Forms of sulfur extracted from soils after different methods of sample preparation

Abstract

Three forms of sulfur (S) were determined in extracts from field-moist, air-dried and conditioned (i.e. maintained at 20°C and 75% field moisture capacity for 2 weeks) soils by using four extractants-water, CaCl2, KH2PO4 and Ca(H2PO4)2. The amounts and proportions of inorganic sulfate (SO2-4-S), organic HI-reducible S (HI-S) and carbon bonded S (C-S) in extracts varied with soil type, method of sample preparation and the extractant used. Substantially less S overall was extracted from field-moist soil compared with air-dried and conditioned soils. For soils with pH values close to or above 6, water and phosphate extractants removed similar amounts of SO2-4-S. However, phosphate solutions removed greater amounts of SO2-4-S than water from soils with relatively low pH or with a large capacity for sulfate adsorption. The CaCl2 solution extracted the smallest amounts of sulfate from the soils studied. There was no consistent ratio between extractable sulfate-S and extractable organic S. Extracts from air-dried soil contained the greatest amounts and proportions of organic S, particularly in phosphate extracts. The conditioning of soils decreased the extractable organic S and increased the sulfate-S, indicating that extractable organic S is readily mineralizable. The relative proportions of extractable organic S present as HI-S and C-S varied considerably. Comparison of the conditioned and air-dried soils indicated there was no difference between the two forms of organic S in their ease of mineralization.