Humic substance

Humic substances (HS) are colored relatively recalcitrant <a href="/facts/Organic_compound/gqCNaN45">organic compounds</a> naturally formed during long-term decomposition and transformation of <a href="/facts/Biomass/4VHjaya3">biomass</a> residues. The color of humic substances varies from bright yellow to light or dark brown leading to black. The term comes from <a href="/facts/Humus/vlhws4I0">humus</a>, which in turn comes from the Latin word humus, meaning "soil, earth". Humic substances represent the major part of organic matter in <a href="/facts/Soil/5Ddpr3QM">soil</a>, <a href="/facts/Peat/F0kJcsLt">peat</a>, <a href="/facts/Coal/83kV3qxg">coal</a>, and <a href="/facts/Sediment/zsIH1A6U">sediments</a>, and are important components of dissolved <a href="/facts/Natural_organic_matter/t17tVCvk">natural organic matter</a> (NOM) in lakes (especially <a href="/facts/Dystrophic_lake/hDU72azU">dystrophic lakes</a>), rivers, and <a href="/facts/Sea_water/fRBJjbws">sea water</a>. Humic substances account for 50 – 90% of <a href="/facts/Cation_exchange_capacity/4DoXx9Jk">cation exchange capacity</a> 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 <a href="/facts/Franz_Karl_Achard/PMBha6y0">Franz Karl Achard</a> treated peat with a solution of <a href="/facts/Potassium_hydroxide/NBhPBuXc">potassium hydroxide</a> and, after subsequent addition of an acid, obtained an amorphous dark <a href="/facts/Precipitate/s3BgsLLY">precipitate</a> (i.e., humic acid). Aquatic humic substances were isolated for the first time in 1806, from <a href="/facts/Spring_(hydrology)/blPBY4VG">spring</a> water by <a href="/facts/J%25C3%25B6ns_Jakob_Berzelius/HakSv3sP">Jöns Jakob Berzelius</a>.
In terms of chemistry, FA, HA, and humin share more similarities than differences and represent a <a href="/facts/Continuum_(measurement)/oz3r6kvm">continuum</a> of humic molecules. All of them are constructed from similar <a href="/facts/Aromatic/wK0DzRY6">aromatic</a>, <a href="/facts/Polyaromatic/K52xSfNC">polyaromatic</a>, <a href="/facts/Aliphatic/9usN3neo">aliphatic</a>, and <a href="/facts/Carbohydrate/Geuuzxsf">carbohydrate</a> units and contain the same <a href="/facts/Functional_group/GXkSPsjM">functional groups</a> (mainly <a href="/facts/Carboxylic/U4xuMosy">carboxylic</a>, <a href="/facts/Phenols/xs1CtNTa">phenolic</a>, and <a href="/facts/Ester/FoUAIIw4">ester</a> 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, <a href="/facts/Molecular_weight_distribution/Bh3Fsd4r">molecular weight distributions</a> of HA and FA overlap significantly.
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 <a href="/facts/Leonardite/lmKq4oRM">leonardite</a> (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. 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 It is the large molecular complexity of the soil humeome that confers to humic matter its bioactivity in, its stability in ecosystems, soil and its role as plant growth promoter (in particular plant roots).
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 to analyze the soil directly.

Humic substance open-in-new

Humic substance