An assessment of the bioavailability of metals in soils on oil palm plantations in Nigeria

We investigated the speciation, bioavailability, and mobility of metals in soils from selected states on oil palm plantations in southwest Nigeria. Soils were analysed for total metal content and speciation on all sampling locations. Metal concentrations were compared against pH, electrical conductivity (EC) and soil organic matter (SOM), which were high at all the sampling locations. The speciation results deduced that the concentrations of the different fractions vary widely at each sampling location. Cd is mostly abundant in the exchangeable phase in all the samples analysed in the various locations. Ni in the Iresa-apa plantation (29.9%) and Co in the Acharu plantation (29.9%) also showed appreciable concentrations in the Fe/Mn oxide fractions. The concentrations of Pb and Cu were highest in the organic fraction. Metals in the organic phase are more released into soil solution when compared to the residual phase fractions. Chromium and Zinc were mostly associated with the residual phase when the values of the residual phase were compared with other geochemical fractions. The plantations under study, which recorded the highest concentrations of Cr and Zn in the residual fractions, were 87.9% (Okitipupa), 86.3% (Ikire), 82.7% (Apoje), 90.3% (Onishere), 89.7% (Benin City), and 85.6% (Nsukka).The results of the sequential extraction show that mostly Cr and Zn were strongly bound to the residual/inert phase in all the soils under investigation. The potential mobility of the metals with high fractions in the residual phase is as follows: Cr (74.8%), Zn (74.0%), Pb (73%), Fe (69.5%), Ni (67.0%), Cu (63.8%), Mn (30.5%), Co (30.7%), and Cd (25.9%). These values were high when compared to the values of other metals in the other mobile and potentially mobile fractions.Cape Peninsula University of Technolog