and sulphate are known to be sorbed by the same soil components: amorphous Al and Fe compounds. The reten- 41 JOURNAL OF AGRICULTURAL SCIENCE IN FINLAND https://www.c-info.fi/en/info/?token=HSvn7v9YDakkgUCk.SLpIjvbvYLQ9azfUiJsetw.UjyEQ_NIY5dA5XSfAGlQGCCrUn9-yerEpAc8Gd_c2i2sI4OjupDZ0iyfXIxBs280MH5woXNJ-YiT-r-k49xypqXOKXHcHc50eKBigd4Tq9isOkVfg1zkxZs6BTew4Sreac_LFD7aKUeD1cvKr_D27qHxjjzKHTlFJ1sWNb3V tion of phosphate is, however, much stronger than that of sulphate (Barrow 1970). Labo- ratory experiments on the competition of anions for sorption sites have shown phos- phate to reduce sulphate sorption (Kamprath et al. 1956, Metson and Blakemore 1978). It has not been established, however, whether the sulphate sorption properties of the soil are influenced by phosphate within normal con- centration ranges as opposed to the higher concentrations used in laboratory experi- ments. The purpose of this study was to evaluate the ability of Finnish cultivated soils to retain sulphate and the influence of soil properties on the quantities retained. Special attention was paid to the effect of soil phosphorus status on sulphur sorption. The results were to give additional information on the suffi- ciency of sulphur nutrition of plants in Fin- land. Materials and methods The soil material consisted of 38 samples col- lected from the plough layer of cultivated fields in different parts of Finland. The sam- ples were air-dried and ground to pass a 2 mm sieve. Particle size analysis was done by a pipette method. Organic carbon was deter- mined by a wet digestion method. Soil pH was measured in a 0.01 M CaCl2 suspension at a soil-solution ratio of 1:2.5 (v/v). Amorphous Al and Fe were extracted with 0.05 M am- monium oxalate solution, pH 3.3, at a soil- solution ratio of 1:20 (w/v) and shaking time of two hours (Hartikainen 1982). Fe and Al were determined by AAS. The soil properties are presented in Table 1. Phosphorus, extractable with 0.5 M CHjCOOH—O.S M CH 3 COONH4 (pH 4.65), abbreviated AAAc in this paper, was determined according to Vuorinenand Mäki- tie (1955). Water-soluble phosphorus was determined by the method of Hartikainen (1982). Inorganic phosphorus was fraction- ated by the Chang and Jackson method as modified by Hartikainen (1979). The phos- phorus sorption index of the soils was deter- mined by shaking 1.0 g of soil in 60 ml of solution containing P 0.5 mg/1 as KH 2P04 . Shaking time was 23 hours. The amount of native sulphur, extracted with 0.01 M CaCl2, was determined by shak- ing 10 g of soil in 50 ml of 0.01 M CaCl 2 solution for one hour. The extracts were clear- ed withactivated charcoal which does not ab- sorb or release sulphur in the extractant con- cerned. Sulphur determinations were made using an indirect AAS method proposed by Galindoet al. (1969). Adsorption of sulphate was determined by shaking 10 g of soil in 50 ml Table 1. Chemical characteristics of the experimental soils. Means with the confidence limits at the 95 per cent level. Clay soils n= 16 Silt soils n= 11 Coarse soils n = 11 Clay % mean range mean range mean range mean range mean range 45 ± 5 30 —65 3.410.7 1.0—7.4 5.110.3 4.3—6.5 5117 30 —B9 90 117 59 —175 19 ± 6 5 —29 8 ± 4 2 —lB Organic C % 3.4 ±0.9 3.4±0.8 1.5—6.2 1.9—4.6 pH 5.2 ±0.3 4.7—6.1 68 ±3l 5.6±0.6 4.0—7.2 Oxalate-extr. Al mmol/kg 86 ±36 28 —lB6 27 —195 Oxalate-extr. Fe mmol/kg 63 ±lO 43 —96 65 ±29 27 —lB4 42 of 0.01 M CaCl2 solution which contained S 20 mg/1 as K2S0 4. The sorption index was calculated from the decrease in concentration during equilibration. Results The amounts of sulphur sorbed, shown in Table 2, ranged from 2 to 32 % mean 19 % of the quantity added. Although the sorption index seemed to be lower in silt soils, the ranges were nearly similar in various textural classes and, thus, the mean sulphur sorption indices did not deviate statistically significant- ly from one another in various soil groups. The amounts of sulphur sorbed were of the same magnitude as the quantities of phos- phorus retained. One should take into account the fact that the amount of sulphur available for adsorption was 100 mg/kg and that of phosphorus only 30 mg/kg. Thus, the per- centages of phosphorus sorbed were consider- ably greater than those of sulphur ranging from 31 to 89 %, mean as high as 63 %. The relations between sulphate sorption in- dex and soil properties were studied with the correlation analysis. Corresponding calcula- tions were also made for phosphate sorption index. Statistically significant correlation coef- ficients were as follows: r S sorption P sorption index index CaCl2-soluble S —0.32* n.s. Oxalate- extractable AI n.s. 0.39* pH —o.46** n.s. NH4F-P/A1 —o.62*** —o.s2*** P sorption index o.69*** AAAc- soluble P —o.7o*** —o.sl*** Water- soluble P —o.6B*** —o.73*** The sulphate sorption index exhibited the highest correlation coefficients with the phos- phate sorption index as well as with variables describing the phosphorus status of the soil. The molar ratio of NH 4F-soluble phosphorus to oxalate-extractablealuminum indicates the degree of phosphate coverage of amorphous aluminum compounds of the soil. This is reflected in the values of water-soluble phos- phorus (Hartikainen 1982) which might be used as an estimate of phosphorus concen- tration in soil solution (Amarasiri and Olsen 1973). In the present material, the same obvi- ously holds also for AAAc-extractable phos- phorus. Table 2. Sulphate and phosphate sorption indices and amounts of 1 CaCl2 -extractable sulphur and water-soluble phosphorus in the experimental soils, all expressed as mg/kg. Clay soils n= 16 Silt soils n = 11 Coarse soils n = 11 CaCl2 -extractable S 9.6±3.0 4.5—26.8 21.2 ±4.8 1.7—32.0 9.8 ±4.0 7.712.3 2.9—15.8 16.315.7 6.8—28.4 14.318.3 3.1—41.2 17.713.3 9.5—23.5 9.1±3.7 3.9—20.0 17.414.9 mean range mean range mean range mean range S sorption index 1.9—30.0 10.9rt 6.1 2.4—24.8 19.6±3.4 13.3—26.7 Water-soluble P 3.1—32.3 19.112.3 9.3—24.9 P sorption index 43 The strong correlations mentioned above markedly masked other relationships between the two indices measured and the soil char- acteristics. Therefore, the partial correlation coefficient was calculated between S sorption (1) and oxalate-extractable Al (2) so that the effect of NH4F-soluble phosphorus (3) was eliminated. This correlation coefficient (r 123 = o.s7***) proved statistically significant. The corresponding partial correlation coefficient between phosphate sorption index (4) and alu- minum (r243 = o.64***) was also higher than the total one. Partial correlation coefficients remained low between soil pH and both sorp- tion indices. It was interesting to observe that oxalate- extractable iron and the content of NaOH- extractable phosphorus (»Fe-P») seemed to have no correlation with sulphate or phos- phate sorption as far as total or partial cor- relation coefficients were concerned. The results were analysed also by the re- gression analysis. AAAc-extractable phos- phorus as mg/1 (X,) explained 49 % of the variation of sulphate sorption (Y). When oxalate-extractable aluminum as mmol/kg (X 2) was added to the regression model, the coefficient of multiple determination increased by 7 %. The equation was as follows: Y = 0.45X, + 0.056X2 + 20.44 (F = 22.624***) R 2 =56 % In another regression model, the phos- phorus sorption index as mg/kg (X 3) alone explained 48 % of the variation of sulphate sorption (Y) (Fig. 1). Inclusion of soil pH (X 4) in the model increased the coefficient of multiple determination by 14 %. The model was as follows: Y =1.15Xj—4.58X4 + 21.22 (F = 27.226***) R 2 =6l % S =5.42 Discussion In the present study, the soil samples dis- played great variation in their ability to sorb added sulphate, sorption ranging from nil to more than a third of the amount added. The absolute quantities of sulphate sulphur and phosphate phosphorus sorbed by the samples were rather similar, which apparently differs from the results of Barrow (1970) and Scott (1976). One has to take into account two fac- tors which in the present study contributed to the unusually high sorption of sulphate com- pared to that of phosphate. Firstly, phosphate sorption took place in a soil suspension where no electrolyte was added, while sulphate sorp- tion was determined in a CaCl2-containing environment. Both sulphate and phosphate sorption are known to be enhanced markedly in the presence of neutral salts (e.g. Barrow 1972). Secondly, the concentration of phos- phorus in the added solution was only 0.5 mg/1, or forty times less than that of sulphur in the corresponding solution, 20 mg/I. The quantities of a certain ion sorbed are usually the greater, the higher the concentration in the solution added (e.g. Kamprath et al. 1956, Rajan 1974, 1978). The absolute sorption in- dex values, depending greatly on the test con- ditions, are thus of minor importance. Atten- tion is to be paid rather on their correlation with soil properties. Some significant obser- vations can be made, even though the varia- tion in the sulphate sorption index could only partly be explained by the soil properties. In the soils used in this study, the abundance of oxalate-extractable aluminum seemed to Fig. 1. Dependence of sulphur sorption index on phos- phorus sorption index. 44 control sulphate sorption, as has been report- ed also by Barrow (1967) in Australian soils. In Scottish soils, amorphous iron was consid- ered at least as responsible for sulphate sorp- tion as aluminum (Scott 1976). This dis- agreement can be due to the difference in the pH in which sorption was measured. Scott (1976) used pH 3.0, but in the present study the determinations were made in unbuffered suspensions where pH ranged from 4.0 to 7.2. Hartikainen(1981) has suggested a theory which tends to explain why Fe compounds, as compared to Al compounds, are relatively less important sorbents for phosphate at normal soil pH than at very low pH. This theory is also applicable to sulphate sorption. The theory is based on the fact that iron is a harder Lewis acid than aluminum. Consequently, at a certain pH, there are more H2O ligands coordinated with Al than with Fe which is cor- respondingly greatly surrounded by hydroxyls. Water ligands are more easily displaced by sulphate than hydroxyls (Rajan 1978), and therefore Al compounds offer in Finnish soils within normal pH range more easily accessible sites for sorption of sulphate than do Fe com- pounds as was observed in the present study. At pH 3.0 also Fe is mainly coordinated with water ligands and, thus, Fe compounds may well serve as important sorbents for sulphate at this exceptionally low pH as was demon- strated by Scott (1976). Elevation in soil pH also affects sulphate sorption more directly. With rising pH the concentrationof hydroxyls in the soil suspen- sion increases and sulphate sorption decreases due to anion competition (Kamprath et al. 1956), the net negative charge of the soil also increases, and the surfaces begin to exercise electrical repulsion on sulphate, leading to declining sorption of sulphate (Scott 1976). Actually, Barrow (1970) has pointed out that sorption of sulphate decreases more than that of phosphate with increasing pH. This net ef- feet of soil pH on the sulphate sorption was also seen in the present study. Phosphorus status of the experimental soils, practically covering the wholerange found in Finland (Kurki 1982), seemed to affect, in addition to phosphate sorption, decisively also sulphate sorption in the soil. Soils low in easily soluble phosphorus sorbed the largest amounts of sulphur. On the other hand, in soils rich in easily soluble phosphorus, sulphur sorption was reduced as the sorption sites were already occupied by phosphorus. In earlier studies, Kamprath et al. (1956) and Metson and Blakemore (1978) have shown that phosphate can prevent sulphate sorption. In those experiments, the quantities of phos- phorus added were at least several hundred milligrams per kilogram of soil. Metson and Blakemore (1978) equilibrated the soil sam- ples in a solution which contained 500 ppm P and 500 or 1 500 ppm S. These concentra- tions inevitably produce extracts very different from the soil solution, wherephosphorus con- centration seldom exceeds 1 mg/1 (e.g. Men- gel et al. 1968, Wiklander and Andersson 1974). In thepresent study, it was shown that phosphate levels met in ordinary cultivated soils may reduce sulphate sorption as well. The sorption capacity of sulphate was, how- ever, negligible only in soils unusually rich in easily soluble phosphorus. According to Kurki (1982), the average content of AAAc-extractable phosphorus has increased in Finnish fields due to fertilization, from about 4 mg/1 to 11 mg/1 in less than three decades. The frequency of very high phosphorus contents has necessarily increased, too. The observations of the present study suggest that the capacity ofFinnish field soils, at least in the plough layer, to sorb sulphate has diminished accordingly. Acknowledgement.The author wishes to thank the Au- gust Johannes and Aino Tiura Agricultural Research Foundation for the financial support of this study. 45 References Amarasiri, S.L. & Olsen, S.R. 1973. Liming as related to solubility of P and plant growth in acid tropical soil. Soil Sei. Soc. Amer. Proc. 37: 716—721. Barrow, N.J. 1967. Studies on the adsorption of sulfate by soils. Soil Sci. 104: 342—349. —, 1970. Comparison of the adsorption of molybdate, sulfate and phosphateby soils. Soil Sci. 109: 282—288. —, 1972. Influence of solution concentration of calcium on the adsorption of phosphate, sulfate and molybdate by soils. Soil Sci. 113: 175—180. Galindo, G.G., Appelt, H. & Schalscha, E.B. 1969. Sulfur determination in soil extract by an indirect atomic absorption spectrophotometric method. Soil Sci. Soc. Amer. Proc. 33: 974—975. Hartikainen, H. 1979. Phosphorus and its reactions in terrestrial soils and lake sediments. J. Scient. Agric. Soc. Finl. 51: 537—624. —, 1981. Effect of decreasing acidity on the extractability of inorganic soil phosphorus. J. Scient. Agric. Soc. Finl. 53: 16—26. —, 1982. Water soluble phosphorus in Finnish mineral soils and its dependence on soil properties. J. Scient. Agric. Soc. Finl. 54: 89—98. Kamprath, E.J., Nelson, W.L. & Fitts, J.W. 1956. The effect of pH, sulfate and phosphate concentration on the adsorption of sulfate in soils. Soil Sei. Soc. Amer. Proc. 20: 463—466. Kurki, M. 1982. Suomen peltojen viljavuudesta. 111. Summary: On the fertility of Finnish tilled fields in the light of investigations of soil fertility carried out in the years 1955—1980. 181 p. Helsinki. Mengel, K., Grimme, H. & Nemeth, K. 1969. Potentielle und effektive Verfiigbarkeit von Pflanzennährstoffen in Boden, Landw. Forsch. 23/1 Sonderheft: 79—91. Metson, A.J. & Blakemore, L.C. 1978. Sulphate reten- tion by New Zealand soils in relation to the competitive effect of phosphate. N.Z. J. Agric. Res. 21; 243—253. Nodvin, S.C., Driscoll, C.T. & Likens, G.E. 1986. The effect of pH on sulfate adsorption by a forest soil. Soil Sci. 142: 69—75. Rajan, S.S.S. 1978. Sulfate adsorbed on hydrous alu- mina, ligands displaced, and changes in surface charge. Soil Sei. Soc. Amer. Proc. 42: 39—44. —, Perrot, K.W. & Saunders, W.M.H. 1974. Identifi- cation of phosphate-reactive sites of hydrous alumina from proton consumption during phosphateadsorption at constant pH values. J. Soil Sci. 25: 438—447. Scott, N.M. 1976. Sulphate contents and sorption in Scottish soils. J. Sci. Fd. Agric. 27: 367—372. Singh, B.R. 1984. Sulfate sorption by acid forest soils: 1. Sulfateadsorption isotherms and comparison of dif- ferent adsorption equations in describing sulfate adsorp- tion. Soil Sci. 138: 189—197. Vuorinen, M. & Mäkitie, O. 1955. The method of soil testing in use in Finland. Agrogeol. Pubi. 63: 1—44. Wiklander, L. & Andersson, A. 1974. The composition of the soil solution as influenced by fertilization and nutrient uptake. Geoderma 11: 157 —166. Ms received February 2, 1987. SELOSTUS Suomalaisien kivennäismaiden sulfaatinpidätyskyky Markku Yli-Halla Kemira Oy, Espoon tutkimuskeskus Luoteisrinne 2, 02270 Espoo Sulfaatin pidättymistä kivennäismaihin tutkittiin 38 muokkauskerroksesta otetun maanäytteen aineistolla. Pi- dättyneet sulfaattirikkimäärät suurenivat maan pH:n ale- tessa (r =—o.46***) ja maan fosforinpidätyskyvyn kas- vaessa (r =o.69***). Mitä enemmän maassa oli happa- maan ammoniumasetaattiin (pH 4.65) uultuvaa fosforia, sitä vähemmän maa pidätti sulfaattirikkiä (r=—o.7o***). Amorfiset alumiiniyhdisteet, jotka ensisijaisesti säätele- vät helppoliukoisen fosforin määrää maassa, ovat myös tärkein sulfaattia sitova ainesosa. Jos maan alumiini- yhdisteidenanioninpidätyspaikat ovat suureksi osaksi fos- faatin miehittämät, voi maa pidättää vain niukasti sul- faattia, joka sitoutuu maahan paljon heikommin kuin fos- faatti. Viime vuosikymmeninä peltojemme muokkausker- roksen helppoliukoisen fosforin varat ovat kasvaneet tun- tuvasti, mikä on ilmeisesti pienentänyt maittemme sul- faatinpidätyskykyä. 46