Agroclimatological Studies on the Regional Difference of Soybean Yield

Abstract

This study was made to clear the dependence of the yield of soybean crop on climatic conditions. In the analysis, the yield data of soybean crop obtained at about fifty districts in Japan during the period from 1954 to 1973 were used together with weather records for the same period.The results obtained can be summarized as follows:(1) The average yield of soybean crop during this period was higher in northern part of Japan, especially in Hokkaido than in other part of Japan. The average yield decreased gradually with the movement to lower latitude and reached the lowest value at a south-western part of this country. As shown in Fig. 1, the value of coefficient of variance of the yield in the middle part of Honshu was lower compared with those in northern and western part of this country.(2) To characterize the climatic dependence of the yield of soybean crop (Y), the yield was divided by the mean value of solar radiation during the same responsible period (Q) and plotted against air temperature (T). The relationships presented in Fig. 2 were expressed approximately by the following quadratic equationsY/Q1=-0.51+0.0821T1-0.00197T12, andY/Q2=-1.01+0.1369T2-0.00336T22, where suffixes 1 and 2 denote the quantities related to July-August period and August-September period, respectively. Although the value of coefficient of determination was not very high in the two cases, the mean air temperature during the period from August to September to which corresponds the stage of the pod developing, seems to be more influential on the yield of soybean.(3) Figure 4 shows that the yield of soybean crop correlates inversely with the wetness index defined as the ratio of latent heat equivalent of daily rainfall (Lr, ly day-1) to daily solar radiation (Q, ly day-1). As can be seen in Fig. 4, the yield of soybean crop decreased slightly linearly with the increase of wetness index, in the range above 0.5 of Lr/Q, implying that soil moisture is less effective to the growth and the yield of soybean crop under the so humid climatic conditions as Japan.(4) Figure 6 shows the dependence of the coefficient of variance of yield of soybean crop on the coefficient of variance of mean temperature during the period from July to August. The magnitude of correlation coefficient between both coefficients of variance is significantly higher in a northern district of Japan than that calculated for the whole area of Japan. This fact indicates clearly that the growth and the yield of soybean crop in a northern part of Japan with lower mean and higher variance of air temperature are greatly affected by yearly temperature regime in that district.