EFFECTS OF SECONDARY SOIL FORMING PROCESSES AFTER DEPOSITTION ON THE CLAY MINERALOGICAL COMPOSITION OF SALINE SOILS AND ACID SULFATE SOILS IN VIETNAM

Abstract

The clay mineralogical composition was examined for 4 profiles of each of saline soils and acid sulfate soils collected from some important river systems of Vietnam. Predominant clay minerals of saline and acid sulfate soils were found to be mica and kaolinite, accompanied with mica/vermiculite/smectite∼mica/smectite mixed-layer mineral, vermiculite, chlorite and smectite. The clay mineralogical composition of these soils was compared with that of alluvial soils from the corresponding river systems/agro-ecological regions in order to clarify the effects of the secondary soil forming processes of salinization and acidification (as a result of pyrite oxidization) on the clay mineralogical composition of the soils. It was hardly observed the mineralogical differences based on the distribution pattern of major clay minerals in the clay fraction. Then, examination of correlation between mineral contents was taken for the selected pairs of clay minerals, and the clay mineralogical alteration was somewhat indicated. In saline soils, it was the transformation of mica into expansible 2:1 layer silicate minerals. In alkaline saline soils, the transformation of mica into vermiculite occurred principally due to presence of a large amount of Ca and Mg salts in soil. In case of acid sulfate soils, the transformation into kaolinite (1:1 layer mineral) was suggested. However, such clay mineral transformations have occurred limitedly in soils. This is probably due to prevention of the development of salinization and oxidation by the secondary soil forming processes under paddy-rice cultivation and mangrove forestation. Based on these results, it could be asserted that saline soils and acid sulfate soils, in general, still kept the essence of the original alluvial soils.