Patterns of Soil Nitrogen Release in Paddy Fields of Warm Regions in Japan and Diagnosis of Fertilizer Application : I. Influence of soil nitrogen release from paddy soils on growth and yield of rice plants of warm regions

The Influence of nitrogen release from paddy soils on the grain yield of rice plants was investigated, and the optimum amount of nitrogen fertilizer application was estimated for each of the soil nitrogen level. The amount of soil nitrogen released (SNR) from paddy soil was estimated theoretically by the kinetic method. The result showed that the amounts differed remarkably among soils. The amount of soil nitrogen absorbed (SNA) by rice plants was estimated from the nitrogen accumulated in the rice plants grown in the field with no nitrogen fertilizer. The average value of SNA/SNR was 70-80%. The yields of Nishihomare increased in proportion to the amount of SNA when the amount of SNA was less than 8 kg/ 10 a. The yield reached the plateau when it was about 9 kg/10 a and the yield did not increase in the field having above 9 kg/10 a of SNA. Consequently, it was recognized that the decrease of the amount of nitrogen fertilizer application was needed in that field. In the case of Hinohikari, a high quality rice variety, the optimum amount of basal nitrogen was 6 kg/10 a in the field where soil nitrogen level was low, 4.5 kg/10 a in the field where SNA was 7-8 kg/10 a, and less than 4.5 kg/10 a where SNA was more than 8 kg/10 a.