CLAY MINERALOGY OF ALLUVIAL SOILS AND GREY DEGRADED SOILS DERIVED FROM THE SAME RIVER SEDIMENT IN THE RED RIVER DELTA, NORTHERN VIETNAM

Abstract

Five samples of alluvial soils and 3 samples of grey degraded soils, both derived from the sediments of the Thaibinh River system in the Red River Delta, northern Vietnam, were subjected to clay mineralogical analysis in addition to measurement of pH, organic matter content and particle-size distribution. The pH was comparable between the two soil types. In contrast, the organic matter and clay contents were remarkably lower for grey degraded soils than for alluvial soils. Predominant clay minerals in both soil types were mica and kaolinite; the mica content was higher for alluvial soils while the kaolinite content was higher for grey degraded soils. Chlorite-vermiculite intergrade was detected only in grey degraded soils, whereas vermiculite was present in alluvial soils instead. Absence of goethite in grey degraded soils might be a cause of their grey color. A sign of degradation on the amount (clay content) and type (clay mineralogical composition) of clay minerals was indicated in grey degraded soils in the Red River Delta, leading to decrease in the organic matter content and then to decline in the soil fertility.