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.
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