ABSTRACT
Soil humic substances (HS) compose the largest part of soil organic matter (SOM) and have a deep impact on soil properties such as buffering capacity, metalbinding ability, stability of aggregates of soil particles, water-holding capacity, and sorption of hydrophobic organic compounds, most of which in turn have a large impact on soil fertility (Senesi and Loffredo 1998; Baldock and Nelson 1999; Hayes and Malcolm 2001; González-Pérez et al. 2004; Clapp et al. 2005). It is a highly heterogeneous mixture (in chemical and physical sense) of organic compounds, many of colloidal and/or polyelectrolytic nature, having a medium to high molecular weight, yellow to black color, and a mixed aliphatic/aromatic composition. These substances are distinctive of soil systems, being the result of degradation processes of dead organisms (mainly vegetal, but also including animal) as a result of microbial activity, possibly followed for secondary synthesis/ condensation reactions, in what is called the humification process (Clapp et al. 2005). It is interesting to note that even when their composition is location dependent (due to local vegetation, fauna, and climate), their properties are similar everywhere.
