Evaluation of Selected Heavy Metal Concentrations in Soils Affected by Agricultural Waste Disposal in Akwa Ibom State, Nigeria

Abstract

This study evaluated selected heavy-metal concentrations and soil physicochemical properties at agricultural-waste disposal sites in Abak Local Government Area, Akwa Ibom State, Nigeria. Soil samples were obtained from sites affected by piggery manure, poultry manure, cassava mill effluent and palm oil mill effluent, together with unaffected control sites, at depths of 0–15, 15–30 and 30–45 cm. Particle-size distribution, pH, organic carbon, total nitrogen, available phosphorus, exchangeable cations, exchangeable acidity, effective cation exchange capacity, base saturation, and total and extractable concentrations of Ni, Fe, Cd, Cr, Cu, Pb and Zn were determined using standard laboratory procedures. Agricultural-waste-affected soils generally had higher pH, organic carbon, total nitrogen, exchangeable bases, effective cation exchange capacity and base saturation than the control, while available phosphorus was highest in the control. Most fertility indicators declined with increasing soil depth. Total Ni, Cd and Cu concentrations exceeded the permissible limits applied in the study at several sites, whereas total Fe, Cr, Pb and Zn remained below their respective limits. Extractable metal concentrations were generally low or present in trace amounts, although extractable Ni was comparatively elevated in poultry-affected soil. The findings demonstrate that agricultural wastes can improve several soil-fertility characteristics while contributing to the accumulation of potentially harmful metals. Regular monitoring and controlled waste application are therefore required to maintain soil productivity and reduce long-term environmental risks.