Rock phosphate and superphosphate compared as pasture fertilizers on acid soils

Abstract

The results of a study comparing rock phosphate and superphosphate as pasture top-dressings on two acid soils are presented. The trials were designed to examine the immediate response to, and the residual effects of, an initial application of both fertilizers. At equivalent levels of applied phosphorus, superphosphate was a better fertilizer than rock phosphate in the early years of the trial. Rock phosphate and superphosphate both had a good residual value, and the yields obtained with both fertilizers were not significantly different after the first few years. The total yield of pasture over the 7 year period was similar for both fertilizers. A low residual value of gypsum as a sulphur fertilizer is demonstrated. The evidence shows that where a deficiency limits plant growth and inhibits the uptake of phosphorus, the fall in the residual value of a phosphorus fertilizer is less than where plant growth is not restricted. This suggests that a major factor governing the availability of residual phosphorus to plants, from both superphosphate and rock phosphate, is the conversion of available inorganic phosphorus to organic forms, through uptake by the plants themselves. It would appear that this effect on residual phosphate can be more important than "fixation" of inorganic forms. Mixtures of rock phosphate and superphosphate, both physical and by under-acidulation of rock phosphate, were used. The results are presented and discussed.