Studies on mineralization of sulphur from soil organic matter

Abstract

Determination of the sulphate in water extracts of soil by the method of Freney (1958) was shown to overestimate available sulphur. Further, changes in sulphate as measured by this procedure sometimes bore little relationship to presumed changes in available sulphur. Extraction of the soil with a 0.15% solution of calcium chloride seemed to give a better indication of available sulphur. When soils were dried there was an immediate increase in inorganic sulphate. It was suggested that this inorganic sulphate split from organic sulphates present in soil and it was shown that it was available to plants. This can greatly affect the results of pot experiments with sulphur, for even if two soils are dried at the same temperature the relative availability of sulphur may be no indication of the relative availability when the soils were fresh. Incubation of a soil taken from beneath unfertilized native pasture did not result in any mineralization of sulphur. It was concluded that this was a consequence of a low sulphur content in the decomposing portion of the soil organic matter. When the organic matter content of the soil was increased by applying superphosphate and growing clover, sulphur was mineralized when the soil was incubated.