Nature of Humic Acids

THE idea that humic acids are heteropolycondensates1of phenolic substances with or without the participation of amino-acids is now widely accepted. Three possible sources of the phenols are postulated: (1) phenolic materials such as flavonoids leached from the plant debris; (2) phenolic units formed during the decomposition of lignin; (3) phenolic substances synthesized by soil micro-organisms which may have been utilizing carbohydrates. However, attempts to identify the units in the condensate have been hampered by the absence of degradative procedures, which yield monomers with unchanged substitution patterns and intact carbon side-chains. Previous chemical investigations have, therefore, been primarily concerned with elemental analysis and the determination of active groups1,2which are mainly carboxyl and phenolic hydroxyls and possibly smaller quantities of quinonoid and primary alcohol groupings.