Sodification of Soils in Relation to Sodium Activity Ratios and Ca: Mg in Soil Solutions

A study of the exchange equilibrium between Na+ and Ca⁺²+Mg⁺² on four soils differing in texture and CEC was conducted with five sodium adsorption ratios, SAR, (10, 20, 30, 40, 60 mmole ^1/2 1^−1/2) and two proportions of Ca⁺²: Mg⁺² (75: 25 and 25: 75 on equivalent basis). The results obtained were interpreted using the empirical and the classical thermodynamic approaches. A good agreement between the Gapon's empirical approach and the thermodynamic approach was noted. The results also indicated the absence of specific exchange sites for Na+. By and large the soils showed a little more preference for Ca⁺² to Mg⁺². The relatively higher constancy of K_v (Vanselow coefficient) compared to K_kdo (Krishnamoorthy-Davis-Overstreet coefficient), both empirical approaches, revealed that the cation exchange sites on these soils were apart enough to behave relatively independently. An increase in the slope of the exchange isotherm (exchangeable sodium ratio versus sodium adsorption ratio) was noticed in the present experiment. The presence of some unidentified Na+ releasing minerals was also observed in these soils and the exchangeable sodium percentage (ESP), was corrected for the solubility of Na+ from such sources.