DISAPPEARING TREND OF SMECTITE IN GANGES FLOODPLAINS SOILS OF BANGLADESH

Eleven soils from different Ganges Floodplains of Bangladesh were studied to investigate the clay mineralogical composition and to clarify the relationship between disappearance of smectite and decalcification of calcareous floodplain soils occurring in these areas. In general, mineralogical composition of calcareous floodplain soils were dominated by mica and smectite, and in some cases by vermiculite. Chlorite and kaolinte were also present in minor amounts. Amjhupi and Gangni soils on ridges and Ghior (1) soil in a basin had the lower smectite as well as carbonate contents in Ap and B2 horizons than in the corresponding C horizon. However, all horizons having low carbonate content did not always show the lower smectite content, possibly due to near-neutral reaction or due to difference in the rate between decalcification and disappearance of smectite. Complete disappearance of smectite was observed in Tahirpur soil and partial disappearance in Baradi soil; both are situated on the non-flooded highland and thoroughly decalcified and considered to be in the oldest soil formation in Ganges Floodplains of Bangladesh. Based on the assigned age of the soils, smectite disappearance was estimated to be in the order of 2500-3000 years.