カンボジアの水田土壌作土の粘土鉱物組成と土壌の風化の程度

Abstract

The clay mineralogy of the surface horizon sample of 16 paddy soils from Cambodia was studied in relation to weathering features, such as pisolitic iron concretions and sand separations in the soil profile. The occurrence of these weathering features generally agreed with the geomorphological setting of soil sampling sites. Results indicated that the clay mineral composition reflects the weathering history of a soil in such a way that the relative content of kaolin minerals increases while the illite content and other 2 : 1 type clay mineral species decrease as the soil material is more highly weathered. It was also shown that recent Mekong sediments contain quite a high amount of illite, while recent lacustrine sediments around the Tonle Sap Lake are characterized by a higher content of montmorillonite and vermiculite relative to the illite content. Soil developed in acidic swamp conditions contains a fairly high amount of 2 : 1 type minerals, which consist almost exclusively of aluminium interlayered vermiculite. Montmorillonite is the dominant clay mineral species in grumusolic soil developed residually on a basalt plateau.