Akira Miyasaka

Studies on the Causes of Rice Yield Decrease Resulting from Continual Direct Sowing Culture : II. In relation to phosphorus nutrition

In some cases of direct sowing culture (direct seeding culture of paddy rice on upland field) 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 the previous paper, the cause of this problem has been reported with reference to the movement of inorganic nitrogen in the soil. In this paper the cause related to phosphorus nutrition is described. Two kinds of paddy soil were chosen, one was alluvial which is rich in available phosphorus and the other was diluvial which is not so rich as the former. In the case of direct sowing culture, the experimental plots were divided by the number of years and the amount of phospsorus applications. Especially the difference between direct sowing and transplanting culture, caused by phosphorus applications, was examined. The results are presented as follows: 1. Direct sowing culture in diluvial paddy soil. The content of available phosphorus in the soil and of the phosphorus in the rice plants in diluvial soil were lower than those in alluvial soil. Heavy phosphorus application gave the remarkable good effect to the growth and yield of rice in diluvial soil. It was observed that the growth and yield of rice tend to be retard year by year by continuous direct sowing culture under different levels of phosphorus application. However, the tendency was not so much remarkable in heavy phosphorus application plots. 2. Transplanting culture in diluvial paddy soil. The effect of heavy phosphorus application on the growth and yield of rice plant was less in the transplanting culture than in the direct sowing one. It was attributed to the difference of phosphorus absorption between transplanting and direct sowing culture. 3. Direct sowing culture in alluvial paddy soil. The incrcased levels of phosphorus applications were not effective to the growth and yield of rice plant in this soil due to the high content of available phosphorus which was sufficient for the growth. 4. From the above facts it could be concluded that the decrease of yield in the continuous direct sowing culture caused by the shortage of phosphorus absorption could be recognized only in such soils that were low in available phosphorus content. This yield decrease could be lightened to some degree by increasing the amount of phosphorus application. On the contrary, in alluvial paddy soil which was high in available phosphorus (most part of paddy field in Japan is occupied by this soil), such decrease could not be found, even under continuous direct sowing culture.