EFFECTS OF MAGNESIUM ON CATION SELECTIVITY AND STRUCTURAL STABILITY OF SODIC SOILS

This study was conducted to evaluate the effects of Mg on the cation-e xchange relationships and structural stability of sodic soils. Samples of prairie soils (Haploborolls) from Saskatchewan, Canada, were equil ibrated with sodic solutions [sodium adsorption ratio (SAR = Na/(Ca Mg)0.5, where concentrations are expressed in millimoles per liter) ra nging from 5 to 401 with different Ca/Mg ratios. Exchangeable Na incre ased as the Mg/Ca ratio increased. Averaged from all SAR treatments, t he Mg- Na systems had between 17 and 37% more exchangeable Na than did the Ca-Na systems. The Mg-induced increase in exchangeable Na was dir ectly related to a preference by the soils for Ca over Mg, which makes Na more competitive against Mg than against Ca. The influence of Mg o n structural stability was examined by measuring saturated hydraulic c onductivity (K) using Mg-Ca-Na solutions (Mg/Ca ratios ranging from 0: 1-1:0) with SARs of 10 and 20. The Mg-Na system developed considerably lower K than did the Ca-Na system when leached with solutions with to tal electrolyte concentration less-than-or-equal-to 100 mmol(c) L-1. S ystems with intermediate Mg/Ca ratios (i.e., Mg/Ca of 1:2 and 2:1) beh aved more like the Ca-Na than the Mg-Na system. Clay concentration in leachate from the soil columns increased as Mg/Ca ratio increased, sug gesting that a major effect of exchangeable Mg was that it increased t he tendency for clay dispersion. Dispersion tests confirmed that the e ffect of Mg was greater than could be explained by the higher exchange able Na level in the Mg system, i.e., Mg had a specific effect on clay dispersion. Our results suggest that exchangeable Mg is about 5% as d ispersive as exchangeable Na.