Soil sulphur fractions as chemical indices of available sulphur in some Australian soils

Abstract

A range of soils from eastern Australia hare been examined for a number of fractions of soil sulphur. These have been compared with plant sulphur uptake and yield of oats grown in pot culture in order to assess their possible values as indices of available sulphur. In all of the soils examined, most of the sulphur was present in organic forms and in most cases at least 50 per cent. could be extracted by 0.1N sodium hydroxide. Similar amounts were released as sulphate by ignition of the soil and similar amounts were also estimated by a reduction method. These three fractions were highly correlated with one another and with the total sulphur, but each was poorly correlated with the sulphur uptake and yield of oats in pot culture experiments. The results indicated that an appreciable proportion of the soil sulphur may consist of organic sulphates. The amounts of free water-soluble sulphate were small and were unsatisfactory as indices of yield. In addition to free sulphate, aqueous extracts also contained some organic sulphur which was easily oxidized to free sulphate by dilute hydrogen peroxide. The amount of water-soluble sulphur could be substantially increased by heating the soil prior to extraction. The fraction of the soil sulphur which was extracted after heating with water on a boiling water-bath, followed by heating for 1 hr in a hot-air oven (heat-soluble sulphur), was found to be highly correlated with both sulphur uptake and yield of oats grown in pot culture. It seems likely that this heat-soluble fraction ma3 provide a satisfactory index of available sulphur.