The geochemistry of soils on a catena on sedimentary rock at Nam Phong, north-east Thailand

Abstract

The elemental composition of size fractions of soils on the Nam Phong catena, north-east Thailand, has been determined. The catena can be divided into 6 geomorphic positions: summit, shoulder, upper midslope, lower midslope, footslope and toeslope positions where soils have developed on sedimentary rocks under a tropical savannah climate. Factor analysis was used to interpret the large dataset and to determine profile and spatial trends in geochemistry. Silica is the major component of the soils on this catena, reflecting the presence of much quartz in the silt and sand fractions in soils at all landscape positions. Smaller amount of Al and Fe are also present; these elements are associated with the presence of kaolin and iron oxides. Factor analysis shows systematic differences in chemical composition between soils on higher positions and the soil on the lowest position in the landscape. Small variations in the chemical compositions of the whole soil, fine sand and silt between upslope soils are recognised. Soils on the summit, shoulder, midslope, and footslope exhibit little within-profile variation in chemical composition and the compositions of the profiles overlap closely. The Al affinity group (Al, Co, Ca, Mg, K, Sr, Cs, Rb, Ga, Zn, Ni, Li, Mn, Ti) increases in abundance in the toeslope soil, which is clearly different in chemical composition from the soils on higher positions. For the clay fraction, the differences in concentration of both the Si group (Si, Ni, Mn, Co, Mg, K, Ba, Pb) and Ca group (Ca, Zn, Cu, Sr, Cr, P) result in soils on toeslope and footslope positions being distinctly different from upslope soils, which have similar compositions. Soils at all positions show moderate variation in chemical composition of the clay with depth. The small variations in the chemical compositions of upslope soils on the Nam Phong catena are due to different degrees of weathering of the same parent rock, whereas soil on the toeslope position has a quite different elemental composition, possibly due to a different parent rock and the authigenesis of minerals in this landscape position where leached ions accumulate.