The geochemistry of Thai paddy soils Nattaporn Prakongkep , Anchalee Suddhiprakarn, Irb Kheoruenromne ,Michael Smirk , Robert J. Gilkes Geoderma 144 (2008) 310–324

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.

OBJECTIVES To determine the elemental composition of seventeen Thai paddy soils. to determine if differences of chemical composition exist within and between soil profiles that may reflect sedimentological discontinuities or pedogenic processes.

MATERIALS AND METHODS Soil sampling and geological setting 17 soil profiles 6 soil series from northeast Thailand 5 soil series from north Thailand 4 soil series from the Central Plain, Thailand 2 soil series from east Thailand Soil profiles were described and sampled by genetic horizon according to standard field study methods The samples include 7- 10 samples per profile comprising 32 samples of topsoils (A horizon) and 114 samples of subsoils (B horizon).

MATERIALS AND METHODS Analytical methods Soil texture was determined using the pipette method Soil pH was determined with a pH electrode in 1:1 suspension (soil: 1MKCl and soil: H2O) Organic carbon was determined by the Walkley–Black wet oxidation method Cation exchange capacity by the ammonium saturation method at pH 7. Minerals (Si, Al, Fe, K, Ca, Mg, Na, Ti and S) were identified by powder X-ray diffraction (XRD) Other elements (Mn, Cr, V, P, Zn, Rb, Li, Ni, Sr, Cu, Ga, Pb, Co, As, Be, U, Ag and Cd) were determined by ICP-MS

RESULTS AND DISCUSSIONS The typical appearance of Thai paddy soils profiles

RESULTS AND DISCUSSIONS The solid bar (▪) is for the surface horizon and the open bar (□) is for all subsurface horizons Sai, Cb and Kl and Re, Tt, Ki (top soil) : loam to sandy clay loam (subsoils of Re, Tt and Ki: sandy clay texture) Other soil series: silty clay to clay Pm2: clay in topsoil and sandy clay loam in the subsoil

RESULTS AND DISCUSSIONS All of the soils are poorly drained 2.5Y to 10YR hue with low chroma colour (≤2) were presented Redox generated concentrations occur in the form of iron oxide mottles in all profiles Soil pH for the topsoil averages 6.0±1.0 and is 6.4±1.1 for subsoil The organic carbon (OC) concentration decreases with depth CEC of subsoils is ≥10 cmol(+)/kg for almost all of the soils

RESULTS AND DISCUSSIONS Mineralogical characteristics Soil samples Major mineral Clay fraction Silt Fraction Central Plain Smectite Quartz Northeast Kaolinite Northern Eastern The Central Plain is a basin that developed in the Plio-Pleistoscene epoch. This area is naturally poorly drained and was saturated for a long period each year iron was reduced and dissolved ions including Si, Al and Mg were introduced from the adjacent leached uplands enabling the neoformation of smectite most paddy soils in other part of Thailand are on terraces where leaching of Si and basic cations has occurred in the humid tropical climate. This condition favours formation of kaolin

RESULTS AND DISCUSSIONS Statistical analyses of the geochemical data Silicon, Al and Fe are the only major elements in the paddy soils The median concentrations of Zn, Rb, Sr, Pb, As and Cd are lower than worldwide normal soils and Cr concentrations are above values

RESULTS AND DISCUSSIONS Statistical analyses of the geochemical data Factor analysis and principal component analysis were used to determine elements of similar geochemical behavior and also to group soil samples on the basis of their geochemical affinity.

RESULTS AND DISCUSSIONS Specific elemental relationships Al, Fe, Ga and V clay mineral oxide structures Li, Co and Mn manganese oxides Phosphate and As oxyanions in soils Sr vs Ca divalent elements

RESULTS AND DISCUSSIONS Specific elemental relationships Soil group Sr to Ca ratios Parent materials Group 1 (Re, Tt, Pm1, Pm2, Ki, Ud, Ms, Hd, Sai, Cr, Cs, Cb and Kl) 0.0093 alluvium Group 2 (Tr and Kk) 0.0070 riverine and marine sediments Group 3 (Lb and Bph) 0.0025 calcite has crystallised within the soil profile

CONCLUSIONS Thai upland paddy soils have properties that mostly reflect the nature of parent materials and that depositional layering is common. The paddy soils of the Central Plain are different in that authigenesis of smectite and calcite has occurred however here too depositional layering persists