The ability of the soil to produce crop is dependent not only upon the proper supply of nutrients but also upon air and water relations that would make possible the most efficient usage of available nutrients by crop. The composition of the soil air depends primarily upon the extent of biological processes and the ease with which it is renewed by exchange with atmospheric air. Growing plants as well as most soil micro-organisms utilize oxygen of the soil air and give off carbon dioxide. These values change with season, soil, crop, tillage and activity. In this paper, therefore, the authors study especially the mechanism of moisture decrease in the field soil, as the first step to establish seasonal moisture management of field in Kyushu region. From this study the following results were obtained. 1) It was confirmed that at the low soil moisture content, crop controls the evapotrnspiration rate itself with disregard of the climate condition. 2) The evapotranspiration rate during the vegetative period changes accordingly to kind of crop, the vegetative stage and period of the vegetative season. 3) In expressing the soil moisture content, the volumetric expression has following advantages in comparison with the gravimetric expression. First, the relationship among soil, air and water can be known easily. Secondly, in the case of two soils of different weight, even if soil moisture contents of these soil in term of gravimetric expression are equal, soil moisture contents in term of volumetric expression may not be equal (Table-2) . 4) The validity of dividing Kyushu region into the north and south districts, on the estimation of the evapotranspiration rate of crop as criteria in Kyushu region was investigated, as a result it was concluded that this division is valid practically. 5) The result of this study can be used as basic data in deciding the consumptive use of the crop and irrigation interval in the main field in Kyushu region.
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