The total metal content of a soil is distributed among all possible chemical forms (speciation) in the solid, liquid or the biotic phases. Use of total concentration as a criterion to assess the potential risks may be misleading as the risks may be over-estimated, implies that all forms of a given metal have an equal impact on the environment. Despite this, the bioavailability of metals and the toxicity of metals are more closely related to bioavailable metal, rather than total metal concentrations in soil. Knowing the distribution and transformation of metals under changing condition of environment is therefore very useful to understand the processes whereby metals are held in soil and the condition under which they can be released.