CHARACTERIZATION AND CLASSIFICATION OF MAJOR AGRICULTURAL SOILS ALONG TOPOSEQUENCE OF TUJI GEBISA SUBWATERSHED, EAST HARARGHE, ETHIOPIA

Any study concerning agricultural use of the soil resources has to begin with the characterization, genetic assessment and classification of the soils since these aid to identify constraints and potentials of soils that play great role in the exchange of knowledge among agricultural experts. The soil resources of East Hararghe, due to different environmental factors, especially due to the strongly rugged topographic condition and anthropogenic influences over a long period of time, are strongly variable. Soils of Tuji Gebisa subwatershed as part of East Hararghe zone are not exclusive of such conditions and hence soil studies that aid to identify their spatial variation, constraints and potentials are of paramount importance. The present study was conducted with the main objective of characterizing and classifying the major agricultural soils along a Toposequence in Tuji Gebisa subwatershed to generate information that aid the management of the soils and related land resources. Four representative pedons (P1, P2, P3, and P4) were opened alongside the toposequence and sampled, characterized and classified. Results showed that the soils were morphologically and physio-chemically variable along the slope positions and in relation to soil depth. Morphologically, the shallowest pedon was observed at the convex-angular summit position and the deepest at the foot- and toe-slope positions. All the pedons opened in the study area had granular type structure at their surface horizon that changed to sub-angular blocky to angular blocky type as soil depth increased. Pedon 3 and 4 had friable consistence (moist) throughout their depth, while pedons 1 and 2 had a firm consistence only in the second horizon. Texturally, the soils were sandy loam and sandy clay loam at least in their upper two horizons and the percentage of sand was relatively high in all the horizons of all the pedons. The soil pH of all pedons was in the range of moderately to strongly alkaline (7.4 to 8.2); and the analysis results of soil carbonates indicated that almost all the soils sampled were calcareous. The base saturation of the pedons 2, 3, and 4 was above 50%, whereas very high Ca2+ (7 to 30.1 cmol(+)kg-1), K+ (0.6 to 1.9 cmol(+)kg-1 and medium to very high CEC (23 to 53.6 cmol(+)kg-1) were recorded; which indicated that the soils had appreciable chemical fertility status. The recorded values of extractable phosphorus ranged from very low to high (2.6 - 47.9 mgkg-1) and that of %TN from very low to medium (0.07 – 0.21%), indicating that phosphorus and nitrogen may be among the critical plant nutrients in the study area, which needs consideration in fertilizer application practices. In all pedons sampled, the extractable Cu and Zn contents were in the low to very low range (0.08 to 0.71and 0.06 to 0.9 mgkg-1) respectively, and hence need attention in fertilization practices within the study area. The soil classification indicated that except the Regosol pedon identified at the summit position, all the soils sampled in the present study area belonged to the reference soil groups with high base status and organic matter accumulation (Chernozems and Kastanozems); which can be considered as slight variation most likely due to the differences in slope position, slope gradient and parent material.