ION PRODUCTS AND SOLID-PHASE ACTIVITY TO DESCRIBE PHOSPHATE SORPTION BY SOILS

Solution equilibria and regular solid-solution concepts were combined to relate phosphate solubility to amounts of added phosphate in 8 acidic Missouri [USA] soils. Phosphate sorption studies were done by shaking KH2PO4 solutions with soil for 16 days and measuring pH, Eh [redox potential], electrical conductivity, Ca, Mn, Al, Fe and P contents of the solution. These data were used to compute ion products and to estimate the activities of solid-phase phosphate minerals. Solubility relationships for the reaction of H2PO4 with Al(OH)3 could be interfaced with estimates of solid-phase activities and used to predict phosphate in solution as a function of a soil constant (C), soil pH, and the amount of phosphate added. Activities for the equilibrium equation, p(H2PO4) = 10.50 - pH - p[Al(OH)3] + p(AlPO4), were estimated. The value of p[Al(OH)3] was determined from the data to be a linear function of pH, 2.41 - 0.42 pH, for soils used in this study. The value of p(AlPO4) was determined by statistically fitting observed p(AlPO4) and Padded to a polynomial equation similar in form to the one for a regular solid solution: p(AlPO4) = C - 9.4 .times. 10-3 (Padded) + 6.9 .times. 10-6 (Padded)2, with r2 = 0.96. Values of p(H2PO4) were calculated from pH and Padded by these combined equations, and compared with measured p(H2PO4) with an r2 of 0.96. The system had solubility features similar to those described for a regular solid solution. Combining solution equilibria and regular solid-solution concepts was useful in providing a chemical basis for interpreting phosphate sorption by soils.