The general term that includes (large-molecule) humic acid, fulvic acid, and humin, all of which form the natural organic matter (NOM) of some soils, sediments, peats, coals, and shales. Non-humic materials, such as amino acids, peptides, and sugars, may also be present in some soils as NOM. Humic substances impart the brown or black color to some soils and surface waters as well as biological productivity. Humic substances do not further biodegrade readily. Humic substances are quite heterogeneous and complex, and studies often provide average properties and chemical compositions. However, Lehmann and Kleber (2015) have suggested that large-molecule humic substances do not actually exist in soils, and NOM form from a progressive breakdown of organic compounds.
Cf., fulvic acid, humic acid, humification, humin