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Various marginal soils derived from different parent materials were collected from different landforms and agro-ecological regions in Vietnam and were subjected to clay mineral analysis in addition to particle-size analysis. The result showed that the particle-size distribution of the soils had a close relationship with the landform. The clay content was highest for the soils from the meander floodplain and inland valley, followed by the soil from the hill and lowest for the soils from the mountainous area. The clay mineralogical composition was different among the soils and was found to be controlled by the parent material and soil forming process.

Alluvial-affected gley soils, carbonate black soils, black soils on basaltic tuff, and brownish grey soils in the semi-arid region had the clay mineralogical composition comparable with that of the soils originating from the same parent material or distributed in the same agro-ecological region, as reported in the previous papers (Nguyen and Egashira, 2005, 2007a, 2007b, 2008b). Neo-formation of smectite resulted in its predominance in the clay fraction of black soils on basaltic deposit. Alitization process taking place in humus soils on mountain caused accumulation of aluminum, leading to the high gibbsite content in the clay fraction of the soils. In contrast, podzolization process has removed iron and aluminum, outside the profile. As a result, podzols lacked gibbsite and goethite with mica as a predominant clay mineral. Inherent potentiality of the soils was assessed based on the type and amount of clay minerals. The assessment showed a large variability in the inherent potentiality of the soils ranging from the low to high levels.