Humic acid is an organic chemical produced by decaying plants and animals. Humic acid is one of the major components of humic substances which is dark brown in colour and the major constituents of soil organic matter humus that contributes to soil chemical and physical properties and are also precursors of some fossil fuels. Humic acid can also be found in peat, coal, many upland streams and ocean water.
Humic substances make up a large portion of the dark matter in humus and are made up of complex colloidal supramolecular mixtures (Piccolo, 1996, 2001; MacCarthy, 2001) that have never been separated into pure components. Since the end of the 18th century, humic substances have been designated as either humic acid, fulvic acid or humin. These fractions are defined strictly on their solubility in either acid or alkali, describing the materials by operation only, thus imparting no chemical information about the extracted materials.
The term ‘humic substances’ is used in a generic context to distinguish the naturally occurring material from the chemical extractions named humic acid and fulvic acid, which are defined “operationally” by their solubility in alkali or acid solutions. It is important to note, however, that no sharp divisions exist between humic acids, fulvic acids and humins. They are all part of an extremely heterogeneous supramolecular system and the differences between the subdivisions are due to variations in chemical composition, acidity, degree of hydrophobicity and self-associations of molecules. When humic substances are characterized, especially when functionality is studied, there is always the problem that one usually has to separate the huge number of different bio-organic molecules into homogenous fractions.
Humic substances arise by the microbial degradation of biomolecules (lipids, proteins, carbohydrates, lignin) dispersed in the environment after the death of living cells. A modern structural description regards humic material as a supramolecular structure of relatively small bio-organic molecules (having molecular mass <1000 Da) self-assembled mainly by weak dispersive forces such as Van der Waals force, π-π, and CH-π bonds into only apparently large molecular sizes.
A large class of humic molecules is represented by hydrophobic compounds (long alkyl-chain alkanes, alkenes, fatty acids, sterols, terpenoids, and phenyl-alkyl residues of lignin degradation) which allow their self-association into supramolecular structures separated from the water medium and, thus, their long residence time in the environment. Humic substances may chelate multivalent cations such as Mg2+, Ca2+, and Fe2+. By chelating the ions, they increase the availability of these cations to organisms, including plants.