Heavy metals in soil Both geological weathering and anthropogenic activities have introduced point and diffuse sources of heavy metals to the environment. Mining, smelting, industrial processing and waste disposal have impacted on heavy metal concentrations in urban and rural environments alike, whilst fertilizers, herbicides and pesticides have contributed to the prevalence of high concentrations of heavy metals in some agricultural systems (Ross, 1994). In excessive concentration the heavy metals regarded as the most toxic and environmentally damaging are cadmium (Cd), chromium (Cr), copper (Cu), mercury (Hg), nickel (Ni), lead (Pb) and zinc (Zn) (Ross, 1994). Several of these, especially the transition metals, are essential for plant metabolism (e.g. Cu, Ni, Zn). Heavy metals, by defi nition, are a group of elements with specifi c gravities of > 5g cm-3 (Ross, 1994) which are both industrially and biologically important (Alloway, 1995). Although not a

heavy metal by this defi nition, the metalloid arsenic (As) is given the status of ‘risk element’ or ‘potentially toxic element’ due to its carcinogenic effect on humans and toxicity to plants (Moreno-Jimenez et al, 2012). Excessive concentrations of heavy metals and As that, through direct or secondary exposure, can cause a toxic response to biota or humans resulting in an unacceptable level of environmental risk (Adriano, 2001; Abrahams, 2002; Vangronsveld et al, 2009) may be classed as pollutants.