PEDOGENESIS AND SOIL·GEOMORPHIC RELATIONSHIPS ON THE PIEDMONT SLOPES OF WURGO VALLEY, SOUTHERN WELO, ETHIOPIA

Abstract

The morphological, physical, chemical and mineralogical characteristics of six pedons were investigated to establish relationships that exist
between soils and geomorphic processes on the piedmont slopes of the Wurgo valley of southern Welo, Ethiopia. The results revealed that slope processes have played major role in the development of Eutric Cambisols with typical AlBw/Bb horizon sequences on the convergent footslopes. Luvic Phaeozems with A/Bt/Cr or A/Bt/Bb sequences on the non-convergent footslopes, and Eutric Vertisols with AIClCr or AI AC/Bb arrangements on the toeslopes. The Cambisols were weakly acid to neutral soils with loam to clay loam texture and cation exchange capacity (CEC) of 139 to 253 cmol(+)kg-! clay. The sand fraction in these soils is dominated by pyroxenes and volcanic clasts. The Phaeozems were marked by clay loam to clay texture, CEC of 113 to 203 cmol(+)kg-1 clay and sand fractions dominated by feldspars, volcanic clasts and laterite nodules. The Vertisols are heavy clay soils having CEC of 75 to 139 cmoI(+)kg-1 clay and sand fractions dominated by weathered volcanic clasts and laterite nodules. All of the characteristics used as
indices of weathering suggest that the three soil units are at different stages of pedogenesis - the Cambisols being the youngest and the Vertisols the oldest soils. The major slope-related soil building process that resulted in the formation of Cambisols on the convergent footslopes was "cumulization" resulting from intermittent addition of alluvial and colluvial materials. Absence of "cumulization" on the other types of footslopes has allowed an uninterrupted operation
of "lessivage" with subsequent development of argic B horizons within the subsoils of the Phaeozems. The active operation of "argilli pedoturbation" and "haploidization" and the subsequent development of Vertisols on the toeslopes was encouraged by the gentle to almost flat surfaces and the fine textured parent materials.