The aim of the present studies was to make clear the productive efficiency of nitrogen absorbed at different growth periods for the rice plant. The whole growth stage of the rice plant was divided into 9 periods which were experimentally made by adopting the water culture method and by controlling the nitrogen supply. The results of this investigation showed that there were two periods in which maximum efficiency of nitrogen appeard. The first one showed itself when the total nitrogen absorbed in 3 plants amounted to 170mg for the grain and 100g for the straw production, regardless of the stage of the growth. On the contrary, the second one appeared in a definite period of the growth stage about 14 days before heading indepently of the amount of nitrogen absorbed; and it is interesting to note this second maximum efficiency appeared only in the case of grain production. Basing upon these data, the authors obtained an iso-efficiency map of nitrogen for the grain production in regard to the amount of nitrogen absorbed and the whole growth period of the rice plant. By using this map, the relation between the grain yield and the course of nitrogen absorption may be examined.

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