Study on physical and hydraulic properties of major soil series in northeast, Thailand

Most important soil series in the Northeast are Korat and Roi-et soil series which cover 20 percent and 17 percent of the whole area respectively. Korat soil series represents upland soil while Roi-et soil series corresponds to lowland soil. Since the physical and hydraulic properties of soils are necessary for calculation, design, and modeling of many kinds of work, e.g. irrigation, drainage, hydrology, and the likes, this project was set up to study, experiment and compile physical and hydraulic properties of both soil series. The study was done to a depth of 100 cm soil. Texture of both soils were analysed by sedimentation analysis for fine particles and sieve analysis for coarse particles. After analysis, the textural name given to Korat soil is sandy loam and Roi-et soil is loam. The soil water characteristic curves were determined by the hanging column method for the soil samples at low water potential and by the pressure plate method for those soils with high water potential. From the curves, the water content at field capacity of Korat soil is about 0.12, for Roi-et soil the value is 0.18 at the soil surface and about 0.30 deeper than 75 cm. The water content at permanent wilting point of Korat soil is 0.05 and of Roi-et soil is 0.06 at the soil surface and 0.16 at a depth of greater than 75 cm. Bulk density values, measured from soil cores of 100 cubic m, were about 1.47 gm/cubic m and 1.55 gm/cubic m for Korat and Roi-et soil series respectively. Total soil porosity values, measured by using cored samples of 100 cubic m which were weighed at saturation and weighed again after oven drying, were found to be 0.25 for Korat soil and 0.36 for Roi-et soil. The falling head method was used to obtain the values of saturated hydraulic conductivity (K sat) of both soils. The average K sat-value for Korat soil is 103 cm/day but for Roi-et soil the values vary from 29 cm/day at soil surface to 0.9 cm/day at a depth lower than 75 cm. By using the double ring method, the constant infiltration rate of Korat and Roi-et soils are 75 mm/hr and 6 mm/hr respectively. The hydraulic conductivity and infiltration rate values of Roi-et soil are much lower than those of Korat soil. This means that water loss by percolation is greater in Korat soil than Roi-et soil. Roi-et soil series is more suitable for paddy rice cultivation than Korat soil series.