Balanced and adequate nutrition of phosphorus and sulphur in pasture

Pasture production responses and nutrient concentrations from 5 rates of ph osphorus (P) (0, 10, 20, 40, 80 kg ha(-1)) and sulphur (S) (0, 5, 10, 20, 4 0 kg ha(-1)) applied three times in two years were measured on a direct-dri lled ryegrass/white clover pasture in a small plot mowing trial on a site w ith low fertility (initial soil Olsen P 5 mu g ml(-1), soil sulphate-S 3 mu g g(-1), soil organic-S 4 mu g g(-1)) over two years. There were responses in total pasture and clover DM (dry matter) production to P and S in the s econd year. The response to S was much greater than to P. A bivariate Mitsc herlich-related equation accounted for 88-96% of the variation in measured values of clover DM production and ratios of P, S, and nitrogen (N) concent rations. The fitted equations were used to identify patios 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 it s 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 sec ond 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], a dequate nutrition was achieved at 100 x P/N of 7.10 and 100 x SIN ratio of 5.64. Clover S/P ratios derived from economically optimal fertiliser SIP ra tios 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. Over all, the results from the trial reported here confirm that ratios of nutrie nt concentrations are useful indicators of balanced and adequate P and S nu trition for white clover.