SOIL N SUPPLY AND PLANT N UPTAKE BY IRRIGATED RICE IN TAMIL-NADU

This study was undertaken to assess the N supply capacity of some irri gated rice soils of India and its relationship with N uptake of crops with and without fertilizer N supply and to simulate effects of differ ent soil-N supply patterns on optimizing fertilizer N application usin g the MANAGE-N model. Soil samples were collected from N-0 plots (no f ertilizer N applied) of a multilocation field experiment with rice (Or yza sativa L.) conducted in Tamil Nadu, India, during the southwest mo nsoon season (July-October 1994) with cultivar IR64. The N supply capa cities of soil samples were assessed by static soil test methods and a naerobic incubation with and without K-saturated cation exchange resin . The experiments had a common set of treatments of different N applic ation strategies. Plant samples were collected at different growth sta ges and N uptake was measured from the biomass and N content, MANAGE-N was used to optimize N application with different soil N supply regim es, The results revealed that soil-N supply capacities assessed by bot h static and anaerobic incubation methods were related to plant N upta ke up to first flowering (r(2) = 0.64 to 0.85) but they failed to corr elate with grain yield. Actual N uptake rates of the N-0 crops during different growth periods ranged from 0.28 to 1.17 kg ha(-1) day(-1). S ome of the soils could supply N equal to that of a sufficiently fertil ized crop in the initial period. Based on the soil-N supply capacities , the soils could be classified into those not requiring basal N and t hose requiring basal N. Simulation results revealed that if the soil N supply regime observed at Ambasamudram were available at Paiyur, the grain yield of N, crop would increase by 38% and there would be 7 to 1 1% increased yield for 100 and 200 kg N applications and less fertiliz er N would be required for different yield levels obtained with the Pa iyur soil-N supply regime. MANAGE-N generated fertilizer N recommendat ion curves that identified different optimal timing of N application f or different soil N supply regimes.