The influence of subterranean clover pastures on soil fertility. III. The effect of applied phosphorus and sulphur

Abstract

The total nitrogen content of the soil was significantly increased at each of four levels of superphosphate applied to a densely sown subterranean clover pasture on a coarse-textured sand at Kojonup, W.A.

During the first 4 years of the experiment nitrogen build-up was similar for all four treatments, but over the 11 year term the build-up was significantly slower at the lowest level of superphosphate.

The relationship between the age of the pasture and soil nitrogen was linear at the lowest level of fertilizer but curvilinear at all other levels. The relationship between total superphosphate applied and soil nitrogen was curvilinear.

There were no differential effects of the fertilizer treatments on either the bulk density or the pH of the soil.

At the two lower levels of superphosphate a high percentage of both phosphorus and sulphur was retained in the major plant root zone (0–10 cm) of the soil. At the two higher levels most of the phosphorus was retained but there were heavy losses of sulphur. The results of pot trials with soil taken from the experiment at the end of the 11 year period showed that the different levels of superphosphate had induced large differences in the phosphorus status of the soil but had had much smaller effects on the sulphur status. The possible effect of this loss of sulphur on the accumulation of soil organic matter is discussed.