Effect of quantity and frequency of application of superphosphate on the seasonal yield of annual pasture

Abstract

An experiment compared the effectiveness of annual and quadrennial applications of superphosphate (totalling 24,48 or 72 kg P/ha over four years) in increasing the winter and spring yield of annual pasture. Initial levels of superphosphate of 12, 84 or 156 kg P/ha had been applied in the preliminary year of the experiment. Over the next four years there was no response to subsequent application of superphosphate after an initial application of 156 kg P/ha, or early in the experiment, after an initial application of 84 kg P/ha. However, whenever the pasture did respond to subsequent topdressing, annual applications produced a greater total response in winter over four years than did a single initial application of four times the annual rate; with the low initial rate of application, this benefit was 1.5 t dry matter/ha overthe four year period. Frequency of application did not affect responses in the spring. Pasture yield was described by the model: yield =a - b exp (- c.SP) where SP is available soil phosphorus based on the model SPn = +i =i0 Pi (1 - V) n -I n being the year of experiment, Pi the rate of phosphorus applied in year i, and V the annual decay rate of applied phosphorus in the soil. The level of soil phosphorus, extractable by acetic acid at the end of the experiment, was related to the total amount of fertilizer applied during the previous five years, but was not shown to be related to the frequency of application. Estimates of the annual decay rate of the applied phosphorus ranged from 19 to 75%.