Sodium-calcium exchange selectivity as influenced by clay minerals and composition

Knowledge of exchangeable cation chemistry can provide clues about tendencies toward either soil dispersion or flocculation as exchangeable sodium accumulates. The accumulation of sodium might be influenced by soil texture and degree of salinity. The effects of clay minerals, composition, and salinity of solution on Na-Ca exchange selectivities of montmorillonite and illite were studied. Regression equations for the exchangeable sodium ratio (ESR)-sodium adsorption ratio (SAR) differed according to clay minerals, composition, and salinity of equilibrated solutions. The experimental Na-Ca exchange selectivities in this study differed from the values reported by U.S. Salinity Laboratory Staff. Our values were influenced by clay minerals, composition, and salinity of equilibrated solutions. Compared with montmorillonite, the affinity of illite for Na was high and decreased with increasing clay content. This phenomenon was obvious at the low SAR and high salinity of equilibrated solutions, based on colloidal properties. These findings suggest that the affinity of the soil for Na increases with soil salinization. In soil with low clay contents, Na adsorption/Ca release in the soil colloids was enhanced, as was soil sodicity. Therefore, the exchangeable sodium percentage of the soil increased rapidly, leading to the formation of sodic soils. Conversely, Na adsorption/Ca release in the soil colloids was comparatively suppressed in soil with high clay contents. The affinity of the soil for Na increased with increasing salinity of soil solution through the accumulation of salt in the soil. It can therefore be concluded that sodic soils are formed during the process of soil salinization in soil with high clay contents.