EXCHANGE EQUILIBRIA OF POTASSIUM IN SOILS

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Abstract
We studied K-Ca exchange equilibria on three calcareous soils after incubating them with four levels of farmyard manure (FYM 0, 2.5, 5, and 10%) for 10 mo. Calcium-saturated soil samples were equilibrated with KCl + CaCl2 solutions at a constant chloride concentration of 50 meq L-1 in the equilibrium solutions. We used a thermodynamic approach and various selectivity quotients to describe the experimental results. The positions of the normalized exchange isotherms for potassium vis-à-vis the diagonal suggested higher “apparent K specificity” in the first 40 to 50% of K saturation. FYM treatments resulted in a small but consistent increase in the shift of isotherms toward a greater K preference, the shift being more conspicuous in the low K-saturation zone. The standard free energies of K-Ca exchange (ΔG0) were strongly negative (i.e., −1554 to −1968 cal eq−1), suggesting strong preference for K over Ca in these soils. ΔG0 became more negative by 256, 228, and 151 cal eq−1 with 10% FYM treatments over controls in Pehowa, Hissar, and Panipat soils, respectively. The reasons for this result being at variance with the conclusions of other workers in earlier work are discussed. The values of all the selectivity quotients scrutinized, viz., Gapon (KG), Krishnamoorthy-Davis-Overstreet (KKDO), Vanselow (KV), and thermodynamic (KN), were strongly K-saturation-dependent. Their values decreased with the increase in K saturation. The order of dependence was KV > KKDO > KN > KG. Experimental values of KN (i.e., 1.55 to 546 L M−1) were much higher than the “nonpreferential‘’ value of 0.368 L M−1 needed for the “no preference‘’ condition of ΔG0 = 0, showing strong K preference throughout experimental K saturation (i.e. ≈ 2 to 60% of CEC). Application of FYM increased K selectivity in all three soils, possibly due to an increase in the proportion of K-preferential internal:external exchange sites. © Williams & Wilkins 1986. All Rights Reserved.

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