Comparison of the Moisture Extraction Patterns of Some Crops

[摘要] To advance the efficiency of water usage in irrigation practice, it is necessary to determine the depth of soil from which the crop withdraws most of its water. In the present paper a study of water use by the upland rice, sweet potato and soybean crop was reported. Namely, the pattern of moisture extraction under these crops was traced up by using gypsum blocks and tentiometers installed to a depth of 50 cm in the field. The movable shed was used to protect the crop from rain so that the whole water supply was under control, and the plots designated as wet were irrigated whenever the soil moisture tension at l0 cm depths rose to about 2 atm., and the plots. designated as dry were irrigated when upland rice showed some signs of wilting. The results obtained were as follows: 1. The consumptive use of upland rice were consistently highest, followed by soybean and sweet potato, and the peak use rate per day of these crops was 7, 6 and 6 mm, respectively. Although under upland rice nearly all of the moisture was extracted from the first 15 cm of soil, under sweet potato less amount of water was depleted from this depth and relatively more moisture was lost from the 15-30 cm depth than under upland rice. 2. The extraction pattens for a crop differ materially throughout the growing season. Noticeable variations in extraction pattern also result from the soil moisture content. There was an increase in the rate of extraction from the deeper depth (40∼50 cm) when about fifty per cent of available moisture in the shallow depths had been exhausted. Judging from this fact, it seems that in this soil (volcanic ash soil, clay loam) when about fifty per cent of available moisture is exhausted it becomes vary difficult for crops to absorb the water. 3. After the irrigation, water was again removed more from the surface and by far the greatest amounts of water lost from the first 30 cm of soil under both irrigation conditions. But although progressively less amounts were lost from deeper depth under the "dry" condition, relatively more moisture was used from the middle (about 10∼20 cm) depths of soil under the frequent irrigation treatments.

[学科分类] 农业科学（综合）