Assessment of Heavy Metals Mobility in Selected Contaminated Cocoa Soils in Ondo State, Nigeria

Phytophtora pod rot is currently the most important limiting factor in the Nigeria cocoa Industry with a total losses of 30-90% crop loss. To overcome this problem, Nigerian cocoa farmers solely use copper-based fungicides to control the disease. Copper being a metal is not biodegradable. Hence, it accumulates in the soil as a result of continual use. Heavy metals are potentially toxic to soil microbes and soil environment in general. Toxicity depends on chemical association in soil. For this reason, determining the chemical form of a metal in soils is important to evaluate its mobility and bioavailability. Sequential extraction was used to fractionate four heavy metals (Cu, Zn, Cd and Pb) in soils from ten contaminated cocoa plantations across Ondo State, Nigeria into six operationally defined pools: water soluble, extractable, carbonate, Fe-Mn oxide, organic and residual. The organic fraction was the most abundant pool for copper and zinc, extractable fraction for Lead and residual for cadmium. Mobility and bioavailability of these four metals in the studied soils were in the order : Pb>Zn>Cd>Cu. This indicates that lead had the highest chance of being accumulated in cocoa beans.