Spatial variability of acid sulphate soils in the Plain of Reeds, Mekong delta, Vietnam

At all scales (delta-wide to individual fields) the acid sulphate soils of the Mekong delta show high spatial variability and closely intertwined soil types. Focusing on the field level in the Plain of Reeds, clear correlations are observed between soil physical and chemical characteristics, natural vegetation, groundwater table and microelevation. On "high" locations (higher than 85 cm above mean sea level), Typic Sulfaquepts are covered with Ischaemum spp. (grass). Highly organic and hydromorphic Hydraquentic Sulfaquepts are found in "low" locations (lower than 75 cm above mean sea level), where Eleocharis spp. (reeds) are dominant. In between, soils and vegetation present intermediate characteristics. These correlations, the high spatial variability and the soil patterns are explained by soil genesis. Because of longer and stronger evaporation on high locations as compared with low locations, small differences in elevation can lead to: important differences in water table level and therefore redox condition. Over long periods, these differences greatly influence soil development and thus, soil types. Hydraquentic Sulfaquepts can be considered at an intermediate stage of development and are expected to develop into Typic Sulfaquepts upon further drainage. Rice growth is strongly influenced by soil characteristics and redox conditions. As a consequence, rice yields are correlated with microelevation. Correlations between elevation, soil characteristics and natural vegetation can facilitate mapping of these highly variable soils. Integration of soil and water variability in the research programmes and use of adapted methods not only increase research efficiency but also make it possible to use this variability to better understand soil genesis and agronomic processes. (C) 2000 Elsevier Science B.V. All rights reserved.