Formation of soils with contrasting textures by translocation of clays rather than ferrolysis in flooded rice fields in Northeast Thailand

This paper describes the conditions for dispersion and flocculation of clays, and the impact of this process on soils of contrasting textures cropped with rice. Clay seems to be translocated down the profiles and along a topographic sequence. The clays are mixed kaolinite-smectite. The cation exchange capacity of these clays exceeds 20 cmol(c) kg(-1). Both the proportion of smectite and clay content increase with increasing depth and from the top to the lower part of the sequence. The pH ranges from 5 to 8. The aluminium oxide content is small. The soil solutions collected during the rainy season were analysed for Fe2+ and major cations. We calculated the sodium adsorption ratios (SAR) taking into account Fe2+ and compared them with the critical coagulation concentration (CCC) found in a previous study. The Fe2+ contributed to a decrease in the SAR of cropped soils. The comparison between SAR, total electrolyte concentrations and CCC values showed that the dispersible clays are likely (i) to disperse in the abandoned and non-saline fields, (ii) to flocculate in the saline and uncropped soils as a result of the large salt content and in the cropped soils because of either large salt or Fe2+ content, and (iii) to disperse in the flood water and at the surfaces of abandoned fields under rain. No evidence for ferrolysis was found. The observed contrasting textures and clay mineralogy can be explained by clay translocation controlled by salinity and rice farming.