Soil classification of humid Western Ethiopia: A transect study along a toposequence in Didessa watershed

Little has been studied on genesis and properties of soils in Western Ethiopia. This study was conducted to understand the pedogenesis, and to classify benchmark soils in Didessa watershed along a toposequence. The six soil profiles are representative of low, mid and highland positions within an elevation range from 1273 to 2700 m above sea level along a 53-km long transect. Their pedological processes, soil properties and classification are discussed in context of soil forming factors and sustainable soil use. Melanization, cheluviation (chelation) of organo-mineral substances, oxidation of iron and leaching of iron compounds and basic cations, clay translocation, de-alkalization, and acidification were major pedological processes at the upland soils while erosion took place at shoulder positions. Calcification and pedoturbation (vertization) were major pedological processes in the lowland and melanization was also observed there. The surface horizons of these very deep soils had loam a loamy texture while the subsurface horizons were clayey. Leaching of basic cations increased with elevation. The U.S. Soil Taxonomy classification identified a soil sequence consisting of Typic Hapludults, Typic Ferrudalfs, Typic Rhodudalfs and Typic Hapluderts while the World Reference Base classified the corresponding soils as Ferric Rhodic Alisols, Ferric Rhodic Luvisols, Ferric Chromic Luvisols, and Calcic Pellic Vertisols. Numerical hierarchical cluster analysis identified similar and dissimilar horizons in the soil catena. Ultisols were developed on tertiary basalt at midland, Alfisols were developed on tertiary basalts and granitic gneisses at midland and highland, and Vertisols were developed on alluvium and colluvium at lowland. The relief characteristics, lithologic units, agro-ecology and land use covered by the current study are typical of much of the Western and Southwestern Ethiopia. The study indicated that Didessa toposequence has three soil orders (reference groups) that need different management requirements for sustainable soil use. Thus, the transect serves as a model of soil development and soil management in humid Western Ethiopia.