Phosphatea dsorption characteristics of some Malaysian soils III. Relationships with crop yield and P fertilizer requirements

A. R. Ahmad and H. L. Foster

Abstract

Phosphate adsorption isotlerms described by Freundlich equation were used to calculate P concentrations in solution, P_l and the P buffer capacities dP_S/dP_l of soils sampled from nine sites in Malaysia where fertilizer trials on maize and soybean crops were conducted. The potential available water holding capacities of the soils were determined from field capaciry, wilting point and bulk density values. The fractions of the pore volumes occupied by the available water were used to calculate the impedance factors which in turn were used to calculate D, the P diffusion coefficient of each soil. Simple statistical correlations indicated that there were no relariorships between maize md soybean yields, urd P_l ,dP_S//dP_l  and D. The relationships between the crop yields and the adsorprion and diffusion variables could only be explained when physicochemical processes in the soils were considered. The soils could be divided into two groups based on the relationship between the impedance factor and bulk densities, and the interaction between bulk densities and potential available water fractions. Group I soils showed no yield responses while group II soils did show responses, with increases in P_l, dP _l /dP_S and D. A procedure for the calculation of P fertilizer requirements of group II soils was suggested.

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