Division of Agricultural Chemistry and Soil Science, College of Agriculture, Calcutta University, 35 Ballygaunge Circular Road, Calcutta, 700019.
Present addresses: 1ICAR Research Complex for NEH Region, Manipur Centre, Lamphelpat, Imphal, 795004.
Detailed characteristics of the nonexchangeable K fraction of soils of different orders, namely, Entisol, Alfisol, Mollisol and Inceptisol were studied. The non-exchangeable K reserves have been categorized into step K and constant rate K. The threshold level of K release from the intermediate K in terms of activity ratio, K concentration and exchangeable K has also been determined. The mineralogical composition of the soils varied widely. The amounts of nonexchangeable K and step K were significantly correlated with the illitic fraction content and the percentage clay present in the soil. The CR-K and specifically held K were also significantly correlated with the percentage of illitic minerals present in the clay fraction. The threshold values in terms of K concentration and activity ratio changed considerably for all the soils as the electrolyte concentration decreased from 0.01 to 0.002 M. Soil samples studied released intermediate K at a very low activity ratio except Inceptisol.
Nonexchangeable K fraction, intermediate K, threshold level, Entisol, Alfisol, Mollisol, Inceptisol
