Humic substances (HS) are colored relatively recalcitrant organic compounds naturally formed during long-term decomposition and transformation of biomass residues. The color of humic substances varies from bright yellow to light or dark brown leading to black. The term comes from humus, which in turn comes from the Latin word humus, meaning "soil, earth".[1] Humic substances represent the major part of organic matter in soil, peat, coal, and sediments, and are important components of dissolved natural organic matter (NOM) in lakes (especially dystrophic lakes), rivers, and sea water. Humic substances account for 50 – 90% of cation exchange capacity in soils. "Humic substances" is an umbrella term covering humic acid, fulvic acid and humin, which differ in solubility. By definition, humic acid (HA) is soluble in water at neutral and alkaline pH, but insoluble at acidic pH < 2. Fulvic acid (FA) is soluble in water at any pH. Humin is not soluble in water at any pH. This definition of humic substances is largely operational. It is rooted in the history of soil science and, more precisely, in the tradition of alkaline extraction, which dates back to 1786, when Franz Karl Achard treated peat with a solution of potassium hydroxide and, after subsequent addition of an acid, obtained an amorphous dark precipitate (i.e., humic acid). Aquatic humic substances were isolated for the first time in 1806, from spring water by Jöns Jakob Berzelius. In terms of chemistry, FA, HA, and humin share more similarities than differences and represent a continuum of humic molecules. All of them are constructed from similar aromatic, polyaromatic, aliphatic, and carbohydrate units and contain the same functional groups (mainly carboxylic, phenolic, and ester groups), albeit in varying proportions. Water solubility of humic substances is primarily governed by interplay of two factors: the amount of ionizable functional groups and (mainly carboxylic) and molecular weight (MW). In general, fulvic acid has a higher amount of carboxylic groups and lower average molecular weight than does humic acid. Measured average molecular weights vary with source, however, molecular weight distributions of HA and FA significantly overlap. Age and origin of the source material determine the chemical structure of humic substances. In general, humic substances derived from soil and peat (which takes hundreds to thousands of years to form) have higher molecular weight, higher amounts of O and N, more carbohydrate units, and fewer polyaromatic units than humic substances derived from coal and leonardite (which takes millions of years to form). Isolation of HS is the result of an alkaline extraction from solid sources of NOM the adsorption of HS on a resin.[2][3][4] A newer view of humic substances is that they are not mostly high-molecular-weight macropolymers but rather represent a heterogeneous mixture of relatively small molecular components of the soil organic matter auto-assembled in supramolecular associations and are composed of a variety of compounds of biological origin and synthesized by abiotic and biotic reactions in soil. and surface waters[5] It is the large molecular complexity of the soil humeome[6] that confers to humic matter its bioactivity in, its stability in ecosystems, soil and its role as plant growth promoter (in particular plant roots).[7] The academic definition of humic substances is under debate and some researchers argue against the traditional concepts of humification and seek to forgo alkali extract method and directly analyze the soil, [8] Concepts of humic substances The formation of HS in nature is one of the least understood aspects of humus chemistry and one of the most intriguing. Historically, there have been three main theories to explain it: the lignin theory of Waksman (1932), the polyphenol theory, and the sugar-amine condensation theory of Maillard (1911).[9][10] Humic substances are formed by the microbial degradation of dead biota matter, such as lignin, cellulose. ligno-cellulose and charcoal.[11][12] Humic substances in the lab are resistant to further biodegradation. Their structure, elemental composition and content of functional groups of a given sample depend on the water or soil source and on the specific procedures and conditions of extraction. Nevertheless, the average properties of lab extractes HS from different sources are remarkably similar.