Studies on two times cropping of rice in North Kanto district and tentative classification of the zone of two rice crops a year in Japan, from the point of temperature.

Abstract

(1) From the results of this experiment, if we transplant early the rice seedlings in the hot bed of the high thermo-sensitive varieties as Hokukaido's Norin-15 etc. and after the 1st time rice harvest, transplant the temporary planted rice seedlings of the varieties capable of bearing good yield under late transplanting as Norin-25 etc., it has been found that the rice culture of two times a year is possible at Mito, Ibaraki Prefecture. On this case, it is key point in Mito that the harvest of the 1st time rice culture is possible by July 20th. The total yield of the 1st and the 2nd time rice culture was about 150 % of the normal rice culture. (2) Generally speaking from these experiments ; 2 places besides Mito in Ibaraki Prefecture and 4 places in Koti, Mie, Shizuoka and Kanagawa Prefecture, the two time rice culture is possible in the land where the mean air temperature in May is higher than 16°. But the higher the mean temperature of May, the easier the practice of 2 times cropping of ride is possible. On the two time rice-culture in Japan, the writer will call tentatively the land where the mean air temperature in May is 16°∼17°C is the North-limit zone (North Kanto, Tosan including North half in Gifu Prefecture and Hokuriku districts) and the land where higher than 17°C is the zone where 2 times of cropping may be made without particular difficulty (South Kanto, Tokaido, Kinki, Tyugoku, Shikoku and Kyushu districts). The latter tentatively may be classified into three districts, according to the manner of the cultural practices.