ABSTRACT
Heavy metals in soils are in forms that are strongly determined by their origin and history. Although native metals are frequently in highly immobile forms,1 anthropogenic forms are often more reactive and thus are more available to plants. In the latter case, however, a number of reactions with soil components2 contribute to a progressive insolubilization of metals entering the soils. Tagami and Uchida3 concluded that, although exchangeable and adsorbed fractions of several metals were observed after being freshly introduced in soil samples, the exchangeable fractions decreased to almost zero within a few days. Ma and Uren4 also found that when water-soluble heavy metals were added to a soil they are rapidly retained by the soils, and the reactive forms then slowly transform into highly stable forms.
