Effects and interactions of phosphorus and sulphur on a mown white clover/ryegrass sward

Abstract

The pattern of yield response to phosphorus (P) and sulphur (S) fertiliser applied in a factorial design involving 5 rates of each nutrient on a mown white clover/ryegrass sward was studied over 2 years. Large responses to both P and S occurred with white clover and only small responses with ryegrass in both years. Over the whole period, in white clover the response to S was twice the response to P whereas in ryegrass the response to P was twice the response to S. A bivariate Mitscherlich‐related equation was developed to model the response surface and was found to account for 92.5–95.5% of the variation in white clover and total DM yields. The fitted equation was used to identify combinations of P and S fertiliser rates giving rise to balanced nutrition. Nutrients were defined as being in balance when maximum possible responses to each applied alone were equal. Once the initial imbalance between nutrients was eliminated by appropriate application of one nutrient, the ratio of further S and P required to provide balance at higher levels of production was relatively constant over application rates, being 0.56:1 S:P for the full period of the experiment. Economically optimal fertiliser S:P ratios were calculated from fitted response surfaces and individual nutrient costs. These ratios were always higher than those which supported balanced nutrition and this was attributed to the much lower cost of S and its greater agronomic efficiency on a per kg basis.