Predicting the response of irrigated perennial pasture to superphosphate in Victoria

Abstract

Experiments were conducted at 42 sites in the northern and south-eastern irrigation districts of Victoria to determine the relationship between extractable phosphorus (P) using the Olsen P soil test, and response of irrigated perennial pasture to an annual application of superphosphate. Relative responses (the response relative to the maximum yield, P non-limiting) were measured over 4 seasons (summer, autumn, winter, and spring). At 12 of these sites, selected from a range of soils with different clay contents, the effect of clay content on the curvature of the pasture dry mass (DM) response was determined. Different soil-sampling methods, with the surface organic mat included or excluded from the sample, were used to measure Olsen P.

A well-defined linear model described the relationship between Olsen P values of soil sampled using these two methods. There was no significant effect of season on the relationship between relative response and Olsen P value. There was also no significant relationship between the curvature of the response to applied P and clay content of the soil. The derived relationship between Olsen P soil test level and relative response to applied fertiliser accounted for only 14 · 6% of the variation and it appears that other factors are markedly affecting irrigated pasture response to applied superphosphate. The accumulation of organic matter on the surface of irrigated perennial pastures may be influencing nutrient availability and pasture growth, and warrants further investigation.