Phenols humic substances synthesis

Synthetic humic acids have been prepared by a number of investigators over the past 50 years. Most of these preparations are based on a preference for a specific hypothesis for the origin of humic substances (1) partial degradation of lignin (2) chemical polymerization of microbial phenols or lignin monomers and amino acids (3) cell autolysis products (4) microbial synthesis (see Stevenson, 1982, Chapter 8 for review). 

Phenolic compounds of low molecular weight, particularly P hydroxybenzoic, vanillic, p-coumaric and ferulic acids, are of widespread occurrence in soils and occur mainly but not entirely in chemically-bound forms. They are believed to be important intermediates in the formation of humic substances and, at certain concentrations, they may influence the growth of plants and the activities of soil micro-organisms (see chapter 6). There is evidence that the phenolic acids and related compounds in soils originate in part from the decomposition of plant residues and in part from synthesis by soil micro-organi sms. 

Trusov (1915,1916), as indicated in Section 1.4.1, introduced the concept that polyphenols and quinones contributed to the synthesis of HS. Subsequently, as the result of enzymatic oxidation (with oxidase enzymes from microorganisms), the phenols oxidize to quinones and these, through further condensation, are converted into dark-colored humic-type substances. 

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