Abstract:
This study investigated the concentrations of cadmium (Cd), copper (Cu), chromium (Cr), cobalt (Co), zinc (Zn), manganese (Mn), nickel (Ni) and lead (Pb) in soils from Cedara Research Farm in KwaZulu-Natal, South Africa. Soil samples were collected using a beta soil auger from areas representing different land uses and analysed for total and bioavailable metal concentrations. Total metals were determined through complete acid digestion, while bioavailable fractions were assessed using the Ambic-2 extraction method. Metal concentrations were quantified using inductively coupled plasma–atomic emission spectroscopy (ICP-AES) and atomic absorption spectroscopy (AAS).
The mean concentrations of metals followed the order Mn > Pb > Zn > Cr > Ni > Cu > Co > Cd, with values of 245.57, 206.94, 127.19, 98.76, 23.80, 21.83, 2.16 and 0.97 mg·kg−1, respectively. The concentrations of Cr, Cu, Pb and Zn exceeded the allowed limits for agricultural soils, while Co was undetectable at all sites. Cadmium was detected in only three fields and Ni remained within acceptable limits across all sites. Manganese levels were consistently higher than those at the control site. The pollution indices (Igeo, EF and CF) indicated significant contamination by Cu, Zn and Mn, particularly in non-agricultural fields. However, the cropping areas exhibited comparatively lower concentrations, suggesting the mobilisation and partial removal of metals through agricultural activity. Comparison with background values from the 1970s confirmed ongoing soil contamination.
Between the two extraction methods tested, Ambic-2 extracted higher and more consistent metal concentrations than ammonium-EDTA. Elevated levels of heavy metals in these agricultural soils pose potential risks to food safety and human health. Routine monitoring and inclusion of heavy metal analyses in standard soil fertility assessments are recommended to improve soil management and mitigate heavy metal pollution.
