Genesis and classification of sesquioxidic soils from volcanic rocks in sub-humid tropical highlands of Ethiopia

Abstract

A number of studies have been conducted on the genesis of soils from pyroclastic deposits under humid tropical climate. However, little has been studied on the mineralogical assemblage and genesis of sesquioxidic reddish soils from volcanic rocks under sub-humid tropical highland climate. This study was conducted to understand the genesis of sesquioxidic reddish soils from ignimbrite and basalt under sub-humid tropical climate in the Sidamo highlands of southern Ethiopia. Eight pedons developed from the two parent material groups belonging to Tertiary and Quaternary were sampled down to the bedrock and examined. The soils are deep, well drained and friable. The texture of all the soils is clayey. The soils are characterized by predominant neogenesis of kaolinite and residual accumulation of Fe and Al oxides in the solum of all the pedons while halloysite was detected only in the saprolitic layers of the ignimbrites. This indicates that the conditions for the formation of halloysite are met only in the deeper less permeable layers and it transforms to kaolinite at earlier stage of soil formation. Large amounts of Fe liberated by weathering from primary minerals both in basalt and ignimbrites are transformed into crystalline form whereas free Al is dominantly in poorly crystalline form. Hematite was found to be the dominant crystalline Fe-oxide in this sub-humid tropical climate, as opposed to goethite in humid tropics, due to the prevailing drier conditions. The relatively lower degree of leaching in this sub-humid climate with alternating dry and wet seasons, together with the slow permeability of the ignimbrite caused an increase in Si concentrations in the soil solution and favored neogenesis of crystalline clays such as kaolinite consuming a large amount of Al and Si derived from the parent material as opposed to poorly crystalline colloidal constituents such as allophane. The soils developed from basalt, and those from ignimbrite under udic regime are classified as Oxisols, whereas those from ignimbrite under ustic regime are classified as Ultisols/Alfisols. Time of pedogenesis, microclimatic variations affecting leaching regime and internal drainage are the important factors governing the formation and development of these soils.