Release pattern of non-exchangeable potassium reserves in Alfisols, Inceptisols and Entisols of West Bengal, India

Abstract Eight surface soil samples representing three soil orders viz., Inceptisols, Alfisols and Entisols were analyzed to characterize the non-exchangeable potassium (K) reserves. The mineralogical composition of the experimental soils varied widely. The reserves of Step-K and Constant rate K were computed by repeated extraction of soils with boiling1 M HNO3. The cumulative release of non-exchangeable K by such repeated extraction followed a semi-logarithmic behavior with number of extractions, suggesting that the release of non-exchangeable K decreased with successive extractions. The threshold levels of K in soil solution below which the release of K from the initially non-exchangeable K reserves starts were also evaluated for the selected soils in terms of K activity ratio, K concentration and exchangeable K in 0.01 M and 0.002 M CaCl2 solution. Higher threshold value of Entisols and Inceptisols compared to Alfisols indicates less tenacity with which K is held in wedge zones of micaceous minerals. These threshold values changed considerably for all the soils as the electrolyte concentration decreased from 0.01 M to 0.002 M. Specifically held K, determined as exchangeable K below which the Gapon constant (KG) showed a sharp rise, varied from soil to soil in almost the same manner as noted for threshold K levels in these soils.