Humus
[2] In agriculture, "humus" sometimes also is used to describe mature or natural compost extracted from a woodland or other spontaneous source for use as a soil conditioner.
[10] When not judged contaminated by pathogens, toxic heavy metals, or persistent organic pollutants according to standard tolerance levels, it is sometimes composted and used as a soil amendment.
[11] The primary materials needed for the process of humification are plant detritus and dead animals and microbes, excreta of all soil-dwelling organisms, and also black carbon resulting from past fires.
Fully humified humus, on the contrary, has a uniformly dark, spongy, and jelly-like appearance, and is amorphous; it may gradually decay over several years or persist for millennia.
[16] However, humus can be considered as having distinct properties, mostly linked to its richness in functional groups, justifying its maintenance as a specific term.
[19] Radiocarbon and other dating techniques have shown that the polyphenolic base of humus (mostly lignin and black carbon) can be very old, but the protein and carbohydrate attachments much younger, while to the light of modern concepts and methods the situation appears much more complex and unpredictable than previously thought.
Researchers in the 1940s and 1960s tried using chemical separation to analyze plant and humic compounds in forest and agricultural soils, but this proved impossible because extractants interacted with the analysed organic matter and created many artefacts.
[27] Microorganisms decompose a large portion of the soil organic matter into inorganic minerals that the roots of plants can absorb as nutrients.
Depending on the conditions in which the decomposition occurs, a fraction of the organic matter does not mineralize and instead is transformed by a process called humification.
Organic matter is humified by a combination of saprotrophic fungi, bacteria, microbes and animals such as earthworms, nematodes, protozoa, and arthropods (see Soil biology).
[30] Lignin, which is quickly transformed by white-rot fungi,[31] is one of the primary precursors of humus,[32] together with by-products of microbial[33] and animal[34] activity.
The humus produced by humification is thus a mixture of compounds and complex biological chemicals of plant, animal, and microbial origin that has many functions and benefits in soil.
[37] The mixing activity of soil-consuming invertebrates (e.g. earthworms, termites, some millipedes) contribute to the stability of humus by favouring the formation of organo-mineral complexes with clay at the inside of their guts,[38][39] hence more carbon sequestration in humus forms such as mull and amphi, with well-developed mineral-organic horizons, when compared with moder where most organic matter accumulates at the soil surface.
