Clay mineralogy of grey degraded soils and sandy soils as problem soils in Vietnam

Abstract

Four profiles of grey degraded soils and of sandy soils were collected from different agro-ecological regions in Vietnam and subjected to clay mineral analysis in addition to particle-size analysis. The result showed that those soils had the low clay while high sand contents in the upper layers or throughout the profile. This was ascribed to decomposition of layer silicate minerals in the upper layers and/or downward shifting of clay particles in a profile for grey degraded soils and to inheritance from the parent material for sandy soils. Predominant minerals detected in the clay fraction were kaolin (kaolinite and halloysite (0.7 nm)) and quartz for both soil types. The present study indicated two opposite processes of soil development. For grey degraded soils, reduction in the clay content with decomposition of layer silicate minerals was enhanced with advancement of soil degradation. As a result, they became sandy-like soils with domination of quartz in the clay fraction. For sandy soils, on the other hand, their particle-size distribution and clay mineralogical composition were changed naturally by enrichment with clay particles from surrounding environment. The latter process suggests a possibility of fertility improvement of poor soils by mixing with more fertile materials such as alluvial soils, river sediments and weathered rock materials, in addition to the traditional measures such as intensive application of chemical fertilizers and addition of large amounts of organic amendments.