Abstract
In a previous paper, it was reported that the yield of rice of various districts in Japan was influenced by the weather conditions, particularly by the temperature and the amount of solar radiation in the ripening period. A high correlation was found between the yield and the amount of solar radiation. As to the effect of temperature, however, it was, shown that there exists a certain optimum temperature. The present experiment was carried out to apply this theory to a practical phase and to avoid the ill-effect of high soil-temperature and at the same time to increase the utilization of radiation energy of rice population by increasing reflection at the soil surface. The surface of paddy field was covered with white, foamed styrol board from the panicle-formation-period to harvest. The effect of such treatment on the soil-temperature, reflection of radiation, dry matter production and yield was investigated. A medium-term variety, Norin 29, was used. Three different seasans of cultivation were adopted. The period of mulching treatment was two, a long term one (from 40 days before heading to harvest) and a short term one (from 25 days before heading to harvest). 1. In treated plots, grain yield was increased about 10-15% compared with the non-treated plots in the standard and late cultivations but no increase in early cultivation. 2. In treated plots, daily maximum soil temperature was lowered and the reflection of radiation at soil (mulch) surface was increased. 3. In treated plots, the respiratory activity of root at later growth stages was maintained higher than non-treated plots. 4. Any enhancement of the photosynthetic activity of leaves was not brought about by the treatment. It was supposed that the favorable effects of higher activities of roots were masked by the decrease in nitrogen content in leaves as a result of the retarded decomposition of organic nitrogen in the soil due to lower soil temperature. 5. In spite of this, in treated plots the quantity of assimilated products which were assimilated after heading stage was more than that in non-treated plots. It was considered that this phenomenon was due to the increased reflection of radiation from the soil surface. 6. This treatment was effective only when it was applied at comparatively late growing stage. Before panicle formation period, it was found to be unfavorable for the growth as well as for the grain yield.
