Abstract:
Soil is one of the most important natural resources. However, detailed scientific knowledge and information are lacking on soil resources. Subsequently, this affects agricultural crop productivity and profitability if its potentials and limitations are not realized in the study area. The purpose of this study was to: (1) characterize and classify soils along toposequence; (2) analyze and map the spatial variability of soil fertility status, and (3) assess the spatial variability of soil organic carbon stock (SOCS) in Gurje Subwatershed, Hadiya Zone, southern Ethiopia. Five representative Pedons were opened along toposequence (upper, middle, and lower slope positions). Pedons were described in situ as per the FAO guidelines for soil morphological properties. Soil samples were collected from identified genetic horizons of each Pedon for analysis of relevant soil properties. A total of 40 composite soil samples and 40 undisturbed samples were also collected from randomly pre-defined georeferenced points at depths of 0-20 and extended up to 50 cm depending on the effective root depth of the crops for the determination of selected soil physicochemical properties. Furthermore, for SOCS assessment, a total of 80 composite and 80 undisturbed soil samples (40 soil samples each from the two soil layers) were collected from randomly pre-defined sampling points at the depth of 0-20 and 20-40 cm. The dry soil color of surface horizons varied from dark reddish brown (5YR3/3) for Upper Slope Pedon (USP1) to brown (7.5YR4/2) for Lower Slope Pedon (LSP5) whereas the moist soil color of surface horizons varied from dark reddish brown (5YR2.5/2) for USP1 to dark brown (7.5YR3/2) for LSP5. The dry soil color of the subsurface horizon varied from dark reddish brown (2.5YR3/4) for USP1 and USP2 to light gray (5YR7/1) for LSP5 whereas the moist soil color of subsurface horizons varied from dusky red (2.5YR3/2) for USP1 to dark gray (7.5YR4/1) for LSP5. Clay contents increased with increasing soil depth, which implied the presence of eluvial-illuviation processes. The OC contents decreased consistently with soil depth. Relatively higher CEC values for the surface horizons were recorded in Pedons USP2 and LSP5, whereas relatively lower CEC values were recorded in Pedons USP1, MSP3 and Pedon MSP4. The percent base saturation (PBS) varied from 71.81 (Pedon USP1) to 85.51% (Pedon MSP3), and 55.82 to 86.25% (Pedon LSP5) on the surface and subsurface horizons, respectively. The WRB soil units identified in the Subwatershed were Eutric Rhodic Nitisols (USP1), Eutric Leptosols (Humic) (USP2), Gleyic Luvisols (Clayic, Cutanic) (Middle Slope Pedon (MSP3)), Haplic Luvisols (Clayic, Cutanic) (MSP4), and Eutric Vertic Albic Planosols (Loamic)(LSP5) and equivalent to Typic Rhodustalfs, Lithic Ustorthents, Typic Haplustalfs, Typic Haplustalfs, and Vertic Paleustalfs subgroups of USDA Soil Taxonomy, respectively. The exponential model described the spatial structures of most of the soil properties such as silt, clay, pH-H2O, organic carbon (OC), total nitrogen (TN), magnesium (Mg), cation exchange capacity (CEC), percent base saturation (PBS), cupper (Cu), and K: Mg. Accordingly, %sand, %silt, %clay, bulk density (BD), TN, available phosphorus (Av. P), Ca, Cu, manganese (Mn), and K: Mg ratio revealed a moderate spatial dependence. Av. P, Cu, and boron (B) revealed deficiencies in the soils of the study area. The mean values of SOC and SOCS were 21.51 (g/kg) and 34.06 (t/ha), respectively at 0-20cm and 17.09 (g/kg) and 25.85 (t/ha), respectively at 20-40 cm soil depths. In this study, soil organic carbon (SOC) (g/kg) and SOCS (t/ha) showed a decreasing trend with increasing soil depth. In general, the outcome of analyzing the spatial variability of soil parameters typically yields data that can be used to develop site-specific nutrient management strategies