Both lignin and humic acid are partly soluble in alcohol and pyridine. Both lignin and humic acid are soluble in alkali and precipitated by acids. Both lignin and humic acid contain OCH 3 groups. Both lignin and humic acid are acidic in nature. When lignins are warmed with aqueous alkali, they are transformed into methoxyl-containing humic acids.
Humic acids have properties similar to oxidized lignins.
Although lignin is less easily attacked by microorganisms than other plant components, mechanisms exist in nature for its complete aerobic decomposition. Otherwise undecomposed plant remains would accumulate on the soil surface and the organic matter content of the soil would gradually increase until CO 2 was depleted from the atmosphere. The ability of soil organisms to degrade lignin has been understimated in some quarters and its contribution to humus has been exaggerated. On the other hand, the fungi that degrade lignin are not normally found in excessively wet sediments. Accordingly, it seems logical to assume that modified lignins may make a major contribution to the humus of peat, lake sediments, and poorly drained soils.