Salinization in Northeast Thailand(<Special Issue>Problem Soils in Southeast Asia)

Abstract

Northeast Thailand covers an area of 170, 230(km)^2. This vast area should be agriculturally valuable, but actually its agricultural production per unit area is lower than those of other regions of the country. Low soil fertility, poor soil materials, uncertainty of climate and natural disasters are the main factors reported to reduce agricultural productivity. One prevalent natural disaster in this region is salinity. Saline land is usually barren, for crops cannot tolerate the high salt content of the sub-soil or, sometimes, the surface soil. Certain weeds are tolerant to salinity and are used as salt-indicators : these include nam phee, nam dang, yeak, phla, and mor. Every year the acreage of saline soils is increasing and causing major problems for farmers in managing the land. During April and May 1984, saline areas in the central and northern part of Northeast Thailand were investigated. These were classified into three major types on the basis of their topographic and geologic settings : hill, valley, and basin. A major source of salt wherever it is exposed or lies close to the surface is the Rock Salt Member of the Maha Sarakham Formation, which consists mainly of rock salts. There are, however, other potential salt-sources that were formerly classified as salt-free strata. These are the Upper Clastic Member of the Maha Sarakham Formation and the Plio-Pleistocene Formation, which have recently been reported to contain traces of salts such as gypsum, sulfate, and carbonate, which replace halite. The mechanism of salinization in this region is short-distance interflow of brine in source layers together with capillary rise. Interflow is of short distance because many of the scattered hillocks of the region are underlain by the Maha Sarakham Formation, while the Plio-Pleistocene Formation is also found in small sub-basins. Furthermore, broad, flat, low-lying topographies like the Phimai Plain and the Thung Kula Ronghai are still wrongly classified as alluvial plains, whereas in fact they are Plio-Pleistocene surfaces with alluvial patches and scattered patches of Maha Sarakham Formation. Salt that is weathered and eroded from salt-sources is transported either by surface water or by groundwater to low-lying lands. Whenever the ground surface is dry enough, salt precipitates from saturated brined surface water or rises from saturated brined groundwater.