Estimating calcium-magnesinm selectivity in smectitic soils from organic matter and texture

Selectivity coefficients are essential to describe the distribution of cations between the exchange and solution phases of soil. Because the factors influencing Ca-Mg exchange have not been adequately defined, models commonly incorporate the dubious assumption that soils do not discriminate between these two cations (i.e., the selectivity coefficient is 1). The purpose of this study was to identify key variables influencing Ca-Mg selectivity in smectite-dominated soils of the Canadian prairies. Selectivity coefficients (K) were measured by reacting soils that differed in texture (50-500 g clay kg(-1)), organic matter (4-96 g C kg(-1)), and pH (4.1-7.3) with solutions containing mixtures of CaCl2 and MgCl2. The value of K for the reaction Ca + Mg-soil reversible arrow Mg + Ca-soil ranged from 1.1 to 3.4, depending on soil type and the composition of the reacting solution. Preference for Ca increased as the equivalent fraction of Ca in solution decreased from 0.8 (average K = 1.7) to 0.05 (average K = 2.9). We identified organic: matter as the major source of Ca-preferring sites. The ratio of organic matter to clay was the best single indicator of Ca-Mg selectivity. Measurements on soils whose pH had been altered by laboratory addition of Ca(OH)(2) or field application of anhydrous NH3 indicated that selectivity was independent of pH even though the proportion of organic sites increased as pH increased. A simple two-site model with K of 3.9 for organic matter and K of 1.3 for clay described our data reasonably well, although selectivity for Ca tended to be underpredicted at low Ca loading.