Abstract
In the present study, the weathering process of minerals in ferralitic soils under terrace paddy was identified by comparison of their clay mineralogy with that of ferralitic soils under natural forest. The result showed that clay minerals detected in the soils were vermiculite-chlorite intergrade, kaolinite, gibbsite and goethite. However, the content of these minerals varied with soils depending on the physiographic differences. In the soils under the natural forest and subjected to the terrace paddy field condition only for short-time, ferralitization process is still remaining with decomposition of vermiculite-chlorite intergrade, an easily weatherable mineral. As a result, vermiculite-chlorite intergrade in the clay fraction was in a decreased content with the correspondingly increased content of kaolinite. In contrast, vermiculite-chlorite intergrade was detected with a larger amount in the soils subjected to the terrace paddy field condition for long-time, and its content sometimes exceeded the kaolinite content. Gibbsite was also observed with a larger amount in these soils. This suggests that ferralitization process has been retarded and might be replaced by alitization process under the terrace paddy field conditions.
