Non-exchangeable Potassium Reserves and their Release Pattern in Some Alfisols and Inceptisols of West Bengal

A study was conducted to characterise the non-exchangeable K reserves in three Alfisols and two Inceptisols of West Bengal. The reserves of step-K and constant rate K were evaluated by repeated extraction of soils with boiling molar nitric acid. The reserves of step-K content were, in general, related to the content of the clay fraction of the soils. The cumulative release of non-exchangeable K by repeated extraction with boiling 1 M HNO₃ followed a semi-logarithmic behaviour with increasing number of extractions, suggesting that the release of non-exchangeable K decreased with successive number of extractions. The threshold levels of K in soil solution below which the release of K from the initially non-exchangeable K reserve starts were also evaluated for the given soils in terms of the K-activity ratio, K-concentration and exchangeable K in 0.01 M and 0.002 M CaCl₂ solution. These threshold values registered change with the change of background electrolyte concentration. Specifically held K, determined as exchangeable K below which the Gapon constant (K_G) showed a sharp rise, varied from soil to soil in almost the same manner as noted for threshold K levels in these soils. These trends have been related to the properties of the soils, especially their clay make-up.