水田の適正減水深について (III)

The author practiced an experiment in the irrigation season of 1958, using such an apparatus as shown in Fig. 1 and the picture, by which the percolation amount alone might be changed out of the other several conditions for paddy rise culture. The lysimeter consists of 10 concrete water tanks, 0.9m×0.6m×0.6m (depth) which were filled with 25cm deep top-soil as the first layer, sand as the second layer and gravel as the third layer respectively ; and paddy rice plants were planted in them. A device was made so as to have 0.5mm, 15mm, 25mm, 35mm and 45mm per day as percolation amounts by regulating the size of an orifice which was made on the lower side, And the following items of survey were made : <BR>Growth survey (number of offshoots ; height of plants), water and soil temperatures of paddy fields (for the depth of 15cm from the ground surface), change of nitrogenous concentration of ammonium substance in the percolated solution, and production survey. As the results of the above the following were clarified : -<BR>(1) The production in 5-15mm plot was maximum, and 15mm, 25mm, 0mm, 35mm, and 45mm came next in order.<BR>(2) As for the water temperature of paddy fields, 0 mm plot was higher as compared with the others ; on the contrary, the soil temperature of this plot was lower by 0.5-1.0°C than the others.<BR>(3) The nitrogenous concentration of ammonium substance and the various growing phases of paddy rice were closely related with each other as shown in Fig. 4 and it is especially a matter of great interest that the height of concentration almost corresponded with the paddy rice production after the middle of August. From the phenomenon the fertility of paddy soil is shown with comparative accuracy by the decomposition of total nitrogenous organic substance, or by the degree of availability based on the concentration of percolating ammonium ; and moreover, it may be learned that the degree of percolation into the soil have a great influence upon the above phenomenon. The author clarified that the moderate percolation quantity of paddy fields is 15-25mm per day from the above result, about the cause of which he also investigated to some extent.