Improvement of Salt-affected Soils. 1. Interception of Capillarity

A new method is proposed for soil improvement of salt-affected soils in regions where a sufficient amount of rainfall occurs in summer. A coarse layer is provided in the sub-soil, and the capillarity from groundwater is cut off. Thus, the rise to the soil surface of salts which are dissolved in the groundwater can be prevented, and even if the groundwater level is high, the evaporation of water from the soil surface can be reduced. Moreover, the salts that accumulate in the topsoil can be washed out by rainfall (leaching) during the summer season. Consequently, this method has the same effect as when the groundwater level is lowered. This paper deals with the proof of this concept with a model experiment using cylindrical soil bins in an indoor test room. The results show that after infiltration of a 2% solution of Na2CO3 into pseudogley soil (clay) from Japan, the pH values of every soil horizon increased from 5·9 (acid) to 9·7-10·8. This process formed a sodic soil (solonetz). In the pseudogley soil cylinder without a coarse layer (natural condition), after repeated applications of Na2CO3 capillarity and simulated rainfall, the pH values of all horizons converged at about 9·8, which corresponds to the same condition as the natural solonetz field. The simulated rainfall yielded the soil water content of every horizon of about 50% (dry basis). The rainfall greatly reduced the electrical conductivity (EC) values but Na2CO3 shifted the values upward again. The values for EC finally converged to about 300 mS m-1 which almost the same as the EC values of the natural solonetz horizon. On other hand, in the pseudogley soil cylinder with the coarse layer (our proposed treatment), the pH values of the horizon above the coarse layer decreased gradually after repeated application of simulated rainfall, and there was no increase of pH value upon addition of a Na2CO3 solution. About 3900 mm rainfall (six and a half years' supply) was required to get a pH value of 7·5 in every horizons. The minimum measured soil water content was 17·6% d.b. at the soil surface, and the soil water content of the C horizon below the coarse layer was constant at about 60% d.b. The EC values at the soil surface, the Ap and B horizons above the coarse layer had small fluctuation and converged 50 mS m-1. The EC value at the C horizon below the coarse layer varied sharply the same as that of the pseudogley soil cylinder without the coarse layer.