Humic Acids

dark-colored amorphous organic substances of high molecular weight, the structure of which has not been conclusively established.

Humic acids are present in peats (about 50 percent) and sooty lignites (about 60 percent); they are less abundant in compact and transitional coals, and they vary from 0 to 100 percent of the organic matter (depending on the degree of weathering) in weathered brown and pit coal. In soils the humic acid content is highest in chernozem (to 10 percent). Humic acids are produced by the bacterial decomposition of dead plant residues and by the prolonged action of atmospheric oxygen or stratal water on organic matter.

The structure of humic acids is determined by the presence of weakly condensed and substituted aromatic nuclei interlinked by parts of a nonaromatic nature. The molecules consist of carboxyl and carbonyl groups, alcoholic and phenolic hydroxyls, and (sometimes) methoxyl groups. Dark solutions are formed when an alkali acts on humic acids. The acids are precipitated in the form of swollen gels from these solutions after exposure to acids and salt solutions of higher-valence metals. Humic acids have the capacity for ion-exchange reactions forming soluble and insoluble humates. Humic acids are powerful geochemical agents that help to break down rocks and minerals; they help to concentrate, scatter, and redeposit elements in the earth’s crust. They are the main constituent of the organic matter in soil (humus), which is responsible for soil fertility.

Humic acids are used in the production of lead accumulators and in stabilizing slurries when drilling wells. They are also plant growth stimulants and components of organic mineral (humic) fertilizers.