Abstract
Three varieties of paddy rice, "Musashikogane", "Asanohikari" and "Yumeminori", transplanted late in May, were cultivated an under no-nitrogen condition. The relation between quantily of soil niLrogen absorption and accumulated value of mean air temperature (accumulated temperature) was applied to a logistics curve and the process of soil rlitrogen absorption by paddy rice was quantitatively analyzed. 1) For all three varielies of paddy rice, Musashikogane, Asanohikari and Yumcminori, the actually measured values agreed well with the equation of Soil nitrogen absorption quantity estimated by accumulated temperature, which was applied to the three-dimensional λ-type logistic curve. Hence. it was concluded that this equation accurately expressed the actual soil nitrogen absorbing process. 2) The quantity of Soil nitrogen absorption by paddy rice increased exponentially till the accumulated temperature reached 920℃ for Musashikogane and 1020℃ for Asanohikari and Yumeminori, and then gradually increased. 3) The quantity of soil nitrogen absorption by paddy rice from the time of transplanting to maturing stage estimated by the accumulated temperature of the normal value was 74 kg ha^<-1> for Musashikogane, 79 kg ha^<-1> for Asanohikari and 72 kg ha^<-1> for Yumeminori. 4) The ratio of soil nitrogen absorption by paddy rice among maximum tillering stage, panicle formation stage and heading time estimated by the accumulated temporature of the normal value was 33% till the maximum tillering stage, 61% till the panicle formation stage and 77% till the heading time for Musashikogane. In the same way, for Asanohikari the ratios were 30%, 61% and 82%, respectively, and for Yumeminori 31%, 64% and 80%, respectively. 5) The quantity of soil nitrogen absorption by paddy rice per day estimated by the accumulated temperature of the normal value peaked at the time of maximum tillering stage. It was on the 39th d before heading time for Musashikogane, 35th d for Asanohikari and 41st d for Yumeminori. After the maximum tillering stage, it decreased gradually till the heading time, but the rate of decrease was smaller after the heading time.
