Soil organic matter (SOM) is the most important source of nutrients and regulates the main physical, chemical, and biological properties of different soils. A fundamental set of issues related to the humus formation conditions are the zone-genetic approach, the biogeochemical nature of the humification process, the effects of the thermodynamics and kinetics directing the synthesis and degradation of humic substances (HS), the variability of the properties of SOM and humic substances. In this respect, important features are the physico-chemical diagnostics and identification of humic acids, the special role of mineral components in the humification process, the fundamental questions regarding the authenticity humic acids, and the knowledge derived from the study of altered and denatured products (e.g. industrially manufactured humic-like substances. Five important functions of humic substances provide the opportunity to evaluate the ecological role of humus: 1) accumulative, 2) transport, 3) regulatory, 4) physiological, and 5) protective. The latter involves the ability of humic substances to bind different pollutants: heavy metals, pesticides, radionuclides, and the surplus of fertilizers. Various detoxification mechanisms operate in the soil-plant system in highly diverse natural environments. The relative stability of soil as a whole mainly depends on the stability of humic substances and its fractions, of which humic acids and humin seem to be the most significant.
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