Humic substances (HS) are coloured recalcitrant organic compounds naturally formed during long-term decomposition and transformation of biomass residues. The colour of humic substances varies from yellow to brown to black. 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" is an umbrella term covering humic acid, fulvic acid, humin and hymatomelanic acid which differ in solubility. By definition, humic acid is soluble in water at neutral and alkaline pH, but insoluble at acidic pH < 2. Fulvic acid is soluble in water at any pH. Humin is not soluble in water at any pH. Hymatomelanic acid is part of humic acid that is soluble in ethanol.

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 later, in 1806, from spring water by Jöns Jakob Berzelius.

In terms of chemistry, fulvic acid, humic acid 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) albeit in varying proportions.

Water solubility of humic substances is primarily governed by interplay of two factors: the amount of ionizable functional groups (mainly, carboxylic) and the molecular weight. In general, fulvic acid has higher amount of carboxylic groups and lower average molecular weight than humic acid. However, molecular weight distributions of humic and fulvic acids 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 amount of functional groups, more carbohydrate units and less polyaromatic units than humic substances derived from leonardite (which takes millions of years to form).

Humic matter in isolation is the result of a chemical extraction from the soil organic matter or the dissolved organic matter and represent the humic molecules distributed in the soil or water. A new understanding views humic substances not as high-molecular-weight macropolymers but as heterogeneous and relatively small molecular components of the soil organic matter auto-assembled in supramolecular associations and composed of a variety of compounds of biological origin and synthesized by abiotic and biotic reactions in soil. It is the large molecular complexity of the soil humeome that confers to humic matter its bioactivity in soil and its role as plant growth promoter.

The academic definition of humic substances is under debate as "humification" becomes unsupported as a special case, leading to some radical definitions expanding HS to encompass all difficult-to-characterize soil organic matter, at the cost of clarity. There is also a call to forgo the traditional alkali extract method and directly analyze the soil, but its complexity prevents widespread adoption in agriculture. In practice, this means some sources may apply a traditional acid-base analysis to compost, then state the results in term of "humic substances".