Improvement of Drainage in Clayey Rotational Field by Introducing Winter Crops After Conversion from Paddy

Abstract

Rotational upland fields with heavy clay soil in Hokuriku area often face the problem on drainage of surplus water, due to both the rich precipitation during winter and poor permeability of water in soil. We made some experiments about improvement of the soil pore structure by planting some winter crops, and its effect on drainage of water. Four experimental plots, each of which differs in planted crop or period after conversion from paddy to upland field, were set:(1) wetland rice-Italian rye grass-soybean (2) wetland rice-barley-soybean (3) wetland rice-fallowsoybean (4) wetland rice-Italian rye grass-soybean-barley-soybean. On each plot, tensiometers were set and the suctions of soil moisture were recorded. Soil survey were also conducted before sowing soybean, and some soil physical properties were measured. From these experiments, following results were obtained : 1) During autumn and winter, soil never dried more than the field capacity ; improvement of permeability in plow layer proved to be effective for rapid drainage of soil water to the ditches on the surface of the fields. 2) By planting winter crops, volume of soil macropore in plow layer increased and hydraulic conductivity raised. Especially as the first winter crop after conversion from paddy, Italian rye grass grew well and its root zone contributed to the development of remarkable macropore in plow layer. 3) At the plot No. (4) where one year and a half passed after conversion from paddy, the soil structure at plow layer highly developed. As the result, the surplus water was drained smoothly and soil dried rapidly in spring. 4) Transpiration from winter crops plays an important role in drying of the field in spring. Transpiration rate is so deeply related to the growth of the crops that drying rates of fields depend on the condition of crops.