Balanced and adequate potassium and phosphorus nutrition of pasture

Abstract

Pasture production responses from applications of four rates each of potassium (K) (0, 112.5, 225, 450 kg ha‐1) and phosphorus (P) (0, 60, 120, 240 kg ha‐1) in a factorial design were measured in a mowing trial with 60% of clippings returned at a K‐ and P‐responsive site (54.6 μg K ml‐1 and 9 μg Olsen P ml‐1) at Woodlands in Southland, New Zealand over 2 years. There were large significant responses in total pasture dry matter (DM) production to K and P. A bivariate Mitscherlich‐related equation accounted for 97% of the variation in measured values of total DM yield, 94–99% of the variation in ratios of K, P, and nitrogen (N) concentrations in mixed herbage, and 99–100% of the variation in nutrient ratios in white clover, summed or averaged over 2 years. The fitted equations were used to identify combinations of fertiliser P and K rates and ratios of K and P concentrations in mixed herbage and clover that resulted in balanced and adequate nutrition. Two nutrient elements are in balance when the yield response to one added on its own equals the response to the other added on its own, both in relation to the maximum yield. Balanced nutrition in total DM occurred at a K/P ratio of 5.1–7.4 for mixed herbage and 4.2–7.9 for clover, and were highest at higher rates of K and P. For balanced nutrition at 95% relative yield, K/P ratios were 6.8 for clover and 6.7 for mixed herbage. Fertiliser K/ (K + P) ratios for balanced nutrition increased from 0.5 for fertiliser expenditure of $50 ha‐1 to 0.7–0.8 for expenditure of $100–200 ha‐1. Modelled yield at the fertiliser K/(K + P) ratio for balanced nutrition in mixed herbage and clover DM was similar to maximum modelled yield. Mixed herbage and clover K/P ratios derived from economically optimal fertiliser K/P ratios were 6.2–6.6 and 5.6–6.7, respectively. Using N concentration as an internal standard, nutrient adequacy is defined as the herbage yield with K and P in physiological balance relative to the maximum yield with K and P in physiological balance and expressed as RY [KP]. At 95% RY [KP], adequate nutrition was achieved at K/N of 0.77 for mixed herbage and 0.46 for clover. Overall, the results from the trial reported here confirm that ratios of nutrient concentrations are useful indicators of balanced and adequate K and P nutrition for total pasture production. Mixed herbage or clover nutrient concentrations and ratios were both effective indicators of balanced and adequate nutrition.