Potassium availability, releasing power and uptake by chickpea and rajmash in different soil types of pulse growing regions of India.

Abstract

A large number of experimental studies have been conducted to probe the dynamic nature of equilibria between different forms of soil potassium (K), namely water-soluble K, exchangeable K, non-exchangeable K and the mineral or structural K. Most of these studies have employed a batch technique; allowing the system, namely soil K/solution K, to attain equilibrium at a constant temperature (and pressure). The ensuing distribution ratio (Goulding, 1983) has been related to the approximate thermodynamic equilibrium (K) constant. From the latter, as well as from a study of its temperature dependence, a number of thermodynamic parameters (such as standard free energy change, AG0; enthalpy change, AH; and entropy change, AS) of K* exchange equilibria between soil solid and solution phases, have been computed. The negative sign of AG at a constant temperature and pressure of the ion-exchange reaction is taken to indicate the spontaneity of the given ion-exchange reaction, or the relative preference of the soil colloidal phase for K' ions over other exchangeable ions, notably Ca2 " and Mg 2. Not withstanding the value of such thermodynamic studies, however, it is worth noting that thermodynamic parameters such as a large negative AG TY value merely suggests the formation of a large amount of product for the forward direction of the given reaction at equilibrium. It offers no idea as to how fast or how slow that equilibrium is attained at the given temperature. Although the AG of the reaction, H2 (g) + /202 (g) = H 20(g), is strongly negative at 25°C and I atm pressure, no detectable quantity of water vapour would be obtained on keeping a mixture of hydrogen and oxygen gases at 25°C and I atm pressure for a very long time. This is because the attainment of equilibrium of the forward reaction, being characterized by a high activation energy, is extremely slow. Indeed, for thermodynamic description of the rate processes, one has to have a recourse to irreversible thermodynamics (Katchalsky and Curran, 1965; Sanyal, 1981; Nanda and Sanyal, 1995), but this branch of thermodynamics has seldom been used in the description of soil processes. Thermodynamic considerations, leading to equilibrium constant, are, strictly speaking, applicable only to a closed system. Soil K being subject to fixation, release, leaching, plant uptake, and also addition through fertilizers, irrigation water and crop residues, does not satisfy the description of a closed system.