Effects of previous superphosphate applications on the pasture environment and the response by pasture to a current dressing

Abstract

A study was made of the effect of previous superphosphate applications on the response by pasture to the calcium, phosphorus, and sulphur in a current dressing. The trial was located on the Southern Tableland of New South Wales on soils of granitic, sedimentary, and basaltic origin. In each case the legume component of the pasture was subterranean clover (Trifolium subterraneurn L.). The evidence shows that there is scope for altering the phosphorus and sulphur content of superphosphate to produce more efficient fertilizers for particular stages in pasture development and for particular soils. The residual value of the phosphorus was greater than the residual value of the sulphur in superphosphate. Even on the basaltic soils, phosphate "fixation" was of no practical significance. Nevertheless, any change in the relative phosphorus sulphur requirement of pastures with increasing amounts of superphosphate previously applied, resulted from a decline in the requirement for phosphorus, not from a change in the sulphur requirement. The number of years since the last application of superphosphate was shown to be the best predictor of responses to both phosphorus and sulphur, which indicated that the residual value of both these elements is limited in time. Implications of the lapse rate – the decline in effectiveness of the fertilizer with time – are discussed. The results show that pasture production was limited by phosphorus deficiency on soils where previous evidence suggested that the build-up of organic matter may be limited by sulphur deficiency.