Addition of phosphorus to soils with low to medium phosphorus retention capacities. I. Effect on soil phosphorus sorption properties

Twelve soils with low-medium phosphorus (P) retention capacities were equilibrated for 3 months with soluble phosphate at a rate of 100 mg P kg(-1) soil. The P sorption properties of these soils both with and without added P were studied, including equilibrium P concentration (EPCo), standard P requirement (SPR), soil P sorption capacity (b), maximum buffer capacity (MBC), and P sorption index(P-SI). In general, the soils with no added P showed low values of all the above parameters. Oxalate extractable aluminum appeared to be the major responsible element for the control of P sorption in these soils. The addition of P to these soils had a considerable effect on their P sorption properties. The changes in EPCo were well correlated with P sorption index (r=0.80;p less than or equal to 0.01). The EPCo values of the soils with and without added P were closely correlated to bicarbonate extractable P (P-Olsen) and calcium chloride extractable P (P-CaCl2), with r=0.80, and r=0.99 (p less than or equal to 0.001), respectively. Ninety percent of the variability in EPCo was explained by the corresponding variability in P-Olsen when a curvilinear relationship was adopted. The P sorption properties examined appear to be useful parameters to assess the environmental impact of soil P on the quality of surface waters.