Research area: Thailand

Soils of NW-Thailand

In NW-Thailand, the monsoonal climate leads to clay-illuviation (clay translocation from the topsoil into the subsoil) as major soil forming process. In dependence of parent (rock) material and local climate these clay illuviated soils occur as Acrisols, Alisols and Luvisols. On acid parent rock (e.g. granite, gneiss), Acrisols prevail  from lowest  to high elevations. These Acrisols are characterized by a clay rich texture. The degree and depth of clay illuviation is decreasing with increasing elevation, indicating that clay is mainly mechanically translocated via cracks, which were formed after the soil dried out.

With increasing elevation the precipitation is increasing while evapotranspiration is decreasing, increasingly hampering the formation of cracks and their depth development. Therefore, clay illuviation is fading out at highest elevations, leading to a transition from Acrisols to Cambisols. On limestone, Alisols dominate on elevations below 800 m asl. At 800 m asl Alisols pass over in Acrisols, due to an increase of kaolinite and gibbsite in its clay fraction as a consequence of higher leaching rates, which are related to precipitation increase. Limestone facilitates in comparison to other parent rocks a better drainage of its soil cover. Therefore, Acrisols prevail even at highest elevations, because the soil mass  dries out sufficiently, facilitating crack formation and clay illuvation. On sandstone, Alisols occur at lowest elevations and pass over to Acrisols above 1000 m asl. There Acrisols dominate until the highest elevations of sandstone occurrences. On basic and ultrabasic igneous rocks Luvsiols are the most common reference soil groups.

Minor reference soil groups comprise Anthrosols at sites of paddy rice cultivation, Ferralsols occur either on old relictic land surfaces or are derived from bauxitic parent material, Fluvisols are common along the rivers and streams, Gleysols occur in intramontane basins with high groundwater table, Leptosols and Regosols dominate on extreme steep sloping land, Plinthosols occur mostly within the intramontane basins, Technosols are common within settled areas and are also represented by the road network, Umbrisols occur either at lower slope position and in karst depression or within undisturbed forests.

Exotic reference soil groups comprise Chernozems below karst springs, Histosols on the top of Doi Inthanon, Stagnosols at higher elevations with increased precipitation rates, and Vertisols on lakustrine claystone within intramontane basins.

References

IUSS Working Group WRB (2006): World reference base for soil resources 2006. 2nd  Edition. World Soil Resources Reports No.103, FAO, Rome.