Humic acids, the definition
Humus compounds are complex natural organic compounds that are formed in soils from plant residues, by a process of "humification". Humus materials are complex aggregate of brown to dark colored amorphous substances, which have originated during the decomposition of plant and animal residues by microorganisms, under aerobic and anaerobic conditions, in soils, composts, peat bogs, and water basins.
Chemically, humus consists of certain constituents of the original plant material resistant to further decomposition; of substances undergoing decomposition; of complexes resulting from decomposition, either by processes of hydrolysis or by oxidation and reduction; and of various compounds synthesized by microorganisms. "Humic acid " is the commercial term often used to refer to the combined humic and fulvic acid content found in these naturally occurring deposits. Humic acid is known to be among the most bio-chemically active materials found in soil.
The fraction of humic substances that is not soluble in water under acidic conditions (pH < 2) but is soluble at higher pH values. They can be extracted from soil by various reagents and which is insoluble in dilute acid. Humic acids are the major extractable component of soil humic substances. They are dark brown to black in color.
The fraction of humic substances that is soluble in water under all pH conditions. They remains in solution after removal of humic acid by acidification. Fulvic acids are light yellow to yellow-brown in color.
The fraction of humic substances that is not soluble in water at any pH value and in alkali. Humins are black in color.