Potassium balance and release kinetics under continuous rice-wheat cropping system in Vertisol

In an experiment conducted over 8 years at the research farm of the National Research Centre for Weed Sciences, Jabalpur, we investigated the effects of mineral fertilizer and organic manures on K balances and non-exchangeable K release kinetics in a Typic Haplustert under a rain-fed rice (Oryza sativa L.) and irrigated wheat (Triticum vulgare L.) system. Each year, rice was grown with variable levels of fertilizer N with and without organic manures, while recommended rates of 26 kg P ha(-1) and 33 kg K ha(-1) were applied to both the crops. Wheat was grown without organic manure but with the same levels of fertilizer N as for rice. The apparent K balance was measured as the difference between total K added and that removed by the crop. Increasing levels of fertilizer N resulted in an increase in the negative K balance from 56 kg ha(-1) yr(-1) (control) to 103 kg ha(-1) yr(-1) at 90 kg N ha(-1) and 156 kg ha(-1) at 180 kg N ha(-1). Incorporation of farmyard manure (FYM) or green manure (GM) with fertilizer N reduced the negative K balance. Repeated extraction of soil with 0.01 M CaCl2 revealed that continuous cropping for 8 years either without fertilizer N (control) or with 90 kg N ha(-1) reduced the cumulative K release. However, the application of 180 kg ha(-1) of fertilizer N not only maintained the cumulative K release but also improved it. Incorporation of 5 t FYM or 6 t GM ha(-1) with 90 kg N ha(-1) resulted in an increase in K release by 58 and 37 mg kg(-1) soil, respectively, over the values measured in a soil sample collected at the onset of the experiment. A parabolic diffusion equation described the release rate of the non-exchangeable K. The equation suggested that the application of fertilizer N reduced the release rate of K, whereas the incorporation of manure, with fertilizers, increased it over the values obtained from soil collected at the onset of the experiment. The large cumulative K release at 180 kg N ha(-1) was due to the amount of K released initially. Continuous cropping at a fertilizer level of 33 kg K ha(-1) may pose a threat to the sustainability of the rice-wheat system. Recycling of crop residue or application of higher levels of fertilizer K may provide long-term sustainability to the system. (C) 2002 Elsevier Science B.V. All rights reserved.