Fulvic acid

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 C₁₃₅H₁₈₂O₉₅N₅S₂. 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 Fe³⁺, Al³⁺, and Cu²⁺ 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.

Humic acid

Humic acid, one of two classes of natural acidic organic polymer that can be extracted from humus found in soil, sediment, or aquatic environments. The process by which humic acid forms in humus is not well understood, but the consensus is that it accumulates gradually as a residue from the metabolism of microorganisms. Its structure is unlike that of proteins or carbohydrates, the two most common organic polymers found in biological material; instead, humic acid can be characterized as a loose assembly of aromatic polymers of varying acidity and reactivity.

Humic acid has the average chemical formula C₁₈₇H₁₈₆O₈₉N₉S₁ and is insoluble in strong acid (pH = 1). A 1:1 hydrogen-to-carbon ratio indicates a significant degree of aromatic character (i.e., the presence of benzene rings in the structure), whereas a low oxygen-to-carbon ratio indicates fewer acidic functional groups than occur in fulvic acid, the other acidic organic polymer that can be extracted from humus. Transition and heavy metals—for example, Fe³⁺ or Pb²⁺—as well as other compounds having aromatic or hydrophobic (water-insoluble) chemical structures (i.e., organic pesticides or anthropogenic hydrocarbons), react strongly with humic acid. This property makes it an effective agent in sequestering many of the pollutants in terrestrial and aquatic environments.