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-exchange relationships and structural stability of sodic soils. Samples of prairie soils (Haploborolls) from Saskatchewan, Canada, were equilibrated with sodic solutions [sodium adsorption ratio (SAR = Na/(Ca + Mg)0.5, where concentrations are expressed in millimoles per liter) ranging from 5 to 401 with different Ca/Mg ratios. Exchangeable Na increased as the Mg/Ca ratio increased. Averaged from all SAR treatments, the 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 directly 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 on structural stability was examined by measuring saturated hydraulic conductivity (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 total electrolyte concentration less-than-or-equal-to 100 mmol(c) L-1. Systems with intermediate Mg/Ca ratios (i.e., Mg/Ca of 1:2 and 2:1) behaved more like the Ca-Na than the Mg-Na system. Clay concentration in leachate from the soil columns increased as Mg/Ca ratio increased, suggesting that a major effect of exchangeable Mg was that it increased the tendency for clay dispersion. Dispersion tests confirmed that the effect of Mg was greater than could be explained by the higher exchangeable 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 dispersive as exchangeable Na.