Abstract
The change in salinity distribution accompanied by drainage and drying of soil layers was investigated primarily in an experimental field (20ha in area) whose land draining was completed at the beginning of 1977. Just after this drainage, a large quantity of salt accumulated on the surface layer of the soil. Following this, desalinization advanced from the surface soil layer by the leaching action of rain. Within a period of three years, the salt became distributed in a straight line extending from an upper layer of low salinity to an under layer of high salinity. However, subsequent desalinization became stagnate, and so countermeasures for salt exclusion were examined in order to determine the necessary conditions for salinity distribution that would ensure normal growth of field crops.
1. To promote desalinization, the use of salt accumulation on the soil surface accompanied by dryness of soil layer and the leaching of that salt by rain and irrigation water were found effective.
2. To prevent the concentrated accumulation of salt in a root zone (about 0-30cm layer), the adequate improvement of the physical properties of the soil and irrigation were found to be of the utmost importance.
3. The drainage capacity of underdrain and the quantity of excluded salt were directly in proportion to each other. Based on the above findings, it was concluded that the foremost countermeasure for salt exclusion was the enhencement of the permeability of the under layer and second, the exclusion of saline percolation water and then the control of the rise of the groundwater table by the underdrain.
