Changes in soil sulphur fractions with fertilizer additions and cropping treatments

Abstract

Continued application of fertilizer to a pasture soil over a 4 year period, increased the phosphate extractable, the bicarbonate extractable, and the total sulphur contents. Where the same level of sulphur had been applied during this period, the phosphate extractable sulphur declined exponentially with time since the last fertilizer application. No such relation with bicarbonate extractable sulphur was established. Under a pasture regime, there was an increase in the carbon bonded sulphur fraction with increasing level of sulphur applied. Evidence from cropped soils showed that sulphur in the carbon bonded form was not a permanent sink for soil sulphur. On cropping, more sulphur was withdrawn from the carbon bonded than from the hydriodic acid reducible form. The similarity in composition between new additions to the soil sulphur and that already there, and the mobility of the element between the forms examined, will make it difficult to devise a satisfactory soil test for the sulphur status of a soil.