Humus is an amorphous, dark brown, organic material that is formed by the incomplete decomposition of biomass. Strictly speaking, humus is composed of organic residues that are sufficiently fragmented and decomposed by microbial and other decomposition processes that the original source of the biotic materials is no longer recognizable.
Humus is mostly composed of a very complex mixture of large organic molecules, known as humic compounds, which are resistant to further biological oxidation by microorganisms and are therefore relatively persistent in the environment. Humus is the major component of the organic matter of soil. Soluble humic substances also occur in ground water and surface waters, sometimes giving lakes and rivers a dark, tea-colored appearance.
Humic substances are divided into three functional classes on the basis of their solubility in aqueous solutions of various pH. Humic acids are soluble in strongly alkaline solutions, while fulvic acids are soluble in both alkaline and strongly acidic solutions, and humins are insoluble in either. However, apart from their solubility in these solutions, these fractions of polymeric humic substances cannot be easily differentiated or characterized in terms of chemical structure. All of these humic substances are effective at absorbing water and in binding a wide range of organic and inorganic chemicals. Most of the favorable qualities of humus in soil are associated with these properties of humic substances.
Humus is a very important aspect of soil quality. Some of the most beneficial attributes of humus are associated with its ability to make small, inorganic particles adhere together as loose, friable aggregates. The resulting, relatively coarse physical structure allows oxygen to penetrate effectively into the soil, which is an important benefit in terms of supporting microbial processes related to decay and nutrient cycling, as well as providing for the respiration of plant roots. Humus also improves the water-holding capacity of soils, which helps to mitigate drought because rainwater does not drain rapidly to depths below the penetration of plant roots. Humus is also important in binding ionic forms of nutrients, and in serving as a nutrient reservoir of organically bound nutrients, which are slowly released for plant uptake by microbial nutrient cycling processes.
Because of these characteristics of humus, agricultural and horticultural soils that are composed of a mixture of humus and inorganic minerals usually have a substantially greater capability for supporting a vigorous and healthy growth of plants. Compared with soils that are lacking in humus, such substrates are better aerated and have an improved water and nutrient holding capacity, and they are generally more fertile. These important benefits are why one of the highest priority objectives of organic methods of agriculture is to improve the concentration of humus in soil.
See also Organic farming.