抄録
Nine profiles of ferralitic soils derived from sedimentary and metamorphic rocks were collected from the hilly and mountainous areas in the Northwest, Northern, Northeast, Red River Delta and North of Central agro-ecological regions of Vietnam, and were subjected to clay mineral analysis. The main clay minerals detected were kaolinite, halloysite (0.7nm), and 2: 1-type silicate minerals such as mica, the mica/smectite mixed-layer mineral, and chlorite-vermiculite intergrade. The difference in the clay mineralogical composition among the soils was controlled by the intensity or advancement of ferralitization, a main soil forming process occurring in those areas. Based on the clay mineralogical composition in the clay fraction, nine profiles were divided into 3 groups representing different levels of advancement in ferralitization. In the group 1, in which ferralitization was less advanced due to the young geologic age or retarded by the site-environmental condition or microrelief, halloysite (0.7nm) and 2: 1-type silicate minerals were predominant in the clay fraction. In the group 2 of the intermediate stage of ferralitization, halloysite (0.7nm) was changed to kaolinite, and 2: 1-type silicate minerals were weathered and the decrease in their contents was corresponding with the increase in the kaolinite content. In the group 3 subjected to the most advanced ferralitization, kaolinite occupied up to more than 90% in the clay fraction. Since ferralitization is a prolonged soil genetic process, soils developed from the same parent rock but differing in the geologic age might have the different clay mineralogical composition. In contrast, soils derived from the different parent rocks might have the same clay mineralogical composition at the advanced stage of ferralitization. Inherent potentiality of the soils was assessed based on the type and amount of clay minerals and its variation among the soils was mainly attributed to the amount of 2: 1-type silicate minerals in the clay fraction.
