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

•Non-exchangeable K of soil release K to the soil solution on a long-term basis•Step-K has significant positive correlation with non-exchangeable K.•The cumulative release of non-exchangeable K followed a semi-logarithmic behavior.•Higher threshold value of Entisols and Inceptisols as compared to Alfisols•Higher threshold value indicates less tenacity with which K is held in wedge zones.

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.