Selected chemical properties of alluvial soils in Vietnam

In the present study, 16 profiles of alluvial soils of Vietnam were collected throughout, the country and analyzed for some selected chemical properties. The soil pH was found to be controlled by several factors such as soil forming process, parent material, landform and land management, and the organic carbon (OC) content was mainly dependent on soil forming process and land management. Within soil profile, the pH was a little lower at the surface horizon and was remarkably constant throughout the lower horizons in Eutric Fluvisols and Dystric Fluvisols, whereas it decreased with depth in Cambic Fluvisols and Gleyic Fluvisols. The OC content was higher at the surface and decreased irregularly with depth in case of Eutric Fluvisols and Dystric Fluvisols while increased with depth in Gleyic Fluvisols. The cation exchange capacity (CEC) and the content of exchangeable cations were controlled by sediment sources and showed the regional variations. As a result, soils were divided into three groups based on the river systems. The variability in CEC of soils in the Red River system and the other small rivers system was large, whereas it was relatively small in the Mekong River system. Average of CEC was larger for the latter than for the former. Alluvial soils distributed in the large basins of the Red River and the Mekong River systems had the high content of exchangeable cations, especially Ca2+ and Mg2+, in comparison with alluvial soils from the small drainage basins of the other rivers system.