Soil and landscape data at a regional scale in Lake Tana basin (Ethiopia)

The major soil types found in Lake Tana basin are Nitisols, Vertisols, Luvisols, Regosols and Phaeozems with an outstanding presence of Vertisols and Nitisols. The distribution of these soils in the landscape seems to be related to the topography and drainage. Nitisols appear in the well-drained situation of plateaus. Vertisols appear in flat areas on present floodplains, past floodplains and local depressions at various altitudes. The mineralogical characterization shows that Nitisols and Vertisols are somehow related in this region. Although the relative amounts of the minerals differ strongly, the same constituents are found in both soil types. Vertisols have a dominated smectitic mineralogy, which is in line with the clear slickensides reported. In contrast the Nitisols are more kaolinitic, but they still have some smectites. The mineralogy shows differences between Vertisols and Nitisols which can be explained by a more advanced weathering in the Nitisols than in the Vertisols. The most important parent materials found in the region are mafic rocks and lacustrine deposits. From their position in the landscape the Vertisols of Lake Tana basin are thought to have developed in old lacustrine deposits. Nitisols are supposed to be found on basaltic parent material, but could as well as have developed on strongly weathered old lacustrine deposits. The findings of Vertisols which developed on old lacustrine deposits may indicate possible ancient lake heights for Lake Tana.