Humic acids are humic substances that naturally occur in humus soil, peat and lignite. They are produced by partial degradation of dead organic matter in the soil (humification). Humic acids play an important role as bio-effectors and in the storage of alkaline nitrogen compounds in humus soils. The salts of humic acids, which were first described by the German chemist Franz Karl Achard, are called humate. The molar mass of the high-molecular compounds ranges between 2,000 and 300,000 daltons. They primarily consist of incompletely degraded plant lignin as well as cellulose, to which proteins and carbohydrates are often attached. Easily degradable substances such as sugar are terminally oxidized, while substances that are difficult to break down, such as lignin, waxes as well as fatty and proteinic components, remain in the soil for a long time.
Humus is usually present in the form of a dark mass of organic, poorly degradable substances that are converted into chemically not clearly defined polymeric substances by bacteria, fungi and protozoa. Humic acids play an important role as natural ion exchangers in humus soil. They bind alkaline nitrogen compounds and release them again in exchange for metallic cations. In addition to the carboxyl groups (acid group), they also have other functional groups. They dissociate into an electrically highly-charged polyanion and a corresponding number of cations in water. Humic acids are removed by means of active carbon filters, special ion exchange filters or membrane processes during water treatment, otherwise they may yellow the water and damage downstream ion exchange resins.
Humic substances can be fractionated (i.e. chemically divided) according to their different solubility. According to F. J. Stevenson a distinction is made between a) water-soluble fulvic acids (molar mass below 3,000 daltons), b) water-insoluble but alkaline-soluble humic acids and c) water- and alkaline-insoluble humins. Hymatomelanic acids can be separated from humic acids when they are dissolved in alcohol. Furthermore, it is possible to separate humic acids in gray and brown humic acids by using special methods. Humic acids can be precipitated with dilute mineral acids from their alkaline solutions. The International Humic Substances Society (IHSS) deals with the chemistry, properties and application of humic acids, in particular, in environmental engineering, water management and agriculture. The International Moor and Peat Society (IMTG) is primarily engaged in the research of humic and fulvic acids contained in peat and their application in agriculture (horticulture) and medicine (balneology).