Agro-climate based pedogenetic assessment of soils in Kulumsa Subwatershed, Arsi, Ethiopia
Any particular soil results from the integrated action of soil-forming factors that occur in different kinds and degrees of expression that can be combined in many different ways; so that each combination produces a different soil with its unique properties resulting in the existence of thousands of different soils on earth. Soil genesis brings order to this overwhelming variety of soils aiding their classification. Soils of Kulumsa sub-watershed were surveyed and 19 representative profile locations identified, described, and sampled based on observable site and soil characteristics such as slope, topographic position, vegetation, topsoil color, texture, etc. using a topographic map of 1:50,000 scale as a base map through free survey along altitudinal and latitudinal transects. The sampled 19 pedons were categorized into 10 Reference Soil Groups through pedogenetic assessment; and the relationship of the overall pedogenesis and spatial distribution of the soil groups to altitude and the traditional agro-climates were examined. The pedogenetic assessment of the soils revealed that the degree of weathering and leaching increased with increasing altitude. Accordingly, the least leached and most alkaline Kastanozems in the Lower-Woina-Dega (1800-2100 masl) within the “rift valley” and its border escarpment areas, and the most leached and most acidic Umbrisol in the High-Wurch (>3700 masl) agro-climatic belt on Mount Chillallo, were identified. Even though there were acidic soils with pH as low as 5.15 and very strongly leached profile, the chemical weathering processes appeared to be not so intensive when evaluated in terms of their clay mineralogy. The acidic soil profile condition of these high altitude areas seemed not to be in a position to cause an advanced weathering of the high-activity clay minerals, so that they still dominated the clay fraction of the soils. In relation to the traditional agro-climates, from the lowest towards the highest elevation within the study area, the least leached Kastanozems in the Lower-Woina-Dega, Chernozems and Luvisols in the Proper-Woina-Dega (2100-2250 masl), Planosols and Vertisols in the Upper-Woina-Dega (2250-2400) to Lower-Dega (2400-2550 masl),Vertisols and Luvisols in Lower-Dega, Retisols in the Dega-Proper (2550-3000 masl), Umbric Alisols in the Upper-Dega (3000-3200 masl) and Hyperdystric Umbrisols in the High-Wurch (>3700 masl) agro-climatic belt, were the dominant soil groups. In inference, the results of the pedogenetic assessment denoted that the trend in pedogenesis and the spatial distribution of the soil groups was generally in concurrence with the traditional agro-climatic classification used by the Ethiopian farmers in the E.2.03-type Rainfall Pattern Region of the Arsi-Highlands. However, this needs more detailed investigations that include the other Rainfall Pattern Regions of Arsi and of the country as well for more reliable conclusion.