Soil–plant transfer of trace elements—an environmental issue

The transfer of trace elements within the soil–plant chain is a part of the biochemical cycling of chemical elements—it is an element flow from nonliving to the living compartments of the biosphere. Several factors control the processes of mobility and availability of elements; in general, they are of geochemical, climatic, biological, as well as of anthropogenic origin. The soil is a heterogeneous mixture of different organisms and mineral, organic and organo-mineral substances present in three phases: solid, liquid, gaseous. Thus, trace metals occur in different species according to whether they are external or internal bound to various soil components or in the liquid phase. The solubility of elements, thus, their concentration in the soil solution depend upon the solubility equilibrium. However, the equilibrium constant is constant only under specific conditions. Therefore, the solution equilibrium for soil phases is not easy to predict. So, in order to assess the speciation or binding forms of trace elements in soil material, different analytical procedures have been developed, including successive extraction methods applied broadly for the speciation of trace metals. Criteria for environmental protection related to the trace element status in soils should consider all the major variables that govern their behavior, and in particular those that control their mobility and soil–plant transfer.