RATIONALIZATION OF IONIC STRENGTH AND CATION EFFECTS ON PHOSPHATE SORPTION BY SOILS

The amounts of inorganic phosphate (P) sorbed by four contrasting unfertilized soils during 40 h were influenced by the ionic strength and cation species of the contacting solution (support medium) used, as indicated by isotherms over the final P concentration range of 0 to 1 μg P/ml and 0 to 10 μg P/ml. An increase in ionic strength enhanced P sorption during 40 h but the species of cation also influenced the amount of P sorbed, as shown by the isotherms obtained in 10^?2M Ca and 3 × 10^?2M Na systems. Although pH affected the amounts of P sorbed, pH effects alone could not adequately explain the differences in P sorption. Kinetic studies indicated that within the range of P addition used for each soil, the equilibrium P concentration, at infinite time, was independent of ionic strength and cation species. Consequently, the composition of the solution affected only the rate at which equilibrium was attained. The results are attributed to the effects of ionic strength on the surface charge of retaining components and the thickness of the diffuse double layer, and the effects of specilic sorption of a divalent cation on surface charge, as they relate to the rate of P sorption.