The pedo-geochemistry of Vertisols under tropical savanna climate

Fourteen representative Thai Vertisols on lowland and upland areas under a tropical savanna climate were investigated to ascertain parent material and pedogenic influences on their geochemistry. Physico-chemical, mineralogical and geochemical properties were determined for 98 soil samples. Multivariate analysis was used to interpret the dataset and identify affinity groups of samples and properties. Silicon, Al, Fe and Ca are the major constituents of the soils. Mean concentrations of elements are mostly within the range for worldwide normal soils. The variations in concentration of elements between soils reflect differences in the abundance of the major mineral constituents: beidellite, calcite, iron oxides and quartz. Factor analysis of geochemical data showed that most elements do not cluster into discrete affinity groups. The composition of soils is mostly quite uniform throughout the profiles with no evidence for depositional layers. Beidellite is the dominant clay mineral and acts as a reservoir of many major and trace elements in these soils. The accumulation of pedogenic calcium carbonate and Fe–Mn nodules in distinct horizons has resulted in systematic variations in composition within and between soil profiles for those elements that are associated with these neoformed materials (e.g. Fe, Mn, Co, V, Th, Ba and Ge).