Threshold Levels of Potassium for Intermediate Potassium Release in Relation to Clay Mineralogy and Specifically Held Potassium of Soils

Threshold K levels in terms of K concentration, exchangeable K and activity ratio of equilibrium of soil solution below which release of intermediate K was initiated, were studied at 0.01M and 0.002M CaCl2 solution in relation to soil clay minerals and specifically held K for some Inceptisol and Ultisols of West Bengal. Threshold K levels in terms exchangeable K and activity ratio varied with change in concentration of electrolyte from 0.01M to 0.002M CaCl2 solution, but it remained almost unaffected when measured in terms of K concentration of equilibrium soil solution. Illite-dominant saline soils showed higher threshold K values than the smectite-dominant Inceptisols as well as kaolinite dominant Ultisols. Specifically held K measured in terms of exchangeable K, below which Gapon constant (KG) showed sharp increase. varied withthe same trend of variation from soil to soil as was noticed for threshold K levels in these soils.