Fulvic acid, one of two classes of natural acidic organic polymer that can be extracted from
humus found in
soil, sediment, or aquatic environments. Its name derives from Latin
fulvus, indicating its yellow colour. This organic
matter is soluble in strong
acid (pH = 1) and has the average chemical formula C
135H
182O
95N
5S
2.
A hydrogen-to-carbon ratio greater than 1:1 indicates less aromatic
character (i.e., fewer benzene rings in the structure), while an
oxygen-to-carbon ratio greater than 0.5:1 indicates more acidic
character than in other organic fractions of
humus (for example,
humic acid,
the other natural acidic organic polymer that can be extracted from
humus). Its structure is best characterized as a loose assembly of
aromatic organic polymers with many carboxyl groups (COOH) that release
hydrogen ions, resulting in species that have electric charges at
various sites on the ion. It is especially reactive with metals, forming
strong complexes with Fe
3+, Al
3+, and Cu
2+
in particular and leading to their increased solubility in natural
waters. Fulvic acid is believed to originate as a product of microbial
metabolism, although it is not synthesized as a life-sustaining carbon
or energy source.