Wheat is probably the most important, as a counter-plot for increase of demand of wheat in Japan and failure of the rice-crops of the past few years have sharply emphasized. Although much work has been done to determine the effect of fertilizers or soiltypes or assumption concerning the iufluence of the weather factors on the yield and quality of wheat, yet the actual effects of various kinds of weather upon the quality and yield of wheat are only imperfectly known Relation between the wheat and climate is very much complexer than in the case of the rice-crops and then climatic factors as well as soiltype or manures, may be expected to influence the yield and quality of wheat. This study is an attempt to determine climatic zones for wheat culture, in which it can be grown successfully or unsuccessfully. There are following two general methods by which such a problem may be attacked. One is the experimental method of wheat at the field of experimental station under various kind of weather factors conditions. The other is the collation or statistical method, in which the actual yields under commercial conditions are compared in historical series (A. annual comparative investigation) or local series (B. local comparative investigation) with the recorded weather conditions.
In this study has been investigated relation between the wheat and climate by the latter method. Winter wheat (Triticum Vu'gare, Host) in Japan is planted for the 3 months from September to November and harves_??_ed in June or July, and is therefore subject to the influence of the weather for the 8-10 months.
A. Annual comparative investigation.
Correlations between annual weather elements (monthly mean of temperature, monthly total hours of sunshine and monthly amount of precipitation) and annual yields (1902-1933) per Tan of wheat for the 5 prefectures (Miyagi, Tochigi, Okayama and Fukuoka) are low. Comparative larger coefficient of correlation of the weather elements with the wheat yields shows Following.
B. Local comparative investigation.
(1) Correlations between the yield and precipitation are largest (-0.750 for all period of cultivation) than any other weather elements.
(2) Correlation coefficients between the yield and sunshine are positive and in the case of the middle period (from January to March) the coefficient is largest (+0.621).
(3) In regard to correlation of monthly mean of temperature for cultivated period of wheat with the wheat yield there appears that in the warm locality (more than 11°C) the coefficient in large and negative (-0.750), indicating that the yield was increased by a lowered temperature in the locality.
C. Determination of climatic zones of wheat.
As a result of above both investigation has been determined fourteen climatic zones of wheat in Japan Proper by the various degrees of analogicality or adaptability of climatic environment for the wheatcrops. Wheat grown in Japan Proper, on an average, a higher yields when grown in the region of slightly precipitation (for instance, coast of the Inland Sea, the Kanto district, etc., ) than they did when grown in the humid climate of Japan (for instance, coast of Japanese Sea and southest district).