Plant utilization of native soil potassium reserves by successive rice crops in Entisol, Inceptisol and Alfisols

The reserves of various forms of potassium (K), the threshold values for release of intermediate K, and the quantity/intensity (Q/I) parameters were studied in an Entisol, an Inceptisol and two Alfisols of West Bengal, India. The laboratory experiments were supplemented by a pot-culture experiment in absence of K fertilizer. The purpose was to study the release pattern of K in a rice-rice-rice cropping sequence. The results indicated that the Entisol and Inceptisol contained a higher amount of easily extractable non-exchangeable K, i.e. step-K. This was related to the dominance of micaceous minerals in clay fraction. The step-K content decreased in all the soils on subjecting them to repeated cropping. The Alfisols suffered a higher percentage decline in step-K than the Entisol and Inceptisol as observed after a sequence of three crops. The threshold values for release of intermediate K, showed a decrease in the Entisol and Alfisol studied after the third crop as compared to the first one. This was related to rather low diffusion coefficient of K for its mobility in the interlayer region. The Q/I parameters were favorable for crop growth in the Entisol examined and decreased in all the soils in course of repeated cropping.