Spatial and Depth-Wise Variability of Soil Organic Carbon and Associated Soil Properties for Land Capability and Crop Suitability in the Northern Guinea Savanna, Nigeria

Soil Organic Carbon (SOC) is pivotal indicator of soil health, fertility, and land productivity, particularly in tropical savanna systems prone to degradation. In Niger State, Nigeria, continuous cultivation, low organic inputs, and climate variability have accelerated soil quality decline, affecting crop performance and land capability. However, depth resolved assessments linking SOC, soil physical and chemical properties, and crop suitability remain limited. Soil samples were collected from Gidan Kwano (GK) Campus, Federal University of Technology, Minna, and Umaru Sanda Ahmadu College of Education (COE), Minna, across three depths (0–20, 20–40, 40–60 cm). A total of 54 composite samples were analyzed for pH, electrical conductivity (EC), SOC, total nitrogen (N), phosphorus (P), potassium (K), and cation exchange capacity (CEC), as well as particle size distribution. Descriptive statistics, independent samples t-tests, and repeated measures ANOVA were applied to evaluate site and depth effects. Land capability and crop suitability for maize, soybean, and millet were assessed using the USDA framework and FAO parametric approach. COE soils exhibited higher SOC (11.1–13.97 g kg⁻¹), K (1.85–2.52 cmol kg⁻¹), and CEC (6.46–7.66 cmol kg⁻¹) compared to GK soils (SOC 8.85–10.77 g kg⁻¹; K 1.59–3.48 cmol kg⁻¹; CEC 6.35–7.33 cmol kg⁻¹). pH was significantly higher at COE (6.40–6.79) than GK (5.31–6.20). EC and available P were low across both sites (0.04–0.18 dS m⁻¹; 0.09–0.10 mg kg⁻¹). Depth trends revealed decreasing SOC and CEC with increasing depth. Soil texture varied, with GK soils being sandy (72.9–80.8 % sand) and COE soils finer (61.2–78.8 % sand). Land capability classification indicated moderate to severe limitations (Classes II–III), and crop suitability assessments showed maize and soybean mostly moderately suitable (S2), while millet tolerated lower fertility but with reduced yield potential. Spatial and depth wise variability in SOC, pH, texture, and CEC strongly influenced land capability and crop-specific suitability. Targeted management practices such as SOC restoration, residue retention, and balanced fertilization are recommended to enhance productivity and improve long term soil health in the Northern Guinea Savanna.