Balanced and adequate nutrition of phosphorus and sulphur in pasture

Abstract

Pasture production responses and nutrient concentrations from 5 rates of phosphorus (P) (0, 10,20,40, 80 kg ha−1) and sulphur (S) (0,5, 10,20,40 kg ha−1) applied three times in two years were measured on a direct‐drilled ryegrass/white clover pasture in a small plot mowing trial on a site with low fertility (initial soil Olsen P 5 μg ml−1, soil sulphate‐S 3 μg g−1, soil organic‐S 4 μg g−1) over two years. There were responses in total pasture and clover DM (dry matter) production to P and S in the second year. The response to S was much greater than to P. A bivariate Mitscherlich‐related equation accounted for 88–96% of the variation in measured values of clover DM production and ratios of P, S, and nitrogen (N) concentrations. The fitted equations were used to identify ratios of clover P, S, and N concentrations that resulted in balanced and adequate nutrition. Two nutrient elements are in balance when the yield response to one added on its own in relation to the maximum, equals the possible response to the other added on its own in relation to the maximum. Balanced nutrition in the second year occurred at a clover S/P ratio of 0.81–0.93. Using N concentration as an internal standard, nutrient adequacy is defined as the herbage yield with P and S in physiological balance relative to the maximum yield with P and S in physiological balance and expressed as RY [PS]. At 95% RY [PS], adequate nutrition was achieved at 100 × P/N of 7.10 and 100 × S/N ratio of 5.64. Clover S/P ratios derived from economically optimal fertiliser S/P ratios ranged between 1.03 and 0.82 for P and S fertiliser expenditure of $50–200 ha−1. Clover S/P and S/N ratios were higher than those determined in an earlier trial on a more developed site with higher soil fertility. Overall, the results from the trial reported here confirm that ratios of nutrient concentrations are useful indicators of balanced and adequate P and S nutrition for white clover.