Degraded Sandstone and Siltstone-Derived Soils in Northeast Thailand: The Attempts to Improve Their Quality.

The Northeast of Thailand covers almost one-third of the country’s total area. This region is so important for a production of food crops such as jasmine rice, cassava and sugarcane of which Thailand is among the leading exporters of the world. Geologically, this area is mainly underlain by sandstone and siltstone, intrinsically giving most soils to possess low fertility status coupled with salinity problem. Most soils are classified as Ultisols, Alfisols and Entisols. These soils have long been used for agricultural production and soil degradation has played such a vital part in declining soil quality. Apart from having low organic matter content, low ability to retain soil moisture and plant nutrients, and salinity problem, human-induced soil compaction, topsoil structure easily collapsed and the formation of plough pan in subsoils, has additionally and adversely emerged in vast areas and tended to diminish food security in this region. Many attempts have been carried out to improve these poor soils.  Deep tillage and ripper have been tested to alleviate plough pan problem but positive results are rare or show very short term resolution. Conditioning the soils by using inorganic soil amendments, manures and agricultural wastes including biochar have been trialed. Nonetheless, soil quality has not been improved as much as expected and the average yield of jasmine rice, cassava and sugarcane is still fairly low when compared to those reported from other tropical countries.