Residual effectiveness of phosphorus on white clover on granitic soils

Abstract

Three field experiments were carried out on highly phosphorus-deficient granitic soils in the northern tablelands of New South Wales to determine the residual effectiveness of phosphorus, and the efficiency of its usage by white clover pastures, in relation to the rate and frequency of application of fertilizer.

There was a linear and very large increase in the cumulative yield of white clover up to a maximum rate of about 100 kg phosphorus per hectare. Over the 4 or 5 years of each experiment there was no decrease in the efficiency of usage of phosphorus with increasing initial or cumulative rates of application up to a maximum rate equivalent to the phosphorus adsorption capacity of each soil. Above this rate, loss of efficiency was attributed to leaching of the excess phosphorus.

Phosphorus applied at 98 kg ha^-1 only in the first year gave near-maximum yields in the second year, but by the fourth year yields had fallen to as low as 50% of the yield from equivalent rates of freshly applied phosphorus. The residual effectiveness was greater than that usually reported in the literature, and this may be due to the unusually low phosphate buffer capacities of these soils.

To maintain the high early productivity from a large initial application of phosphorus, a small annual supplementation of 12 kg ha^-1 was required.