Organic sulphur fractions in scottish soils

Abstract

AbstractOver 90% of the sulpher in some acid surface soils derived from different parent materials in north‐east Scotland was in organic combination. N:S ratios in the majority of the soils fell within the narrow limits of 6.0–7.5, and showed less variation than C:S ratios. Organic sulphur was poorly correlated with organic phosphorus, and high correlations with categories of soluble aluminium and iron seemed to reflect similar relationships between these elements and the whole organic matter. A group of calcareous soils derived from shelly sand deposits contained a lower proportion of organic sulphur. An average 64% of organic sulphur in the acid soils occurred as organic sulphate, compared with only 23% in the calcareous soils. Organic sulphate was less well correlated with carbon and nitrogen than was total organic sulphur. Total carbon‐bonded sulphur was best determined as the difference between total organic sulphur and organic sulphate, because direct measurement by Raney‐nickel reduction underestimated the amount present in most of the soils, due to the presence of chemically unreactive compounds. Good correlations between the reactive carbon‐bonded sulphur and carbon, nitrogen and organic sulphur, suggested that a well‐defined group of compounds was being measured. There were no consistent effects of drainage conditions on organic sulphur contents, or on the amounts and proportions of organic sulphur present as sulphate or in carbon‐bonded form. The freely drained soils derived from basic igneous drift, however, contained more organic sulphur than poorly drained samples, but this is attributed to differences in the organic matter content.Several features of the results indicated that organic sulphur, unlike organic phosphorus, was predominantly an integral part of the soil organic matter.