SOIL SURVEY AND FERTILITY ASSESSMENT OF ARGO-GEDILALA SUBWATERSHED IN DUGDA DISTRICT, CENTRAL RIFT VALLEY OF ETHIOPIA

Soil resource inventory through characterization provides an insight into the potentials and limitations of soils for agricultral production. Soil fertility, on the other hand, is one of the most important factors controlling the yield of crops. However, a detailed study on soil resources characterization and soil fertility assessment based on the understanding of the soil system are not available for the most part of Ethiopia. With the cognizance of this fact, the present study was conducted to characterize the morphological and physicochemical properties of soils, classify the soils according to World Reference Base for Soil Resources (WRB), assess the soil fertility status and to prepare the fertility map of Argo-Gedilala Subwatershed in Dugda District of Oromia Regional State, Central Rift Valley of Ethiopia. Four slope classes were considered and one pedon was opened and described in the field on a standard description sheet for each slope category. Soil samples were collected from genetic horizons of each pedon for laboratory analysis of the selected physicochemical properties of the soils.The soils were classified into different Reference Soil Groups following the World Reference Base for Soil Resources (WRB). Eighteen composite soil samples (0-20cm depth) were taken for fertility assessment of the soil from the land mapping units of the subwatershed; and mapping the status of selected soil fertility parameters was done using ordinary kriging technique in ArcGIS 10 software.The soil analysis result showed the dominance of clay fraction and the texture class was predominantly clay both in the surface and subsurface layers. The pH values of the the pedons revealed neutrality to moderate alkalinity (6.84 – 7.98)throughout the profiles.The organic carbon content of the study area ranged from low to medium/moderate (0.64 to 1.83). Total nitrogen (TN),available.phosphorus (Av.P), cation exchange capacity (CEC) and percent base saturation (PBS) of the soils were in the range of (0.066-0.135%), (2.24-4.81 mg/kg), (39.52-43.52) cmol(+)/kg) and ( 77.57- 89.8%), respectively. Accordingly, they were rated as low to medium ,very low to low, high to very high and high to very high ,respectively. The soil classification revealed that pedon 1 and pedon 2 were Cambic Chernic Phaeozems (Pantoclayic, Humic, Hypereutric); and pedon 4, Cambic Chernic Phaeozem (Pantoclayic, Humic); whereas, pedon 3 is found to be a kind of Luvic Phaeozem (Abruptic, Albic, Clayic, Differentic, Hypereutric). On the other hand, the assessment of the laboratory analysis results of the composite samples showed that the soil fertility condition of the study area was considered as fertile in terms of CEC and PBS. The soils of the study area had low organic matter, available P and total nitrogen content,which can be seen as production constraints of all cultivated lands. To overcome the identified limitations, increasing the organic matter levels through continuous application of manure and/or compost should be integrated with chemical fertilizers.