Characterization of Soil Properties in Basement Complex Landscapes of Orile-Igbon/Gambari, Oyo State, Nigeria

This study assessed the physical and chemical properties of soils along a toposequence in the Orile-Igbon/Gambari area of Surulere Local Government Area, Oyo State, Nigeria. The toposequence was divided into upper, middle, and lower slope positions. Soil profile pits were excavated at each position, and samples were collected from all horizons for laboratory analysis. Physical properties evaluated included soil texture, structure, color, and horizon depth, while chemical properties comprised soil pH, exchangeable cations (Ca²⁺, Mg²⁺, K⁺, and Na⁺), effective cation exchange capacity (ECEC), organic carbon, total nitrogen, available phosphorus, and selected micronutrients. Clear variations in soil physical characteristics occurred along the toposequence. Upper slope soils were light-colored, coarse-textured, and well-drained, with sandy clay to sandy clay loam textures and loose consistence. Middle slope soils exhibited darker surface horizons, finer textures, and increased clay accumulation in subsurface horizons, resulting in moderate water retention and nutrient-holding capacity. Lower slope soils showed darker horizons, finer textures, blocky structures, and evidence of waterlogging in deeper horizons, indicating restricted drainage and downslope material accumulation. The soils were slightly acidic, with pH values of 5.0–5.7. Exchangeable calcium dominated the exchange complex, while magnesium, potassium, and sodium occurred at low levels. ECEC values indicated moderate nutrient-holding capacity. Organic carbon and total nitrogen contents were higher in surface horizons, particularly at the lower slope, due to organic matter accumulation. Available phosphorus varied and may limit crop growth in some horizons, while micronutrients were generally adequate. Overall, topography influenced soil properties, land-use planning, and sustainable agricultural production.