The geochemistry of Thai paddy soils

This study has determined the elemental composition of uncontaminated Thai paddy soils, and identified whether variations in chemical composition within and between profiles are sedimentological in origin, or due to pedogenic processes. The chemical composition of the paddy soils is highly diverse. In comparison to typical values for soils worldwide, the median concentrations of Zn, Rb, Sr, Pb, As and Cd in Thai paddy soils are lower, concentrations of Si, Al, Fe, K, Ca, Mg, Na, Mn, Ti, S, V, P, Li, Ni, Cu, Ga, Co, Ag, Be and U are comparable and concentrations of Cr are higher. Using factor analysis, two affinity groups of elements and properties are recognized. The first group consists of sand, Si and Cr and relates to the sand content of the soils which includes chromium magnetite grains. The second group is diffuse and consists of clay, silt, CEC, pH H2O, pH KCl, Al, Fe, K, Ca, Mg, Na, Mn, Ti, S, V, P, Zn, Rb, Li, Ni, Sr, Cu, Ga, Pb, Co, As, Be, U, Ag and Cd. Factor analysis indicates that the compositions of individual horizons in each soil are highly diverse resulting in substantial overlaps in composition between different horizons and soil profiles. The large variations of chemical composition within and between Thai paddy soils are due to the presence of discrete depositional layers within soil profiles with differences in texture of the soil controlling the variations in chemical composition. Pedogenic processes have modified original depositional properties to only a moderate extent but may have included the genesis of smectite. Microprobe analysis of thin sections has shown that the chemical composition of clay coatings is similar to that of the clay matrix and is consistent with the smectitic, kaolinitic or mixed clay mineralogies of particular soils. The composition of clay minerals in iron oxide-rich mottles is similar to that of the clay matrix for kaolinitic soils. Iron oxide-rich nodules in smectitic soils are also kaolinitic rather than smectitic indicating that the smectite formed in the matrix but not in the nodules which are original sedimentary features.