Release Potential of Non-labile Potassium in Some Entisols, Alfisols and Inceptisol

The reserve of non-exchangeable potassium was studied in two Entisols, two Alfisols and an Inceptisol of West Bengal. Step K and constant rate K were evaluated by repeated extraction of the soils with boiling nitric acid. Threshold K levels in tenns of K concentration, exchangeable K and activity ratio of equilibrium soil solution, below which the release from initially non-exchangeable K reserve starts, was also measured in 0.01M and 0.002M CaCl2 solution for the given soils. Threshold K levels in terms of K concentration, exchangeable K and activity ratio varied with change in concentration of the background electrolyte. The reserves of step-K content in the given soils were related to the illitic mineral content of the clay fraction of the present soils. Specifically held K measured in terms of exchangeable K, below which Gapon constant (KG) showed a sharp increase, varied from soil to soil in the same way as observed for threshold K levels in these soils. The importance of the study in tenns of the possible contribution from initially non-exchangeable K (i.e., intermediate K) reserves in the given soils to the plant available pool of soil K has been highlighted.