Experimental evaluation of three woody plant ashes for soil remediation potential and heavy metal transfer in Abelmoschus esculentus

Abstract

Declining soil fertility resulting from continuous cultivation, nutrient depletion and dependence on inorganic fertilizers poses a major challenge to sustainable crop production in Nigeria. Woody plant ashes offer a low-cost and environmentally sustainable alternative soil amendment owing to their rich mineral composition. This study evaluated the nutrient composition, heavy metal characteristics and transfer potential of ashes derived from Azadirachta indica, Eucalyptus globulus and Prosopis africana applied to nutrient-deficient soil cultivated with Abelmoschus esculentus under controlled screen-house conditions. Surface soil (0–15 cm) collected from the experimental farm of Hussaini Adamu Federal Polytechnic, Kazaure, Nigeria, was amended with the three wood ashes at application rates of 0, 0.5, 1.0 and 1.5 kg per 5 kg soil. The ashes were characterized for essential macronutrients (N, P, K, Ca, Mg and S) and selected heavy metals using standard analytical procedures. Heavy metal concentrations in harvested okra samples were determined, while transfer factors and correlation analysis were employed to evaluate metal mobility and nutrient–metal relationships. Potassium was the predominant nutrient in all ash types, with nutrient availability following the order K > Mg > S > P > Ca > N for A. indica and E. globulus, and K > S > Mg > P > Ca > N for P. africana. Metal uptake followed the trend Pb > Cr > Cu > Cd. Prosopis africana recorded the highest Cu transfer factor (2.016), whereas A. indica consistently exhibited higher transfer values for most metals. Significant nutrient associations included a strong negative correlation between Ca and Mg (r = −0.995) and positive correlations between Ca and N (r = 0.977) and S (r = 0.891). The findings demonstrate the potential of woody plant ashes as sustainable nutrient-rich soil amendment materials.