Nitrogen-supplying capacity of soils for rice and wheat and nitrogen availability indices in soils of northwest India

Quantitative assessment of soil nitrogen (N) that will become available is important for determining fertilizer needs of crops. Nitrogen-supplying capacity of soil to rice and wheat was quantified by establishing zero-N plots at on-farm locations to which all nutrients except N were adequately supplied. Nitrogen uptake in zero-N plots ranged from 41.4 to 110.3 kg N ha(-1) for rice and 33.7 to 123.4 kg N ha(-1) for wheat. Availability of soil N was also studied using oxidative, hydrolytic, and autoclaving indices, salt-extraction indices, light-absorption indices, and aerobic and anaerobic incubation indices. These were correlated with yield and N uptake by rice and wheat in zero-N plots. Nitrogen extracted by alkaline KMnO4 and phosphate borate buffer and nitrogen mineralized under aerobic incubation were satisfactory indices of soil N supply. For rice, 2 M KCl and alkaline KMnO4 were the best N-availability indices. Thus, alkaline KMnO4 should prove a quick and reliable indicator of indigenous soil N supply in soils under a rice-wheat cropping system.