Free-radical condensation as a natural mechanism of the formation of humic acids

Recently suggested structural models of humic acids (Schulten, Schnitzer, etc.) do not agree with the existing notions on the biochemical origin of some fragmentary groups composing a significant part of the molecules of humic acids. It is hypothesized that free-radical reactions of the polycondensation of low-molecular organic compounds derived directly from soil biota, from primary products of its posthumous decomposition, and from fulvic acids represent an obligatory stage of natural humus formation. Calculations demonstrate that different natural sources of free hydroxyl radicals (forming in the troposphere and deposited with atmospheric precipitation, produced by soil prokaryotes, etc.) ensure the possibility of free-radical condensation of organic substances and the formation of humic acids in the pedosphere of the Earth; moreover, the actual supply of free hydroxyl radicals exceeds the necessary amount by several orders of magnitude. The development of this concept allows us to assume the existence of a general evolutionary scheme of transformation of organic residues of biological origin on the Earth with the formation of stable products (humic substances, peat, and coal).