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Natural processes, agricultural practice as well as environmental contamination continuously perturb the chemical composition of soil. This pertubation may be intended (liming, fertilization) or inadvertent (pollution). The effect of the pertubation depends on the buffering capacity of the soil system to counter changes. The buffering capacity depends on the chemical reactivity and is compound-specific. This implies that it differs for each compound as the involved reactions and reaction rates differ. Hence, the quantification of the buffering capacity requires understanding of the speciation processes (complexation, adsorption/desorption, precipitation/dissolution, degradation) for the compounds of interest. By these processes, the inflow of relatively large concentrations of solutes is countered: the reactions in soil lead to a decrease in concentration of some solutes, but may result in a simultaneous increase in the concentration of other solutes. In many cases the concentration in solution is the key variable with regard to effects because it usually reflects the biological availability of the compound (for plants) and the leaching hazard to groundwater. During the past decades a massive research effort has been devoted to speciation processes in soils. Since a review is hardly possible in one chapter, we consider only a few aspects in a little more detail, e.g., metal ion binding, heterogeneity at different scales and relations between chemistry and transport. |
