Status of accumulated heavy metals in soils from organic and conventional farms in Akinyele Local Government, Ibadan, Oyo State, Nigeria

The risk associated with the presence of heavy metals in soil is their potential toxicity and ability to enter the ecosystem through the food chain. Total heavy metal content of a soil is inadequate for predicting the toxicity of heavy metals in soil. Therefore, the current study was designed to determine the various forms in which the selected heavy metals: Chromium (Cr), Lead (Pb), Nickel (Ni), and Cadmium (Cd) exist in the soil to ascertain the available forms for plant uptake. Soils samples were randomly collected from selected organic and conventional farms in Akinyele local government, Ibadan, Oyo State, Nigeria, at different depths (0-15, 15-30 and 30-45 cm) using random sampling method. Routine analysis was carried out to determine the textural classes and chemical properties of soil samples. The total heavy metals of the soil samples were determined and their fractions were analysed using sequential extraction method. Cadmium was not detected in most of the soil samples. Residual fraction was predominant in all the analysed heavy metals. In addition, Cr was more associated with the oxidizable fraction having a range of 0.1 mg/kg from Ajibode organic farm (AO) at 30-45 cm depth. Pb and Ni were both more associated with reducible fraction with Pb having a range of 2.7 mg/kg in Ajibode conventional farm (AI) at 0-15 cm depth and 0.1 mg/kg in (AI) at 30-45 cm depth. Reducible values of Ni ranges from 4.3 mg/kg in AI at 0-15 cm to 1.4 mg/kg in AO at 30-45 cm. The apparent mobility and bioavailability for these heavy metals in the studied soils were in the order: Pb>Cr>Ni. The result showed that uptake of heavy metals by the plants would be low as the concentration of the exchangeable form of the heavy metals in the analysed soil sample has the least concentration when compared with the other forms.