a dark brown humic substance that is soluble in water only at pH values greater than 2; "the half-life of humic acid is measured in centuries"
Dark brown acid derived from humus or partially decomposed organic material in soils, particularly peats
Portion of humus that is water insoluble and is extracted from soil with dilute alkali and precipitated upon acidification.
Humic substances that are soluble in strong base solutions but insoluble in acidified (to pH < 2) water, and that affect water quality through exchange of species, such as cations or organic materials.
Usually refers to the mixture of ill-defined dark organic substances precipitated upon acidification of a dilute alkali extract of soil. Some workers use it to include only the alcohol-insoluable portion of the precipitate. The main constituent of humus, composed of proteins and lignins, dark brown to black in color.
Humic acid is one of the major components of humic substances (or Natural Organic Matter (NOM)) which are dark brown and major constituents of soil organic matter humus that contributes to soil chemical and physical quality and are also precursors of some fossil fuels. They can also be found in peat, coal, many upland streams and ocean water.