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 + MgCl 2 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.