Red and yellow soils and laterite formation in the Northeast Plateau, Thailand

Data of a study on red and yellow soils in the Northeast Plateau, Thailand, indicate that the soils are highly leached and well developed under oxidizing condition. They have been developed on slope wash materials and local alluvium underlain by cemented gravels and laterite. Appreciable amounts of Fe-oxides can be found in these soils, mainly in the form of goethite and hematite and in both silt and clay fractions. Other important constituents of these soils are quartz and kaolinite. The soil fabric contains argillan and ferri-argillan on skeleton grains and pore walls, indicating prolonged leaching and oxidizing state of the solum. Laterites found are both massive and pisolithic gravels. Since spot accumulation and separation of argillan and Fe-oxides were not observed in the soils it seems that the soils and laterite have been controlled by different sets of genetic processes.