Integrating Coal-Based Organic and Inorganic Fertilizers to Enhance Cowpea (Vigna unguiculata [L.] Walp) Productivity and Soil Health Across Nigerian Agroecological Zones

The study evaluated the agronomic response and soil health impacts of an improved coal-based fertilizer (Saputra Carbontilizer) on cowpea (Vigna unguiculata) across three agroecological zones in Nigeria: North-West (Kano), North-Central (Benue), and North-East (Borno). A randomized complete block design with seven treatments, including various NPK-Carbontilizer blends, 100% NPK, 100% Carbontilizer, cow dung (5 t/ha), and a no-fertilizer control, was implemented on both on-station and on-farm sites. The 50% NPK/50% Carbontilizer treatment consistently outperformed other treatments in yield performance, recording grain yield of 1.63 t/ha, stover weight of 3.12 t/ha, and pod weight of 2.85 t/ha, exceeding both 100% NPK and 100% Carbontilizer. The 100% NPK treatment produced the highest chlorophyll content (SPAD = 54, p < 0.05) and stem girth (8.2 mm, p < 0.05) in Kano. Soil organic carbon significantly increased under the 50% NPK/50% Carbontilizer treatment (0.45% to 0.69%, p < 0.05), while 100% Carbontilizer yielded the highest rise in soil phosphorus (4.75 to 7.34 mg/kg, p < 0.05). Across treatments, soil pH remained slightly acidic (5.79–5.94), and heavy metal concentrations were within WHO safety thresholds (Pb = 0.002 mg/kg, Cd = 0.002 mg/kg). These findings highlight the dual benefits of integrating coal-based organic fertilizers with mineral nutrients to enhance cowpea productivity and improve soil health. However, the study was limited to short-term trials across three zones; longer-term studies are needed to capture seasonal variability and socioeconomic adoption factors. This integrated approach nevertheless offers a sustainable and scalable strategy for optimizing resource use in low-input farming systems.