In alluvial plains along the northern to central part of the Japan Sea Coast of the Main Island, Japan, namely West Tohoku-and Hokuriku Districts, paddy fields of heavyclay soils are widely distributed. Because of the extremely low water permeability ofthe subsoils, these paddy soils are characterized by ill-drainage and poor bearing capacity.Consequently, it is difficult to use large-sized farm machines and to practice crop rotationincluding upland crops, and the productivity of rice is naturally low. In addition, improvement of these low land heavy clay soils to well-drained paddy soils by theoperation of underdrainage is difficult because of their poor water permeability and largewater holding capacity. Techniques of their conversion to upland state and changesof physical properties of the soil in the process of drying were studied. The resultsobtained are summarized.
1. Gley horizon of low water permeability of the ill-drained clayey paddy soils graduallycracked by drainage and subsequent drying, and finally, became to show a highwater permeability.
2. After drainage and subsequent drying, a horizon of high bulk density was formeddirectly below the plow layer. This horizon obstructed the percoration of gravitationalwater as well as the upward movement of capillary water.
3. Conversion of an ill-drained paddy into an upland field was accompanied by anincrease in the macro pores of the plowlayer and aeration was, therefore, improved.Although the holding capacity of available water was reduced in the early stage of theconversion, it gradually increased with the lapse. of time.
4. As for the growth of a crop (corn), growth and yield were poor in the earlystage of the conversion. This was attributed to a violent fluctuations in soil moisturetensions. Afterwards, however, the yields increased according to the improvement ofthe drainage. It took a few years to complete a conversion into the upland field.
5. It was found that a drainage system including mole drain and block drainagewas required for conversion of an ill-drained clayey paddy soil to an upland state. Inorder to accelerate soil drying, growing crops having deeply penetrable root systemand operation of sub-soiling are desirable.
6. Application of rice straw and aluminum chloride increased macro pores and holding capacity of available water, respectively, and these materials improved the soilconsistency.
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