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

The availability of fertilizer P in six P-deficient tropical soils fro m Brazil, Kenya, Malaysia and Indonesia was assessed by grass in a pot experiment. Grass dry matter yield (D) and fertilizer P (F) were fitt ed to a Mitscherlich equation: D = a - b exp(-cF), and P uptake (U) an d F to the linear equation: U = alpha + betaF. Fitted parameter beta e quals the proportion of P recovered in one crop and it varied widely b etween soils, ranging from 12 to 51%. Quantitative assessments of fert ilizer-P availability could also be made using dry-matter data alone i f the rates of fertilizer used were well distributed along the respons e curve, when Mitscherlich parameter c was correlated well with beta. Chemical measurements were made on uncropped soil. Phosphate sorption isotherms were measured, using P-32 to assess exchangeable and non-exc hangeable phosphate. The availability parameters c and beta were corre lated best with parameter b(e), the phosphate buffer capacity derived from the fitted Freundlich isotherm for exchangeable phosphate, sugges ting that the mobility of exchangeable phosphate is a major influence on P availability. Al and Fe were extracted with acid oxalate, citrate -dithionite and pyrophosphate reagents, and parameters c and beta corr elated best with Al extracted by acid oxalate. These relationships wer e inverse, showing that Al in disordered mineral forms lowers the avai lability of fertilizer P.