Influence of Parent Material on Soil Properties along a Toposequence in Iresa-Apa, Southern Guinea Savanna

The Iresa-Apa toposequence in the Southern Guinea Savanna of Nigeria exhibits a well-defined drainage catena where landscape position significantly influences soil properties. The soils are moderately to strongly developed, with distinct horizons ranging from dark, organic-rich surface layers (5YR–10YR) to yellowish-brown and orange subsoils enriched with iron oxides. Redoximorphic features, including Fe–Mn concretions and mottling in the B and BC horizons, indicate seasonal water saturation, particularly in lower slope positions. Particle size distribution shows clear topographic variation. The summit exhibits coarse-textured subsoil with high sand content (up to 86.3%), while the upper, middle, and lower slopes are dominated by clay (>60%), indicating downslope clay accumulation and advanced weathering. Increasing clay content with depth and the presence of clay cutans confirm active illuviation and argillic horizon development. Soil structure transitions from friable granular surface layers to firm blocky subsurface horizons, restricting permeability and root penetration. Soil pH is moderately acidic (5.0–5.5) and decreases slightly with depth due to leaching. Organic carbon is highest in surface horizons (3.5–4.0%) but declines sharply downward, while total nitrogen remains low. Effective cation exchange capacity is generally low (<2.0 cmol kg⁻¹). Available phosphorus decreases with depth due to fixation by Fe and Al oxides. In conclusion, while the Iresa-apa soils possess moderate inherent fertility, their productivity is limited by subsoil compaction and impeded drainage. Sustainable use requires topography-specific management, emphasizing strategic subsoiling and moisture-tolerant cropping systems to navigate the seasonal hydrological constraints.