連年の乾田直播が水稲収量を低下させる原因について : 第3報 地力消耗の影響について

抄録

In some cases of direct sowing culture (direct sowing of paddy rice under upland condition) 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. In preceding papers, we have reported about the causes of this problem from two points of view, i.e. the movement of inorganic nitrogen in the soil and phosphorus nutrition. In this paper, the cause is described in relation to the fall of nitrogen supplying potentiality of soil. The experiments consist of chemical analysis of the soil (contents of total-N, total-C, ammonification of organic nitrogen in the soil) and yield analysis under the fertilizing condition of basal application but no top- dressing. The results are presented as follows: 1. Chemical analysis of the soil. Both the contents of total nitrogen and total carbon in the soil decreased according to the succeeding year under the continual direct sowing culture. There was a decreasing tendency in the ammonification of organic nitrogen in the soil (temperature-raising effect and pre-drying effect in ammonification), by the continuance of direct sowing culture. From above facts it is recognized that the further continuation of direct sowing culture decreases the amout of easy composable organic matter in the soil, and that it results in the fall of nitrogen supplying potentiality of soil. 2. Growth and yield of rice. There was observed a tendency of rice yield decrease by the continuance of direct sowing culture accompanied with the fall of nitrogen supplying potentiality of soil. 3. From the above facts it could be concluded that the fall of nitrogen supplying potentiality of soil must be one of the causes of the yield decrease in the continuous direct sowing culture. This yield decrease could be lightened to some extent by increasing the ammout of nitrogen application or more frequent top-dressing of nitrogenous fertilizer. However, the supply of such organic substances as compost should be necessary to inhibit the decrease of rice yield in the continual direct sowing culture.