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

2. 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.

3. 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.

4. 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 samples per profile comprising 32 samples of topsoils (A horizon) and 114 samples of subsoils (B horizon).

5. 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

6. RESULTS AND DISCUSSIONS
The typical appearance of Thai paddy soils profiles

7. 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

8. 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

9. 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

10. 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

11. 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.

12. 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

13. 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

14. 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