Studies on the Relationship between the Change of Growing Season and Irrigation in Upland Crops. : II. The relationships among planting time, plant population and irrigation in soybean culture.

Abstract

The relations among planting time, plant population and irrigation in soybean culture were investigated in order to appraise the practicability of irrigation in an early and a late season culture, in 1962. The results may be summarized as follows: (1) The irrigation was able to keep living leaves at the position which perviousness of light is less than the case of non-irrigation. But irrigation increased height of yellowing leaf, because it makes the leaves large and in result lessened remarkably perviousness of light into plant community. However, the thickness of leaves layer was not different between irrigation and non -irrigation plots by the reason which plant height was increased by irrigation, and also was not so much different among plant populations. Number of living leaves was increased rather by irrigation, therefore it was deduced that total leaf area increased by irrigation, and then that the photosynthetic efficiency of leaf of irrigated plot was higher than that of non-irrigation plot during the drought period. (2) In early planting the weight of stem was increased little by irrigation, but in late planting it was increased strikingly by irrigation owing to the increase of stem diameter and length. The elongation of internode is phytosociological phenomenon which occures by the deficit of light perviousness into plant community in principle. But it was affected considerably by soil moisture content, and it was recognized that the influence of soil moisture content was larger in pod-setting period when amount of transpiration increased notably. (3) The irrigation heightened or maintained highly the rate of seed yield to stem weight (seed/stem ratio). Because irrigation mitigated remarkably deficit of soil moisture derived from the vigorous growth in the most important pod-setting period when transpiration showed the maximum amount and number of pods was determined. Therefore irrigation increased pod number per m2 and seed weight per 100 seeds in each cases, and heightened the limiting point of their increase according to the high plant population, and in consequence irrigation brought similar effects to seed yields. (4) In early planting, irrigation increased seed yislds by elevation of seed/stem ratio mainly, and in late planting, increased seed yields by increase of stem weight and maintenance of higher seed/stem ratio. The seed yields of late season culture (planting early in July) were approximated to those of early season culture by irrigation and considerable high population, therefore in the case of irrigation, it was made clear that the combination between wheat or barley and late season culture of soybean was profitable strikingly.