Sodification of Soil in Relation to Organic Matter, Total Electrolyte Concentration and Nature of Cations and Anions

The effect of natural and applied organic matter (n.a.o.m.), total electrolyte concentration (t.e.c.) and nature of cations (Na⁺—Ca²⁺ or Na⁺—Mg²⁺) and anions (Cl⁻ or SO₄²⁻) in the equilibrium solution on sodification of some soils from a semi-arid tropical region of Haryana was investigated. The soils high or low in n.a.o.m. were equilibrated with Cl⁻ or SO₄²⁻ salts of Na⁺—Ca²⁺ or Na⁺—Mg²⁺ at different t.e.c. (25, 50, 75 or 100 m.e./l). The results are interpreted in terms of the approximate free energy change of the excliange reaction (ΔG′_r) and through the Gapon selectivity coefftcient (K_G).

The values of ΔG′_r and K_G suggested that soils high in natural or applied o.m. had higher preference for Ca²⁺ as compared with those lew in o.m. This effect was somewhat pronounced in case of applied organic matter. Larger values of ΔG′_r and smaller K_G for Na⁺—Ca²⁺ than for Na⁺— Mg²⁺ system throughout the exchange isotherm, indicated higher preference for Ca²⁺ to Mg²⁺. ΔG′_r increased with t.e.c. while K_G decreased, which suggest a drop in the preference for Na⁺ to Ca²⁺, when the t.e.c. was increased. For comparable treatments, ΔG′_r was always smaller and K_G bigger for than for Cl⁻ system, showing thereby greater sodification of soil with sulphate dominated solutions. The consideration of ion pairs (i.e., CaSO₄⁰ and NaSO₄⁻) in SO₄²⁻ solution, however, resulted into narrowing down the differences between Cl⁻ and SO₄²⁻ systems.