Author : Aniruth Potichan

Summary : A study was conducted to characterize the morphological, physical, chemical, mineralogical, and micromorphological properties of the skeletal soils in Sakon Nakhon Province, Northeast Thailand. All the properties were used to identify soil forming factors that have influenced formation of the skeletal soils through the specific soil forming processes. Eight representative pedons were selected for the detailed study based on their field morphology, parent materials, elevation, physiography, and vegetation. All the soils studied were classified based on the US Comprehensive Soil Classification System or Soil Taxonomy. Morphological features of all the skeletal soils indicated the intermediate to mature stage of soil development with the A-Bt-C profile. All of them showed the discontinuity of soil materials within the soil profile. The skeletal layers (A and B horizon) of the transported materials were above the mottled clay or C horizon which was an in situ development from siltstone, shale and mudstone. Soils with loose lateritic nodules were formed on the undulating terrains at high position, while soils with massive or sheet laterite were formed on the nearly level to level terrains at low lying position near the base level. Kaolinite was an abundant clay mineral in most of the soils, particularly in the surface and subsoil, except soil on the Phu Phan Range where illite was abundant. Montmorillonite, vermiculite, and illite were also present. Montmorillonite was abundant in the mottled clay and weathering horizon of some soils. Other minor and trace mineral were the interstratified clay minerals between 10 and 14 A degree, quartz, geothite and gibbsite. Quartz was the most abundant mineral in the silt-size fraction, with traces of mica, feldspar and 7 A degree group of clay minerals. The micromorphological study also confirmed the discontinuity of soil materials. It indicated that illuviation was active in the subsoil, and shrinking and swelling were active in the mottled clay horizon.