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 d iscriminate between these two cations (i.e., the selectivity coefficie nt is 1). The purpose of this study was to identify key variables infl uencing 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 C aCl2 and MgCl2. The value of K for the reaction Ca + Mg-soil reversibl e arrow Mg + Ca-soil ranged from 1.1 to 3.4, depending on soil type an d the composition of the reacting solution. Preference for Ca increase d as the equivalent fraction of Ca in solution decreased from 0.8 (ave rage 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 matt er to clay was the best single indicator of Ca-Mg selectivity. Measure ments on soils whose pH had been altered by laboratory addition of Ca( OH)(2) or field application of anhydrous NH3 indicated that selectivit y was independent of pH even though the proportion of organic sites in creased as pH increased. A simple two-site model with K of 3.9 for org anic matter and K of 1.3 for clay described our data reasonably well, although selectivity for Ca tended to be underpredicted at low Ca load ing.