INFLUENCE OF SOIL PROPERTIES ON THE RESPONSE TO PHOSPHORUS IN SOME TROPICAL SOILS .2. RESPONSE TO FERTILIZER-P RESIDUES

The residual values of fertilizer P in six tropical soils were assessed from the response of grass in a pot experiment. The comparison was made between residues of P that had been applied to a previous crop (also grass) and fresh P applied to uncropped soil. Yields of dry matter and P uptakes were fitted to Mitscherlich and linear equations respectively, and the availabilities of fresh and residual P were assessed by fitted parameters of these equations. With sufficient residual P, the maximum yields were close to those obtained with fresh P, but residual P always had a lower availability than fresh P. The residual value was assessed by the ratio of the availability parameters for residual and fresh P and these values showed that there were differences between soils in the decline of P availability. Residual value was not directly related to the initial availability of P. The P uptake responses to both fresh and residual P were correlated significantly with buffer capacity for exchangeable sorption of phosphate and Al extracted by acid oxalate. This suggested that there is an available fraction of residual P, the uptake of which is influenced by the sorption and mobility of phosphate in soil, in the same way as for fresh fertilizer P.