Relationship between a soils ability to adsorb phosphate and the residual effectiveness of superphosphate

Abstract

Twenty-two unfertilized soils from four localities in south-western Australia were used in a pot trial. In March powdered, single superphosphate was applied to the surface of eight of the pots at a range of levels. All the pots were kept moist until August when subterranean clover seed was sown and fresh superphosphate was applied to the previously unfertilized pots. Response curves were fitted to the yields and from the coefficients of the curves the relative effectiveness of the March and the August applications was calculated. For most of the soils, the March application was between 50 and 60% as effective as the August application. There was a small but significant trend for the relative effectiveness to increase as the soils' ability to adsorb phosphate from solution (ÄP) increased. There was a close relationship between the effectiveness of the August application and the inverse of ÄP. Since ÄP is a measure of the distribution of phosphorus between the solid and the solution phases in the soil, it was argued that the decreased effectiveness of the March application could be explained by a change in the distribution of phosphate. An incubation experiment showed that such a redistribution occurred and that the proportional change was little affected by ÄP. This seemed to explain the small effect of ÄP on decrease in effectiveness.