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Saline soils occur extensively in Northeast Thailand. As the climate of this region is clearly differentiated into dry and rainy seasons, salts in the groundwater move up and accumulate on the soil surface during the dry season and are leached down with rainfall during the rainy season. In this paper, the physical and chemical properties concerned with salt movement were investigated for the saline soils in the Khon Kaen Region, Northeast Thailand. Changes in the physical and chemical properties with leaching were also studied in order to determine the effect of rainfall on desaltation. The saline soils in the Khon Kaen Region are generally sandy with low CEC, low content of organic matter and low buffering capacity. The dominant salt is NaCl, which facilitates the dispersion of clay. The main types of clay minerals are I : I type. All the soils contained large amounts of quartz, indicating that they are highly weathered soils. The amount of materials dispersed by water leaching, mainly clay, accounted for the large percentage of clay and silt fractions.

The effects of several amelioration treatments on the physical and chemical properties of the saline soils (Paleustult) in the Khon Kaen Region, Northeast Thailand, were studied. The application of CaS04, CaCOa, converter slag, or compost into the plow layer, the placement of crude oil or rice husks below the plow layer and rice straw mulching were compared.The plots with these treatments were constructed in the field and the moisture content, pH, EC, permeability, clay dispersion, NH4-N and NO~-N contents were determined. The evaluation of each amelioration treatment was as follows. Compost application was the most effective, followed by mulching. The addition of adequate, but not excessive amounts of calcium materials was necessary to increase the soil buffering capacity. The crude oil and rice husk plots showed very wide seasonal pH fluctuations as in the case of the control plot.To simulate rainfall infiltration, a leaching experiment was conducted using soils collected periodically from the above plots. The turbidity level (clay dispersion) of the leachate of CaS04 and CaCOa plot soils was much lower than that of the other plot soils. Compost application also had a suppressive effect on clay dispersion. Clay was dispersed easily in soils from the rice husk and crude oil plots and a high turbidity level of the leachate was observed for the rainy season samples of these plots. By mulching, dispersion was strongly suppressed, which may be due to the stability of the soil aggregates.Leaching speed was negatively correlated with the clay dispersion and it remained high throughout the experimental period (2 years) in the CaS04, CaC03, converter slag, and compost plots.Therefore, the combination of compost and calcium application to the soil with mulching was considered to be a suitable procedure for the amelioration of this kind of soil.

The dynamics of salt movement and the seasonal changes of the physical and chemical properties, especially pH and clay dispersion, of the saline soils in the Khon Kaen Region, Northeast Thailand, were investigated to obtain basic information for the improvement of saline soils. The results obtained are as follows.Not only the salt but also the clay moved upward with the capillary movement of the soil solution, and downward with the percolation of rain water. The soil pH was very low (less than 4) in the dry season and very high (nearly 9) in the rainy season, based on the following reactions: H-clay+NaC1 -*Na-clay + HCI (dry season); Na-clay § H~0-*H-clay § Na0H (rainy season).The soil contained large amounts of dispersible clay in the rainy season, but a small amount in the early dry season. FeS dissolved in the groundwater was considered to be another acidification agent, as it became oxidized to ferric hydroxide (precipitation) and sulfuric acid when it reached the soil surface by capillary movement.Key Words: acidification, alkalinization, clay dispersion, saline soil.The saline soils in the Khon Kaen Region are well weathered sandy textured soils (SL-LS) with a low CEC, low buffering capacity and low organic matter content, and are characterized by the presence of NaC1 as the dominant soluble salt (Patcharapreecha et al. 1989). The climate of this region is clearly differentiated into rainy and dry seasons. Salt moves downward with rainfall during the rainy season while the salt dissolved in the groundwater moves upward and accumulates on the soil surface during the dry season. The concomitant upward and downward movement of clay was also suggested in the model experiment (Patcharapreecha et al. 1987(Patcharapreecha et al. , 1989.In this report, the dynamics of salt movement and the seasonal changes of physical and chemical properties, especially pH and clay dispersion, of the saline soils in the Khon Kaen Region were investigated to obtain basic information for the improvement of saline soils. In addition, the mechanisms of seasonal changes were also studied by leaching experiments.

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