Organic matter in soils resulting from decay of plant and animal materials
This article is about the organic matter in soil. For the food, seeHummus. For the band, seeHumus (band).
Humus has a characteristic black or dark brown color and is an accumulation oforganic carbon. Besides the three majorsoil horizons of (A) surface/topsoil, (B)subsoil, and (C) substratum, most soils have an organic horizon (O) on the very surface. Hard bedrock (R) is not in a strict sense soil.
The primary materials needed for the process of humification are plantdetritus, dead animals and microbes (necromass),excreta of allsoil organisms, and alsoblack carbon resulting from past fires.[16] The composition of humus varies with that of primary (plant) materials and secondary microbial and animal products. The decomposition rate of the different compounds will affect the composition of the humus.[17]
It is difficult to define humus precisely because it is a very complex substance which is still not fully understood. According to the classical conception ofSelman Waksman, long-time reported in most textbooks of soil science, humus is different from decomposingsoil organic matter. The latter looks rough and has visible remains of the original plant, animal or microbial matter, while fully humified humus, on the contrary, is amorphous and has a uniformly dark, spongy, and jelly-like appearance.[18] However, when examined under a lightmicroscope, humus may reveal tiny plant, animal, and microbial remains that have been mechanically, but not chemically, degraded.[19] This suggests an ambiguous boundary between humus and soil organic matter, leading some authors to contest the use of the termhumus and derived terms such ashumic substances orhumification, proposing theSoil Continuum Model (SCM).[20] However, humus can be considered as having distinct properties, mostly linked to its richness infunctional groups, justifying its maintenance as a specific term.[2]
Fully formed humus is essentially a collection of very large and complexmolecules formed in part fromlignin and otherpolyphenolic molecules of the original plant material (foliage, wood, bark), in part from similar molecules that have been produced bymicrobes.[21] Duringdecomposition processes thesepolyphenols are modified chemically so that they are able to join up with one another to form very large molecules. Some parts of these molecules are modified in such a way thatprotein molecules,amino acids, andamino sugars are able to attach themselves to the polyphenol "base" molecule. As protein contains bothnitrogen andsulfur, this attachment gives humus a moderate content of these two important plantnutrients.[22]
Radiocarbon and other dating techniques have shown that the polyphenolic base of humus (mostlylignin andblack carbon) can be very old, but theprotein andcarbohydrate attachments much younger, while to the light of modern concepts and methods the situation appears much more complex and unpredictable than previously thought.[23] It seems that microbes are able to pull protein off humus molecules rather more readily than they are able to break the polyphenolic base molecule itself. As protein is removed its place may be taken by younger protein, or this younger protein may attach itself to another part of the humus molecule.[24]
Soilcarbon sequestration is a major property of the soil, also considered as anecosystem service.[27] Only when it becomes stable and acquires its multi-century permanence, mostly via multiple interactions with thesoil matrix, should molecular soil humus be considered to be of significance in removing the atmosphere's current carbon dioxide overload.[28]
There is little data available on the composition of humus because it is a complex mixture that is challenging for researchers to analyze. 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.[29] Further research has been done in more recent years, though it remains an active field of study.[30]
Microorganisms decompose a large portion of thesoil organic matter into inorganic minerals that the roots of plants can absorb asnutrients. This process is termedmineralization. In this process,nitrogen (nitrogen cycle) and the other nutrients (nutrient cycle) in the decomposed organic matter are recycled. Depending on the conditions in which thedecomposition occurs, a fraction of the organic matter does not mineralize and instead is transformed by a process calledhumification. Prior to modern analytical methods, early evidence led scientists to believe that humification resulted in concatenations of organicpolymers resistant to the action of microorganisms,[31] however recent research has demonstrated that microorganisms are capable of digesting humus.[32]
Humification can occur naturally insoil or artificially in the production ofcompost. Organic matter is humified by a combination ofsaprotrophic fungi, bacteria, microbes and animals such as earthworms,nematodes,protozoa, and arthropods (seeSoil biology andSoil animals). Plant remains, including those that animals digested and excreted, contain organic compounds:sugars,starches,proteins,carbohydrates,lignins,waxes,resins, andorganic acids. Decay in the soil begins with the decomposition of sugars and starches from carbohydrates, which decompose easily asdetritivores initially invade the dead plant organs, while the remainingcellulose andlignin decompose more slowly. Simple proteins, organic acids, starches, and sugars decompose rapidly, while crude proteins, fats, waxes, and resins remain relatively unchanged for longer periods of time.[33]
Lignin, which is quickly transformed bywhite-rot fungi,[34] is one of the primary precursors of humus,[35] together with by-products of microbial[36] and animal[37] 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.[21] Some judge earthworm humus (vermicompost) to be the optimal organicmanure.[38]
Much of the humus in most soils has persisted for more than 100 years, rather than having been decomposed into CO2, and can be regarded as stable; this organic matter has been protected from decomposition by microbial or enzyme action because it is hidden (occluded) inside small aggregates of soil particles, or tightlysorbed orcomplexed toclays.[39] Most humus that is not protected in this way is decomposed within 10 years and can be regarded as less stable or morelabile.[40] The mixing activity of soil-consuming invertebrates (e.g.earthworms,termites, somemillipedes) contribute to the stability of humus by favouring the formation of mineral-organic complexes withclay minerals at the inside of theirguts,[41][42] hence morecarbon sequestration inhumus forms such asmull andamphi, with well-developed mineral-organichorizons, when compared withmoder andmor where most organic matter accumulates at the soil surface.[43]
Stable humus contributes few plant-available nutrients in soil, but it helps maintain its physical structure.[44] A very stable form of humus is formed from the slowoxidation ofsoil carbon after the incorporation of finely powderedcharcoal into thetopsoil, suggested to result from the grinding and mixing activity of a tropical earthworm.[45] This process is speculated to have been important in the formation of the unusually fertile Amazonianterra preta do Indio, also calledAmazonian Dark Earths.[46] However, some authors[20] suggest that complex soil organic molecules may be much less stable than previously thought: "the available evidence does not support the formation of large-molecular-size and persistent 'humic substances' in soils. Instead, soil organic matter is a continuum of progressively decomposing organic compounds.″
Humus has a characteristic black or dark brown color and is organic due to an accumulation of organic carbon. Soil scientists use the capital letters O, A, B, C, and E to identify the mastersoil horizons, and lowercase letters for distinctions of these horizons. Most soils have three major horizons: the surface horizon (A), the subsoil (B), and the substratum (C). Most soils have an organic horizon (O) on the surface, but this horizon can also be buried.[47] The master horizon (E) is used for subsurface horizons that have significantly lost minerals (eluviation).Bedrock, which is not soil, uses the letter R. The richness of soil horizons in humus determines their more or less dark color, generally decreasing from O to E, to the exception of deep horizons of podzolic soils enriched withcolloidal humic substances which have beenleached down the soil profile.[48]
The importance of chemically stable humus is thought by some to be thefertility it provides to soils in both a physical and chemical sense,[49] though some agricultural experts put a greater focus on other features of it, such as its ability to control diseases.[50] It helps the soil retain moisture[51] by increasingmicroporosity[52] and encourages the formation of goodsoil structure.[53][54] The incorporation ofoxygen into large organic molecular assemblages generates many active, negatively charged sites that bind to positively chargedions (cations) ofplant nutrients, making them more available to the plant by way ofion exchange.[55] Humus allows soil organisms to feed and reproduce and is often described as the "life-force" of the soil.[56][57]
The process that converts soil organic matter into humus feeds the population of microorganisms and other creatures in the soil, and thus maintains high and healthy levels of soil life.[56][57]
The rate at which soil organic matter is converted into humus promotes (when fast, e.g.mull) or limits (when slow, e.g.mor) the coexistence of plants, animals, and microorganisms in the soil.[58]
"Effective humus" and "stable humus" are additional sources of nutrients for microbes: the former provides a readily available supply, and the latter acts as a long-term storage reservoir.[59]
Decomposition of dead plant material causes complex organic compounds to be slowly oxidized (lignin-like humus) or to decompose into simpler forms (sugars andamino sugars, andaliphatic andphenolicorganic acids), which are further transformed into microbial biomass (microbial humus) or reorganized and further oxidized into humic assemblages (fulvic acids andhumic acids), which bind toclay minerals andmetal hydroxides.[60] The ability of plants to absorb humic substances with their roots andmetabolize them has been long debated.[61] There is now a consensus that humus functionshormonally rather than simplynutritionally inplant physiology,[62][63] and that organic substances exuded by roots and transformed in humus by soil organisms are an evolved strategy by which plants "talk" to the soil.[64]
Humus is a negativelychargedcolloidal substance which increases thecation-exchange capacity of soil, hence its ability to store nutrients bychelation.[65] While these nutrient cations are available to plants, they are held in the soil and prevented from being leached by rain or irrigation.[55]
Humus can hold the equivalent of 80–90% of its weight in moisture and therefore increases the soil's capacity to withstand drought.[66]
The biochemical structure of humus enables it to moderate, i.e. buffer, excessiveacidic oralkaline soil conditions.[67]
During humification, microbes secrete sticky, gum-likemucilages; these contribute to the crumby structure (tilth) of the soil by adhering particles together and allowing greateraeration of the soil.[68] Toxic substances such asheavy metals and excess nutrients can be chelated, i.e., bound to the organic molecules of humus, and so prevented from leaching away.[69]
The dark, usually brown or black, color of humus helps to warm cold soils in spring.[70]
Humus can contribute toclimate change mitigation through itscarbon sequestration potential.[71] Artificial humic acid and artificial fulvic acid synthesized from agricultural litter can increase the content of dissolved organic matter and total organic carbon in soil.[72]
^Popkin, Gabriel (27 July 2021),A soil-science revolution upends plans to fight climate change,Quanta Magazine, retrieved29 October 2024,"The latest edition of The Nature and Properties of Soils, published in 2016, cites Lehmann's 2015 paper and acknowledges that "our understanding of the nature and genesis of soil humus has advanced greatly since the turn of the century, requiring that some long-accepted concepts be revised or abandoned".
^Senn, T. L.; Kingman, Alta R.; Godley, W. C. (1973)."A review of humus and humic acids"(PDF).Research Series, South Carolina Agricultural Experiment Station.145. Retrieved4 November 2025.
