Trace elements accumulate locally in soils due to weathering of rock minerals. Because trace elements are essential for plants, animals, and human beings, it is necessary to ensure their adequate levels in agricultural products. Apart from trace elements originating in parent materials and entering the soil through chemical weathering processes, soil toxic trace elements have many anthropogenic sources. Campbell et al.  compared natural and anthropogenic quantities of trace metals emitted to the atmosphere and showed that around 15 times more Cd; 100 times more Pb; 13 times more Cu; and 21 times more Zn are emitted by man's activities than by natural processes.
Trace elements and their total concentrations can vary widely in different soils derived from different parent materials. Levinson  and Alloway  proposed that the more basic igneous parent materials contribute highest quantities of Cr, Mn, Co, and Ni to soils; among the sedimentary parent materials, shales potentially contribute the highest quantities of Cr, Co, Ni, Zn, and Pb. Mineral weathering rates determine the release of the elements into soil — initially, in the form of simple or complex inorganic ligands. This process depends on mineral solubility under prevailing soil pH and Eh conditions. In soils, Mn, Ni, and Cr are present in highest quantities, and Cd and Hg are present in the smallest amounts.
The concentration of trace elements such as Mn, Ni, and Cr in the soil profile depends on parent material when the parent material is the main source in the soil. Brooks  found that the concentration of Ni in soils developed on serpentine was as high as 100 to 7000 |g Ni g-1. Because many minerals are resistant to weathering, trace elements are insoluble; higher concentrations can be found in tropical soils where the weathering process is intensive.
Large quantities of Cd are applied to agricultural soils in phosphate fertilizers; the highest concentration occurs in topsoil. Topsoil concentration is maintained by vegetation recycling, and Cd and Pb show very slow movement down the profile . Colbourn and Thornton  calculated the relative topsoil enhancement (RTE) as the ratio of Pb concentration in topsoil (<15 cm) to the Pb concentration in subsoil (>15 cm). They reported RTE values of 1.2 to 2.0 in remote agricultural areas and values of 4 to 20 in areas contaminated by mining.
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