CHARACTERIZATION AND CLASSIFICATION OF SOILS ALONG A TOPOSEQUENCE AT DHENGEGO SUBWATERSHED, EAST HARARGHE ZONE, ETHIOPIA

Abstract

Characterization and classification of soil is the main source of information for precision agriculture, land use planning and management. For an efficient use of the limited land resources, site specific management recommendations based on site specific information are much required. Undulating topography, altitudinal differences and large variations in slope were characteristic features of the present study area (Dhengego subwatershed). The purpose of the present study was to characterize, classify and map soils of Dhengego subwatershed along the hill-slope to generate baseline information. Four representative pedons were opened and described at four topographic positions (summit, upper, middle and lower slope). One pedon was opened for each topographic position and described in the field and sampled basd on standard description sheet. A total of 30 soil samples 15 diturbed and 15 undistarbed were collected from the genetic horizons of each pedon for laboratory analysis of the selected physicochemical properties of the soils. The morphological description of the pedons showed that the effective soil depth of the study area was greater than 200 cm for pedon 3 and 4, but was less than 200 cm for pedon 1 and 2. The soil description and analysis results revealed also variations in the other morphological physical and chemical properties of the soils along different topographic positions. The soil structure of all pedons at the upper horizons was moderate to moderately strong granular that gradually changed to moderately sub-angular to angular blocky aggregates with increasing soil depth. Sand sized fraction dominated the texture of soils in the study area and loose to friable moist consistence was observed at the surface horizons of almost all pedons, which changed to firm consistence with increasing depth only in pedon 4. The soils had a neutral to slightly alkaline reaction (6.74-7.74) throughout their profile. The organic carbon content of the study area ranged from low to hihg (0.71 to 3.07%). Total nitrogen, available phosphorus, cation exchange capacity and percent base saturation of the soils were in the range of (0.13-0.22%), (6.7-25.90 mgkg-1 ), (24-51 cmol(+)kg-1 ) and (60- 99.6%), respectively, which were rated as moderate, high, high to very high and also high to very high, respectively. The concentration of micronutrients in the soils were high for Fe, medium to very high for Mn, very low to low for Zn and medium to high for Cu. The soils were classified into different Reference Soil Groups following the FAO-WRB of 2015 by detecting diagnostic characteristics (horizons, properties and materials). The soil classification revealed that three Reference Soil Groups, namely, Regosols, Phaeozems and Luvisols; the second level detailed naming of which would be Eutric Epileptic Regosol (Humic, Loamic, Mollic) for pedon 1, Cambic Phaeozem (Hypereutric, Profundihumic, Pantoloamic) for pedon 2, Calcaric Phaeozem ( Humic, Pantoloamic) for pedon 3 and Endocalcaric Luvisol (Humic, Katoloamic) for pedon 4. The equivalent names of the USDA-Soil Taxonomy were: Lithic Ustorthents for pedon 1, Typic hapludolls for pedon 2 and 3 and Udic haplustalfs for pedon 4. Since almost all the soils identified were of ample fertility status especially in terms of their base saturation but prone to topographically exacerbated soil degradation, special emphasis should be given to soil OM and integrated soil fertility management coupled with soil and water conservation measures to optimize and sustain crop production in Dhengego subwatershed.