Studies on the soil improvement and desalinization by gypsum dressing in Kasaoka bay polder

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1. In Japan, Kasaoka bay polder was a first case that planned to use a large scale sea bottom polder to upland field from the beginning of prelimirary utilization of polder in 1986 and a growth obstacle caused by salt was feeled uneasy from just after land draining in summer, 1977. After land draining, various effort for a foundation works of upland field and a establishment of technical program about farming were carried out, but the progress of desalinization and soil improvement was stagnated recently. The studies about its reason made clear a vicious cycle that soil physical property grew worse with a progress of desalinization and this relation continuously resulted in a stagnation of desalinization. For its countermeasure, we had pro-posed a gypsum dressing and established a rational theoretical equation for calculation of gypsum puantity.2. We had pursuited and investigated a progress situation of cation exchange of adsorped sodium ion by calcium ion on the construction area of … More gypsum dressing at the end of 1984. The quantity of gypsum was calculated in conformity with the theoretical equation of cation exchange reaction considering space variability. From the results of 4 years investigation, it was found that the improvement of soil physical properties like the heightening of water retentivity, the lowering of consistency index, the increase of permeability and the increase of vapor ratio were gained because soil became floculated condition by gypsum dressing. Though the degree of gypsum effect were different from place to place, it was showed that the soil improvement progressed gradually in eve y places.3. Judging from the electrical conductivity values (EC), the progress of desalinization from 1985 to 1988 was satisfactory and it arised a favorable places that the goal of desalinization (EC become less than 2mS/cm at 40cm depth.) was attained. This effect was found to be exhibited the improvement of physical and chemical properties by gypsum dressing until 50cm depth with deep rotary tiller. Hereafter, in attendance with the progress of environmental consolidation and farming management, it was convinced that the improvement of soil physical properties and the lowering of salinity progress slowly but steadily. Less