Soil Salinization after Deforestation and Control by Reforestation in Northeast Thailand

Abstract

Soil salinity is one of the serious constraints for crop production in Northeast Thailand, where variously salt-affected soils estimated to be approximately 17% of the Northeast are present. However, an additional 18% of the region may be salinized without any proper countermeasures. Source of salt is red siltstone and sandstone, i.e., the upper clastic member of the Mahasarakham Forma-tion in the Northeast. There are the natural and the artificial causes of salinization. However, not the nat-ural causes associated with the geological and topographical factors, but the artificial causes by various human activities have enhanced salinization in recent years. In the present study, salt-accumulation on sandy surface soils after the clearance of the native vegetation in Phra Yun, Khon Kaen was investigated in order to clarify the process of salinization and to identify meth-ods of control. Deforestation changes the balance between rainfall and evapotranspiration, resulting in the increase of percolation and seepage water. Saline seepage water induces salinization in the footslopes and the low-lying lands. Salt-affected soils displayed mottlings in the upper part of the profile in connection with the elevation of the groundwater table in the rainy season. In these soils, salt reached the surface through the upward move-ment of saline water consisted mainly of NaCl, at the beginning of the dry season. Salt could also be easily leached out to the subsoil with rainwater in the rainy season. Accumulation of salt on the soil surface in the dry season was associated with the elevation of the ground-water table to a critical level higher than 70cm from the surface at the end of the rainy season and the con-tinuity of capillary pores above the groundwater level. To control salt-accumulation, the planting of eucalyptus trees which led to the drying of the soils and the lowering of the groundwater table was found to be effective in interrupting capillary rise of saline water and in preventing saline seepage water from reaching the low-lying lands. The low-input technology of re-forestation with eucalyptus trees can be hopeful of controlling soil salinization in the other countries.