Humic Substances: Hypotheses and Reality (a Review)

The term humic substances (HS) refers to a group of heterogeneous, dark-colored polydisperse substances found in soils, peats, natural waters, and sediments. It is hypothesized that HSs are formed as a result of degradation and transformation of biomolecules of organic residues and free radical condensation reactions (a process called humification). Humic substances are classified as a special category of natural compounds that are dissimilar to the biomolecules of plant and microbial tissues, and are resistant to biodegradation. On the basis of their solubility in alkalis, HSs are divided into humic acids (HAs, soluble, precipitated at pH < 2), fulvic acids (FAs, soluble at all pH values), and humin (insoluble residue). The review provides a critical analysis of the HS terminology and nomenclature, the method of extraction of HS from natural objects, and the hypotheses describing their formation. The ambiguity and duality of the concept of HSs (specific compounds and the sum of operational fractions) are demonstrated, as well as an arbitrary character of the division of organic matter (OM) into the dark-colored compounds of poorly defined structure (HSs) and the substances with a known structure (non-HSs). The interpretation of HSs as mass products of a secondary synthesis requires revision. The possibility of extracellular free radical reactions in soils leaves no doubt; however, a quantitative contribution of the corresponding products to the natural OM has not been established yet. The traditional alkaline extraction should be considered as a method for isolation of hydrophilic polar substances, while the precipitation with acid as their concentration for further analysis. The historical names of humic fractions (HAs, FAs, and humin) should be retained as well-established generic terms and names of the preparations obtained in a certain way without attaching unique features and specificity to all components of these fractions. The C-HA/C-FA (or C-HA/C-org) ratios are simple and convenient indicators of the types of humus reflecting bioclimatic conditions of its formation.