Morphology, genesis and characteristics of skeletal soils in Sakon Nakhon Province, Northeast Thailand
1991
Potichan, A.
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 of 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 minerals were the interstratified clay minerals between 10 and 14 A deg, quartz, goethite and gibbsite. Quartz was the most abundant mineral in the silt-size fraction, with few mica, feldspar and 7 A deg group of clay minerals. 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. Massive laterite was formed by the continuous cementing of various nodules by flow and layered ferruginous materials. Two of the five Alfisols were in the subgroup Ultic Haplustalfs, while the rest were in the subgroup Typic Ochraqualfs, Aeric Ochraqualfs, and Aqualtic Haplustalfs. The other three soils including the soil on Phu Phan Range were in the subgroup Plinthic Paleustults.
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