The Effect of pH on Sodium-Calcium and Potassium-Calcium Exchange Selectivity for Cecil Soil
- R. D. Rhue and
- R. S. Mansell
Cation exchange capacity (CEC) and cation-exchange equilibria involving Na, K, and Ca were measured on the <50µm fraction of Cecil sandy loam soil (Typic Hapludult) at pH values between 4 and 8. This was achieved by equilibrating the soil separate in acetate buffers followed by equilibration in 0.1 M chloride solutions. Exchangeable cations were removed by five sequential extractions with 0.5 M Mg(NO3)2. The CEC increased two- to threefold between pH 4 and 8, indicating the presence of a large amount of pH-dependent charge. CEC measured with Ca exceeded that measured with Na and was presumed to result from a greater amount of exchange between Ca ions in solution and hydrogen on the soil organic fraction. In Na-Ca and K-Ca exchange systems, Ca accounted for most of the increase in adsorbed metal cation charge as pH increased from 4.5 to 6.5. Estimates of exchangeable hydrogen were obtained from the relationship between CEC and pH and included in calculations of adsorbed mole fractions. Vanselow selectivity coefficients indicated that the Cecil soil preferred Ca over Na but that selectivity for Na increased sharply at low pH and low Na mole fractions. The selectivity for K relative to Ca also increased with decreasing K mole fraction with K preferred over Ca at low K mole fractions. Potassium added to Na-Ca systems displaced more Na than Ca from the exchanger, resulting in an apparent increase in selectivity for Ca relative to Na.
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