Exchange equilibria of potassium in soils. II. Effect of farmyard manure on K-Mg exchange

Exchange behaviour of potassium versus magnesium was studied on surface soil samples of 3 Ustochrepts from a semiarid tropical region in relation to different levels of cattle farmyard manure (FYM, 0, 2.5, 5 and 10%). Magnesium saturated soil samples were equilibrated with KCl + MgCl₂ solutions having a range of equivalent ion fraction of K from 0 to 1 in the equilibrium solutions. The experimental results were analysed and interpreted, using different exchange selectivity quotients and thermodynamic parameters. Application of FYM caused a small but consistent increase in the K-preference over Mg as depicted from the normalized exchange isotherms. Standard free energy of K-Mg exchange were strongly negative (ΔGo-6.97 to ?9.47 kJ eq^?1 by Gaines and Thomas approach; ΔG'o ?10.85 to ?14.15 kJ M^?1 by Babcock and Duckart approach), suggesting a strong thermodynamic preference for K over Mg in these soils. For comparable treatments, ΔGo were about ?0.84 to ?1.25 kJ eq^?1 more negative for K-Mg, compared to K-Ca system reported earlier. ΔGo for 10% FYM treatments became more negative over the controls by 2.34, 1.40 and 0.53 kJ eq^?1 for Hissar, Panipat and Pehowa soils, respectively. All the selectivity quotients scrutinized in this investigation, viz., Gapon (K_G), Vanselow (K_v) and thermodynamic (K_N) were strongly K-saturation dependent; the dependence becoming more pronounced with the addition of FYM.