Studies on the Causes of Rice Yield Decrease Resulting from Continual Direct Sowing Culture : I. Effects of continual direct sowing culture on the movement of inorganic nitrogen in the soil and on the growth and yield of rice

[摘要] In some cases of direct sowing culture in the Kanto district, there is observed a year by year decrease in yield. The purpose of this experiment is to clarify the reasons for this decrease. The experiment wes carried out on wel1-drained paddy fields from 1968 to 1970. Experimental plots were divided by the number of years of direct sowing culture-first year plot, second year plot, third year plot and long term plot, and the combination of the amount of nitrogen application-0, 12 and 18g per square meter. Results obtained are as follows : 1. Growth and yield. Both the plant growth in the early stage and the yield were greater in the first year plot than in the long term plots. However there were found of apparent differennces in these properties among the second year plot, third year plot and long term plot. 2. Inorganic nitrogen in the soil. The movement of inorganic nitrogen in the soil in the first year plot apparently differed from that in the long term plot. In the first year plot, the amount of ammoniacal nitrogen was higher and the amount of nitrate nitrogen was lower than in the long term plot, in the early stage of upland condition. Accordingly, the ratio of nitrate nitrogen to total inorganic nitrogen in the soil was lower in the first year plot. However there were no significant differences among the second year plot, third year plot and the long term plot, in relation to the amount of inorganic nitrogen and its composition. 3. These results were observed for all nitrogen applicaltion levels in this experment. 4. It may be concluded from the abovefacts, that the better growth and yield of rice in the first year of direct sowing culture are attributed to the retardation of nitrification of inorganic nitrogen inthe soi1 in the early stage of upland conditions.

[学科分类] 农业科学（综合）