Clover nutrient ratios to monitor pasture yield responses to fertiliser nutrients

Abstract

From earlier results of a limited number of trials, ratios of concentrations of phosphorus (P), sulphur (S) and potassium (K) to nitrogen (N) in white clover have been developed as indicators of nutrient adequacy. For 98% of maximum yield, clover herbage ratios of 7.4 (100 × P/N), 5.0 (100 × S/N) and 0.43 (K/N) were required. Mean critical nutrient concentrations have been reported as 0.35% P, 0.28% S and 2.05% K from earlier research on sites at an early stage of pasture development. A series of mowing trials, measuring pasture yield responses to nil or a high rate of P (120 kg/ha), S (100 kg/ha) and K (300 kg/ha) were carried out at 8 sites throughout New Zealand in 1997/98. There were significant (P<0.05) clover or total pasture dry matter (DM) yield responses to P at 7 sites, S at one site, and K at 6 sites. The results indicated that the optimum clover nutrient ratios reported from earlier research are valid over a wide range of environments. Clover P/N and % P, and K/N and % K accurately assessed the effectiveness of P and K applications and were reliable indicators of a pasture yield response. Clover S/N and % S were equally accurate measures of the correction of a S deficiency at the one S-responsive site, but neither measure increased after S application at other nonresponsive sites. Clover S/N was more accurate than % S for indicating a DM response to S. A lower mean critical concentration of 0.24% S in clover would have been more appropriate for these well developed sites. Keywords: nitrogen, nutrient concentrations, nutrient ratios, phosphorus, potassium, sulphur, white clover