Humic Substances, Part 5, A: Characterization Of

CHARACTERIZATION OF HUMIC SUBSTANCES: A

As already pointed out, all the humic substances can be divided into components according to their solubility in different media (Fig. 1). Humic acids and FAs represent alkali-soluble humus fragments; HAs are commonly extracted using diluted alkali and precipitated with an acid, and so are separated from the soluble FAs. Humin represents the insoluble residue (Thorn 1996). From a geological point of view, humic substances are chemical intermediates between plants and fossils. The chemical nature of soils, sludge and sediments can subsequently, continually and selectively vary via the conversion and degradation of organic matter (Ziechman 1993). Extinct vegetation represents the main source of organic matter transforming to humic substances in the environment. Humification is a continuous historical process, and soil humus is a dynamic system of both chemically active and passive components (Gonzalez et al. 2003). The amount of
organic carbon in the Earth in the form of humic substances exceeds that which makes up living organisms (Engel and Macko 1993). Humic acids, one of the most important components of HS, help break up clay and compacted soils, assist in transferring micronutrients from soil to plants, enhance water retention, increase seed germination rates, and stimulate the development of microflora populations in soils. Humic acids also slow down water evaporation from soils. This is especially important in soils where clay is present at low concentration or not at all, in arid areas, and in sandy soils without the capability to hold water.

Humic acids provide also sites for microflora to colonize. Bacteria secrete enzymes which act as catalysts, liberating calcium and phosphorous from insoluble calcium phosphate, and iron and phosphorous from insoluble iron phosphate. The chemical structure of HAs is very complicated and depends on their source. The elemental composition of different FAs and HAs shows that the major elements in their composition are C, H, O, N, and S. These major elements are always present regardless their origin and country or continent (Gajdošová et al. 2001,

Tan 2003, Kurková et al. 2004). Moreover, besides elemental composition, group composition is used
to characterize HS as it gives information about the chemistry and structural properties of HS (Purdue 1988, Tan et al. 2000). Fulvic acids contain more functional groups of an acidic nature, particulary–COOH. The total acidities of fulvic acids (900–400mmol/100g) are considerably higher than for humic acids (400–870mmol/100g). Another important difference is that while the oxygen in fulvic acids is largely in known functional groups (–COOH, –OH, –C=O), with a high oxygen content, the acidity and degree of polymerisation all change systematically with increasing molecular weight. The proportion of oxygen in humic acids seems to occur as a structural component of the nucleus.