CHANGES IN SOME SOIL-PHOSPHORUS AVAILABILITY PARAMETERS AS INDUCED BYPHOSPHORUS ADDITION AND SOIL SORPTION PROPERTIES

Changes in agronomic and environmental soil phosphorus (P) availabilit y parameters, i.e., Mehlich- and Olsen-extractable P, reversibly-adsor bed P, soil-solution P, and equilibrium-P concentration were determine d following equilibration of 13 Italian soils with five rates of P app lication (0, 12.5, 25, 50, and 100 mg P kg(-1) soil). Soil P availabil ity as determined by each parameter increased with added P. The relati ve change in soil P availability with added P was a function of soil s orption index silicon (SI), according to the equation DP=(P-added)(a) exp(b+g()SI). This equation accounted for 94 to 98% of the variance i n soil-P availability. The inclusion of Si in a soil testing program m ay increase the reliability in assessing both soil-P fertilizer requir ements and the vulnerability of a soil to P loss in runoff following l and application of fertilizer or manure P.