Effects of Magnesium on Cation Selectivity and Structural Stability in prominent Vertisols of Karnataka

A study was conducted to evaluate the effects of Mg on ternary (Na-Ca-Mg) exchange and intern structural stability in prominent Vertisols of TBP (Tungabhadra Project) and UKP (Upper Krishna Project) irrigation commands of Karnataka State, India. Surface soil samples representing typical Vertisols derived from granite-gneiss (Gangavati and Kavadimatti), limestone (Devapur and Islampur) and basalt (Kadliwad and Mannapur) were included in the study. These soils were subjected to equilibration with waters of varying sodium adsorption ratio (SAR-5,20,40,60,80 and 100 millimoles per liter1/2) with different Ca/Mg ratio (4:1,2:1,1:1,1:2and 1:4).Exchangeable Na increased as the Mg/Ca ratio increased from 1:4 to 4:1 indicating that Mg charged waters are more detrimental for structural integrity of the soils. Averaged from all SAR treatments, the Mg-Na systems had between 10.5 to 12.1% 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 soil structural stability was examined by measuring degree of dispersion, capillary raise and moisture diffusivity using Na-Ca-Mg solutions varying with Ca/Mg ratios and SAR values. In order to segregate the specific effect of Mg, separate binary systems (exclusive Ca-Mg solutions with varying concentrations without Na) were used to examine the degree of dispersion. The Mg-Na system developed considerably lower soil moisture diffusivity and higher degree of dispersion as well as capillarity than did the Ca-Na system when equilibrated with solutions having total electrolyte concentration 20 m mol liter-1. Dispersion and water transmission tests confirmed that the effect of Mg was greater than could be explained by the higher exchangeable Na alone in the Mg system or else Mg had a specific effect on clay dispersion. It was obviously proved that Mg does have the effect on clay dispersion in the binary system involving diminishing Ca concentration where Na was completelyexcluded. Our results suggest that exchangeable Mg is about 1/10th, 1/20th, and 1/30th as dispersive as Na in granite-gneiss, limestone and basalt derived soils, respectively.