Abstract
It is well known that application of heavy rate of nitrogen fertilizer to obtain higher yield of rice give a negative effect a cool weather year. For adequate nitrogen application, critical nitrogen cantents in leaf blade at the booting stage in relation to booting sterility were surveyed in the materials which were supplied with nitrogen : basal and top dressing at the spikelet differentiation stage. The curves of nitrogen content in leaf blade at the booting stage-relative value of fertility were drawn by free-hand. Relative value of fertility generally decreased slowly at first, then very rapidly with the increase in nitrogen content. The advance of these processes was similar irrespective of different temperatures, varieties and experimental sites (Figs. 5, 6, and 7). The critical nitrogen contents in leaf blade which were calculated from these curves (content at which fertility attained 90% of fertility in minimum nitrogen content plot) were 3.0∼3.3% in Shiokari and 3.3∼3.6% in Ishikari, respectively.
