PORE SIZE DISTRIBUTION OF SOILS IN THE TROPICAL RAIN FOREST AND THE RUBBER PLANTATION IN SOUTHERN THAILAND

Abstract

The study in pore-size distribution and other physical properties of soil (which effected to water storage and drainage) were conducted in the tropical rain forest and the rubber plantation, in the Southern Thailand. The purpose of the study are to find out the pore-size distribution in various land use pattern and the impact of land use change on pore-size distribution of soil. The disturbed and undisturbed soil sample were collected from top soil (0-25 cm) and sub soil (25-50 cm) in each watershed classes to determine the saturated hydraulic conductivity and the relationship between matric potential and soil moisture regime which concerning to pore-size distribution of soil. The result found that the pore size distributions were different. The quantity of large pore-size (8.6-> 28.4 µm) of top soil in the tropical rain forest and in the rubber plantation were 26.20 and 17.96% and in the sub soil were 24.81 and 18.37%, while the quantity of small pore-size (0.2-8.6 µm) of the top soil were 6.06 and 8.57% and in the sub soil were 4.05 and 8.26% respectively. Moreover the change of tropical rain forest to be rubber plantation caused the decreasing of the total porosities of soil about 2.49% in the top soil and about 0.36% in the sub soil. According to the variation of various size of pore-size distribution, it caused the decreasing of pore-size quantity that effected to water storage and drainage, for the large pore-size in the top soil and in the sub soil about 8.24 and 6.44% respectively. In case of small pore-size, percentage of water storage and drainage capability of the top soil was increased 2.51% but in the sub soil was decreased 4.21%. This result effected to the decreasing of soil capability (the drainage capability were confirmed by saturated hydraulic conductivity that would be decreased from 131.87 x 10^-4 cm / second to 76.19 x 10^-4 cm / second in top soil and from 121.95 x 10^-4 cm / second to 71.52 x 10^-4 cm / second in sub soil.).  Therefore, it could be said that, the change of the tropical rain forest to the rubber plantation area caused the change of pore-size distribution and other physical properties of soil that affected to the degradation of hydrological properties, which caused flood more hazardous.