Clay Mineralogy of Paddy Soils in Thailand and Malaysia

The clay mineralogy of paddy soils collected from 28 sites in Thailand and Malaysia that represent 19 soil series was studied by X-ray diffraction analysis. Most soils contain kaolinite, 2 : 1 mixed-layer minerals and mica. Chlorite and chloritesmectite or vermiculite intergrades are rather rare species. The mixed-layer minerals consist of random stratification of smectite/vermiculite/mica or smectite/mica that differ in their major mineral species. All the 2 : 1 layer silicates are dioctahedral and most smectite is a high-charge type. The contents of the minerals in the clay show regional and physiographic variations. No or little hydroxy-Al interlayering was related to a weak “grayzation” (gray coloring) of the paddy soils. The potential productivity of the soils was estimated on the basis of the contents of mica and 2 : 1 mixed layer minerals that increase from soils derived from old to recent alluvium and further to those derived from marine and brackish water deposits