Humus

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.{{cite book |year=2005 |title=Microorganisms in soils: roles in genesis and Functions |editor-last1=Buscot |editor-first1=François |editor-last2=Varma |editor-first2=Ajit |pages=85–106 |chapter=Humification and mineralization in soils |last=Guggenberger |first=Georg |doi=10.1007/3-540-26609-7_4 |isbn=978-3-540-26609-9 |series=Soil biology |volume=3 |publisher=Springer |location=Dordrecht, The Netherlands |url=http://ndl.ethernet.edu.et/bitstream/123456789/75774/1/Franc%C2%B8ois%20Buscot.pdf#page=102 |access-date=8 September 2024 |archive-date=7 July 2024 |archive-url=https://web.archive.org/web/20240707083204/http://ndl.ethernet.edu.et/bitstream/123456789/75774/1/Franc%C2%B8ois%20Buscot.pdf#page=102 |url-status=live }} 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.{{cite journal |last1=Kögel-Knabner |first1=Ingrid|author1-link=Ingrid Kögel-Knabner |last2=Zech |first2=Wolfgang |last3=Hatcher |first3=Patrick G. |date=1988 |title=Chemical composition of the organic matter in forest soils: the humus layer |language=en |journal=Journal of Plant Nutrition and Soil Science |volume=151 |issue=5 |pages=331–40 |doi=10.1002/jpln.19881510512 |url=https://fr.articles.sk/book/34689934/0bcef3 |access-date=14 July 2024 }}

It is difficult to define humus precisely because it is a very complex substance which is still not fully understood. Humus is different from decomposing soil organic matter. The latter looks rough and has visible remains of the original plant or animal matter. 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.{{cite book |last=Waksman |first=Selman A. |year=1936 |title=Humus: origin, chemical composition and importance in nature |language=en |url=https://citeseerx.ist.psu.edu/document?repid=rep1&type=pdf&doi=45020e04c07d0fa28dca0093772951b65197eb2e |access-date=14 July 2024 |isbn=9780598966629 |publisher=Williams & Wilkins |location=Baltimore, Maryland }} It has no determinate shape, structure, or quality. However, when examined under a microscope, humus may reveal tiny plant, animal, or microbial remains that have been mechanically, but not chemically, degraded.{{cite journal |last1=Bernier |first1=Nicolas |last2=Ponge |first2=Jean-François |year=1994 |title=Humus form dynamics during the sylvogenetic cycle in a mountain spruce forest |journal=Soil Biology and Biochemistry |volume=26 |issue=2 |pages=183–220 |doi=10.1016/0038-0717(94)90161-9 |bibcode=1994SBiBi..26..183B |url=https://www.researchgate.net/publication/46312511 |access-date=14 July 2024 }} This suggests an ambiguous boundary between humus and soil organic matter, leading some authors to contest the use of the term humus and derived terms such as humic substances or humification, proposing the Soil Continuum Model (SCM).{{cite journal |last1=Lehmann |first1=Johannes |last2=Kleber |first2=Markus |year=2015 |title=The contentious nature of soil organic matter |journal=Nature |volume=528 |issue=7580 |pages=60–68 |doi=10.1038/nature16069 |pmid=26595271 |bibcode=2015Natur.528...60L |url=https://themarea.org/wp-content/uploads/2018/08/Lehmann-and-Kebbler-2015.pdf |access-date=14 July 2024 }} However, humus can be considered as having distinct properties, mostly linked to its richness in functional groups, justifying its maintenance as a specific term.{{cite journal |last=Ponge |first=Jean-François |year=2022 |title=Humus: dark side of life or intractable "aether"? |journal=Pedosphere |volume=32 |issue=4 |pages=660–64 |doi=10.1016/S1002-0160(21)60013-9 |bibcode=2022Pedos..32..660P |url=https://www.researchgate.net/publication/360175852 |access-date=14 July 2024 }}

Fully formed humus is essentially a collection of very large and complex molecules formed in part from lignin and other polyphenolic molecules of the original plant material (foliage, wood, bark), in part from similar molecules that have been produced by microbes.{{cite journal |last1=Dou |first1=Sen |last2=Shan |first2=Jun |last3=Song |first3=Xiangyun |last4=Cao |first4=Rui |last5=Wu |first5=Meng |last6=Li |first6=Chenglin |last7=Guan |first7=Song |date=April 2020 |title=Are humic substances soil microbial residues or unique synthesized compounds? A perspective on their distinctiveness |journal=Pedosphere |volume=30 |issue=2 |pages=159–67 |doi=10.1016/S1002-0160(20)60001-7 |bibcode=2020Pedos..30..159D |url=https://www.researchgate.net/publication/338991840 |access-date=21 July 2024 }} During decomposition processes these polyphenols 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 that protein molecules, amino acids, and amino sugars are able to attach themselves to the polyphenol "base" molecule. As protein contains both nitrogen and sulfur, this attachment gives humus a moderate content of these two important plant nutrients.{{cite book |year=2017 |title=Plant secondary metabolites. Volume 3. Their roles in stress ecophysiology |editor-last1=Siddiqui |editor-first1=Mohammed Wasim |editor-last2=Bansal |editor-first2=Vasudha |pages=39–61 |chapter=Significance of soil organic matter in relation to plants and their products |last1=Das |first1=Subhasich |last2=Bhattacharya |first2=Satya Sundar |isbn=978-1-77188-356-6 |publisher=Apple Academic Press |location=Palm Bay, Florida |url=https://www.academia.edu/82083954 |access-date=26 August 2024 }}

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.{{cite journal |last=Piccolo |first=Alessandro |date=December 2002 |title=The supramolecular structure of humic substances: a novel understanding of humus chemistry and implications in soil science |journal=Advances in Agronomy |volume=75 |pages=57–134 |doi=10.1016/S0065-2113(02)75003-7 |isbn=978-0-12-000793-6 |url=https://www.researchgate.net/publication/222526145 |access-date=4 August 2024 }} 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.{{cite journal |last=Paul |first=Eldor A. |title=The nature and dynamics of soil organic matter: plant inputs, microbial transformations, and organic matter stabilization |journal=Soil Biology and Biochemistry |date=2016 |volume=98 |pages=109–26 |doi=10.1016/j.soilbio.2016.04.001 |bibcode=2016SBiBi..98..109P |url=https://www.nrel.colostate.edu/assets/nrel_files/labs/paul-lab/docs/Paul_SBBreview2016.pdf |access-date=11 August 2024 }}

The most useful functions of humus are in improving soil structure, all the more when associated with cations (e.g. calcium),{{cite journal |last1=Huang |first1=Xue Ru |last2=Li |first2=H. |last3=Li |first3=Song |last4=Xiong |first4=Hailing |last5=Jiang |first5=Xianjun |date=May 2016 |title=Role of cationic polarization in humus-increased soil aggregate stability |journal=European Journal of Soil Science |volume=67 |issue=3 |pages=341–50 |doi=10.1111/ejss.12342 |bibcode=2016EuJSS..67..341H |url=https://www.researchgate.net/publication/303509978 |access-date=11 August 2024 }} and in providing a very large surface area that can hold nutrient elements until required by plants, an ion exchange function comparable to that of clay particles.{{cite journal |last1=Shoba |first1=V. N. |last2=Chudnenko |first2=K. V. |date=August 2014 |title=Ion exchange properties of humus acids |journal=Eurasian Soil Science |volume=47 |issue=8 |pages=761–71 |doi=10.1134/S1064229314080110 |bibcode=2014EurSS..47..761S |url=https://www.researchgate.net/publication/269385340 |access-date=11 August 2024 }}

Soil carbon sequestration is a major property of the soil, also considered as an ecosystem service.{{cite journal |last1=Lal |first1=Rattan |last2=Negassa |first2=Wakene |last3=Lorenz |first3=Klaus |date=August 2015 |title=Carbon sequestration in soil |journal=Current Opinion in Environmental Sustainability |volume=15 |pages=79–86 |doi=10.1016/j.cosust.2015.09.002 |bibcode=2015COES...15...79L |url=https://www.researchgate.net/publication/283457192 |access-date=18 August 2024 }} Only when it becomes stable and acquires its multi-century permanence, mostly via multiple interactions with the soil matrix, molecular soil humus should be considered to be of significance in removing the atmosphere's current carbon dioxide overload.{{cite journal |last1=Dynarski |first1=Katherine A. |last2=Bossio |first2=Deborah A. |last3=Scow |first3=Kate M.|author3-link=Kate Scow |date=13 November 2020 |title=Dynamic stability of soil carbon: reassessing the "permanence" of soil carbon sequestration |journal=Frontiers in Environmental Science |volume=8 |issue=714701 |doi=10.3389/fenvs.2020.514701 |doi-access=free }}

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.{{cite journal |last1=Kleber |first1=Markus |last2=Lehmann |first2=Johannes |date=8 March 2019 |title=Humic substances extracted by alkali are invalid proxies for the dynamics and functions of organic matter in terrestrial and aquatic ecosystems |journal=Journal of Environmental Quality |volume=48 |issue=2 |pages=207–16 |doi=10.2134/jeq2019.01.0036 |pmid=30951127 |bibcode=2019JEnvQ..48..207K |url=https://acsess.onlinelibrary.wiley.com/doi/pdfdirect/10.2134/jeq2019.01.0036 |access-date=25 August 2024 |doi-access=free }} Further research has been done in more recent years, though it remains an active field of study.{{cite journal |last1=Baveye |first1=Philippe C. |last2=Wander |first2=Michelle |date=6 March 2019 |title=The (bio)chemistry of soil humus and humic substances: why is the "new view" still considered novel after more than 80 years? |journal=Frontiers in Environmental Science |volume=7 |issue=27 |doi=10.3389/fenvs.2019.00027 |doi-access=free }}

Microorganisms decompose a large portion of the soil organic matter into inorganic minerals that the roots of plants can absorb as nutrients. This process is termed mineralization. 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 the decomposition occurs, a fraction of the organic matter does not mineralize and instead is transformed by a process called humification. Prior to modern analytical methods, early evidence led scientists to believe that humification resulted in concatenations of organic polymers resistant to the action of microorganisms,{{cite book |last=Brady |first=Nyle C. |title=The nature and properties of soils |url=https://www.academia.edu/23641831 |date=1984 |edition=9th |publisher=Macmillan Publishing Company |location=New York, New York |language=en |isbn=978-0029460306 |page=265 |access-date=1 September 2024 }} however recent research has demonstrated that microorganisms are capable of digesting humus.{{cite web |url=https://www.quantamagazine.org/a-soil-science-revolution-upends-plans-to-fight-climate-change-20210727/ |title=A soil-science revolution upends plans to fight climate change |quote=Soil researchers have concluded that even the largest, most complex molecules can be quickly devoured by soil’s abundant and voracious microbes |work=Quanta Magazine |last=Popkin |first=Gabriel |year=2021 |access-date=1 September 2024 }}

Humification can occur naturally in soil or artificially in the production of compost. Organic matter is humified by a combination of saprotrophic fungi, bacteria, microbes and animals such as earthworms, nematodes, protozoa, and arthropods (see Soil biology). Plant remains, including those that animals digested and excreted, contain organic compounds: sugars, starches, proteins, carbohydrates, lignins, waxes, resins, and organic acids. Decay in the soil begins with the decomposition of sugars and starches from carbohydrates, which decompose easily as detritivores initially invade the dead plant organs, while the remaining cellulose and lignin 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.{{cite journal |last1=Krishna |first1=M. P. |last2=Mohan |first2=Mahesh |date=July 2017 |title=Litter decomposition in forest ecosystems: a review |journal=Energy, Ecology and Environment |volume=2 |issue=3 |pages=236–49 |doi=10.1007/s40974-017-0064-9 |url=https://www.academia.edu/119720860 |access-date=8 September 2024 }}

Lignin, which is quickly transformed by white-rot fungi,{{cite journal |last1=Levin |first1=Laura |last2=Forchiassin |first2=Flavia |date=9 May 2001 |title=Ligninolytic enzymes of the white rot basidiomycete Trametes trogii |url=https://www.academia.edu/120239930 |journal=Acta Biotechnologica |volume=21 |issue=2 |pages=179–86 |doi=10.1002/1521-3846(200105)21:2<179::AID-ABIO179>3.0.CO;2-2 |access-date=15 September 2024 }} is one of the primary precursors of humus,{{cite journal |last1=González-Pérez |first1=Martha |last2=Vidal Torrado |first2=Pablo |last3=Colnago |first3=Luiz A. |last4=Martin-Neto |first4=Ladislau |last5=Otero |first5=Xosé L. |last6=Milori |first6=Débora M. B. P. |last7=Haenel Gomes |first7=Felipe |date=31 August 2008 |title=13C NMR and FTIR spectroscopy characterization of humic acids in spodosols under tropical rain forest in southeastern Brazil |journal=Geoderma |volume=146 |issue=3–4 |pages=425–33 |doi=10.1016/j.geoderma.2008.06.018 |bibcode=2008Geode.146..425G |url=https://www.academia.edu/14026276 |access-date=15 September 2024 }} together with by-products of microbial{{cite journal |last1=Knicker |first1=Heike |last2=Almendros |first2=Gonzalo |last3=González-Vila |first3=Francisco Javier |last4=Lüdemann |first4=Hans-Dietrich |last5=Martín |first5=Fracisco |date=November–December 1995 |title=13C and 15N NMR analysis of some fungal melanins in comparison with soil organic matter |journal=Organic Geochemistry |volume=23 |issue=11–12 |pages=1023–28 |doi=10.1016/0146-6380(95)00094-1 |bibcode=1995OrGeo..23.1023K |url=https://www.academia.edu/78009567 |access-date=15 September 2024 }} and animal{{cite journal |last1=Muscolo |first1=Adele |last2=Bovalo |first2=Francesco |last3=Gionfriddo |first3=Francesco |last4=Nardi |first4=Serenella |date=August 1999 |title=Earthworm humic matter produces auxin-like effects on Daucus carota cell growth and nitrate metabolism |journal=Soil Biology and Biochemistry |volume=31 |issue=9 |pages=1303–11 |doi=10.1016/S0038-0717(99)00049-8 |bibcode=1999SBiBi..31.1303M |url=https://www.academia.edu/78825632 |access-date=15 September 2024 }} 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. Some judge earthworm humus (vermicompost) to be the optimal organic manure.{{cite journal |last1=Oyege |first1=Ivan |last2=Sridhar |first2=B. B. Maruthi |date=10 November 2023 |title=Effects of vermicompost on soil and plant health and promoting sustainable agriculture |journal=Soil Systems |volume=7 |issue=4 |page=101 |doi=10.3390/soilsystems7040101 |doi-access=free }}

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 tightly sorbed or complexed to clays.{{cite journal |last1=Dungait |first1=J. A. |last2=Hopkins |first2=D. W. |last3=Gregory |first3=A. S. |last4=Whitmore |first4=A. P. |title=Soil organic matter turnover is governed by accessibility not recalcitrance |journal=Global Change Biology |date=14 February 2012 |volume=18 |issue=6 |pages=1781–96 |doi=10.1111/j.1365-2486.2012.02665.x |url=https://www.desmog.com/wp-content/uploads/files/Dungait%20SOM%20article.pdf |access-date=22 September 2024 }} Most humus that is not protected in this way is decomposed within 10 years and can be regarded as less stable or more labile.{{cite journal |last1=Baldock |first1=Jeffrey A. |last2=Skjemstad |first2=Jan Otto |date=July 2000 |title=Role of the soil matrix and minerals in protecting natural organic materials against biological attack |journal=Organic Geochemistry |volume=31 |issue=7 |pages=697–710 |doi=10.1016/S0146-6380(00)00049-8 |url=https://www.academia.edu/78009563 |access-date=22 September 2024 }} 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,{{cite journal |last1=Angst |first1=Šárka |last2=Mueller |first2=Carsten W. |last3=Cajthaml |first3=Tomáš |last4=Angst |first4=Gerrit |last5=Lhotáková |first5=Zuzana |last6=Bartuška |first6=Martin |last7=Špaldoňová |first7=Alexandra |last8=Frouz |first8=Jan |date=1 March 2017 |title=Stabilization of soil organic matter by earthworms is connected with physical protection rather than with chemical changes of organic matter |journal=Geoderma |volume=289 |pages=29–35 |doi=10.1016/j.geoderma.2016.11.017 |url=https://www.academia.edu/80259832 |access-date=6 October 2024 }}{{cite journal |last=Brauman |first=Alain |date=July 2000 |title=Effect of gut transit and mound deposit on soil organic matter transformations in the soil feeding termite: a review |journal=European Journal of Soil Biology |volume=36 |issue=3–4 |pages=117–25 |doi=10.1016/S1164-5563(00)01058-X |url=https://fr.articles.sk/book/18221151/100de8 |access-date=6 October 2024 }} 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.{{cite journal |last1=Andreetta |first1=Anna |last2=Ciampalini |first2=Rossano |last3=Moretti |first3=Pierpaolo |last4=Vingiani |first4=Simona |last5=Poggio |first5=Giorgio |last6=Matteucci |first6=Giorgio |last7=Tescari |first7=Francesca |last8=Carnicelli |first8=Stefano |date=2011 |title=Forest humus forms as potential indicators of soil carbon storage in Mediterranean environments |journal=Biology and Fertility of Soils |volume=47 |pages=31–40 |doi=10.1007/s00374-010-0499-z |url=https://www.researchgate.net/publication/226417489 |access-date=6 October 2024 }}

Stable humus contributes few plant-available nutrients in soil, but it helps maintain its physical structure.{{cite journal |last1=Oades |first1=J. Malcolm |title=Soil organic matter and structural stability: mechanisms and implications for management |journal=Plant and Soil |date=February 1984 |volume=76 |issue=1–3 |pages=319–337 |doi=10.1007/BF02205590|bibcode=1984PlSoi..76..319O |s2cid=7195036 |url=https://edisciplinas.usp.br/pluginfile.php/5168021/mod_resource/content/1/Grupo%206_Oades%2C%201984.%20Soil%20organic%20matter%20and%20structural%20stability%20mechanisms%20and%20implications%20for%20management.pdf |access-date=13 October 2024 }} A very stable form of humus is formed from the slow oxidation (redox) of soil carbon after the incorporation of finely powdered charcoal into the topsoil, suggested to result from the grinding and mixing activity of a tropical earthworm.{{cite journal |last1=Ponge |first1=Jean-François |last2=Topoliantz |first2=Stéphanie |last3=Ballof |first3=Sylvain |last4=Rossi |first4=Jean-Pierre |last5=Lavelle |first5=Patrick |last6=Betsch |first6=Jean-Marie |last7=Gaucher |first7=Philippe |date=July 2006 |title=Ingestion of charcoal by the Amazonian earthworm Pontoscolex corethrurus: a potential for tropical soil fertility |journal=Soil Biology and Biochemistry |volume=38 |issue=7 |pages=2008–9 |doi=10.1016/j.soilbio.2005.12.024 |url=https://www.researchgate.net/publication/44735820 |access-date=13 October 2024 }} This process is speculated to have been important in the formation of the unusually fertile Amazonian {{lang|es|terra preta do Indio}}.{{cite book |date=July 2017 |title=Archaeological soil and sediment micromorphology |editor-last1=Nicosia |editor-first1=Cristiano |editor-last2=Stoops |editor-first2=Georges |pages=345–57 |chapter=Amazonian Dark Earths |last=Arroyo-Kalin |first=Manuel |doi=10.1002/9781118941065.ch33 |isbn=9781118941065 |publisher=Wiley |location=Hoboken, New Jersey |url=https://www.researchgate.net/publication/319444794 |access-date=13 October 2024 }} However, some authors 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 master soil 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). Some soils have an organic horizon (O) on the surface, but this horizon can also be buried.{{cite journal |last1=Gerlach |first1=Renate |last2=Fischer |first2=Peter |last3=Eckmeier |first3=Eileen |last4=Hilgers |first4=Alexandra |title=Buried dark soil horizons and archaeological features in the Neolithic settlement region of the Lower Rhine area, NW Germany: formation, geochemistry and chronostratigraphy |journal=Quaternary International |volume=265 |pages=191–204 |doi=10.1016/j.quaint.2011.10.007 |url=https://www.academia.edu/77065464 |access-date=20 October 2024 }} 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 with colloidal humic substances which have been leached down the soil profile.{{cite journal |last1=Sanborn |first1=Paul |last2=Lamontagne |first2=Luc |last3=Hendershot |first3=William |title=Podzolic soils of Canada: genesis, distribution, and classification |journal=Canadian Journal of Soil Science |volume=91 |issue=5 |pages=843–80 |doi=10.4141/cjss10024 |doi-access=free }}

The importance of chemically stable humus is thought by some to be the fertility it provides to soils in both a physical and chemical sense,{{cite journal |last1=Hargitai |first1=László |date=December 1993 |title=The role of organic matter content and humus quality in the maintenance of soil fertility and in environmental protection |journal=Landscape and Urban Planning |volume=27 |issue=2–4 |pages=161–67 |doi=10.1016/0169-2046(93)90044-E |bibcode=1993LUrbP..27..161H |url=https://fr.articles.sk/book/19958903/07e3a6 |access-date=27 October 2024 }} though some agricultural experts put a greater focus on other features of it, such as its ability to suppress disease.{{cite journal |last1=Hoitink |first1=Harry A. J. |last2=Fahy |first2=Peter C. |date=September 1986 |title=Basis for the control of soilborne plant pathogens with composts |journal=Annual Review of Phytopathology |volume=24 |pages=93–114 |doi=10.1146/annurev.py.24.090186.000521 |url=https://fr.articles.sk/book/18744629/4076fa |access-date=27 October 2024 }} It helps the soil retain moisture{{cite journal |last=Lal |first1=Rattan |date=September 2020 |title=Soil organic matter and water retention |journal=Agronomy Journal |volume=116 |issue=5 |pages=3265–77 |doi=10.1002/agj2.20282 |url=https://www.researchgate.net/publication/341213360 |access-date=27 October 2024 }} by increasing microporosity{{cite journal |last1=de Macedo |first1=José Ronaldo |last2=do Amaral Meneguelli |first2=Neli |last3=Ottoni Filho |first3=Theophilo Benedicto |last4=Lima |first4=Jorge Araújo de Sousa |date=February 2007 |title=Estimation of field capacity and moisture retention based on regression analysis involving chemical and physical properties in Alfisols and Ultisols of the state of Rio de Janeiro |journal=Communications in Soil Science and Plant Analysis |volume=33 |issue=13–14 |pages=2037–55 |doi=10.1081/CSS-120005747 |bibcode=2002CSSPA..33.2037D |s2cid=98466747 |url=https://fr.articles.sk/book/32902177/18be8b |access-date=27 October 2024 }} and encourages the formation of good soil structure.{{cite journal |last1=Hempfling |first1=Reinhold |last2=Schulten |first2=Hans-Rolf |last3=Horn |first3=Rainer |date=June 1990 |title=Relevance of humus composition to the physical/mechanical stability of agricultural soils: a study by direct pyrolysis-mass spectrometry |journal=Journal of Analytical and Applied Pyrolysis |volume=17 |issue=3 |pages=275–81 |doi=10.1016/0165-2370(90)85016-G |url=https://fr.articles.sk/book/8543777/01704b |access-date=3 November 2024 }}{{cite book |date=1996 |title=Humic substances in terrestrial ecosystems |editor-last=Piccolo |editor-first=Alessandro |pages=225–64 |chapter=Humus and soil conservation |last=Piccolo |first=Alessandro |doi=10.1016/B978-044481516-3/50006-2 |isbn=978-0-444-81516-3 |publisher=Elsevier |location=Amsterdam, The Netherlands |url=https://www.researchgate.net/publication/281451183 |access-date=3 November 2024 }} The incorporation of oxygen into large organic molecular assemblages generates many active, negatively charged sites that bind to positively charged ions (cations) of plant nutrients, making them more available to the plant by way of ion exchange.{{cite journal |last=Szalay |first1=Alex |date=October–November 1964 |title=Cation exchange properties of humic acids and their importance in the geochemical enrichment of UO2++ and other cations |journal=Geochimica et Cosmochimica Acta |volume=28 |issue=10–11 |pages=1605–14 |doi=10.1016/0016-7037(64)90009-2 |bibcode=1964GeCoA..28.1605S |url=https://fr.articles.sk/book/19639940/3bac09 |access-date=3 November 2024 }} Humus allows soil organisms to feed and reproduce and is often described as the "life-force" of the soil.{{cite journal |last1=Elo |first1=Seija |last2=Maunuksela |first2=Liisa |last3=Salkinoja-Salonen |first3=Mirja |last4=Smolander |first4=Aino |last5=Haahtela |first5=Kielo |date=February 2000 |title=Humus bacteria of Norway spruce stands: plant growth promoting properties and birch, red fescue and alder colonizing capacity |journal=FEMS Microbiology Ecology |volume=31 |issue=2 |pages=143–52 |doi=10.1111/j.1574-6941.2000.tb00679.x |pmid=10640667 |doi-access=free }}{{cite journal |last1=Vreeken-Buijs |first1=Madelein J. |last2=Hassink |first2=Jan |last3=Brussaard |first3=Lijbert |date=1998 |title=Relationships of soil microarthropod biomass with organic matter and pore size distribution in soils under different land use |journal=Soil Biology and Biochemistry |volume=30 |issue=1 |pages=97–106 |doi=10.1016/S0038-0717(97)00064-3|bibcode=1998SBiBi..30...97V |url=https://www.academia.edu/65368490 |access-date=3 November 2024 }}

• 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.
• 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.{{cite journal |last=Ponge |first=Jean-François |date=July 2003 |title=Humus forms in terrestrial ecosystems: a framework to biodiversity |journal=Soil Biology and Biochemistry |volume=35 |issue=7 |pages=935–45|doi=10.1016/S0038-0717(03)00149-4 |url=https://www.academia.edu/20508983 |access-date=10 November 2024 }}
• "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.{{cite book |date=1991 |title=Advances in soil organic matter research: the impact on agriculture and the environment |editor-last=Wilson |editor-first=W. S. |pages=355–64 |chapter=Soil organic matter: its central position in organic farming |last=Hodges |first=R. D. |doi=10.1016/b978-1-85573-813-3.50040-8 |isbn=978-1-85573-813-3 |publisher=Woodhead Publishing |location=Sawston, United Kingdom |url=https://fr.articles.sk/book/81383430/4b29a3 |access-date=10 November 2024 }}
• Decomposition of dead plant material causes complex organic compounds to be slowly oxidized (lignin-like humus) or to decompose into simpler forms (sugars and amino sugars, and aliphatic and phenolic organic acids), which are further transformed into microbial biomass (microbial humus) or reorganized and further oxidized into humic assemblages (fulvic acids and humic acids), which bind to clay minerals and metal hydroxides.{{cite journal |last1=Gunina |first1=Anna |last2=Kuzyakov |first2=Yakov |date=April 2022 |title=From energy to (soil organic) matter |journal=Global Change Biology |volume=28 |issue=7 |pages=2169–82 |doi=10.1111/gcb.16071 |doi-access=free }} The ability of plants to absorb humic substances with their roots and metabolize them has been long debated.{{cite journal |last1=Senn |first1=T. L. |last2=Kingman |first2=Alta R. |last3=Godley |first3=W. C. |date=1973 |title=A review of humus and humic acids |url=https://www.humintech.com/fileadmin/content_images/agriculture/information/articles_pdf/A-Review-of-Humus-and-Humic-Acids_T.L.Senn__A.R.Kingsmann.pdf |journal=Research Series, South Carolina Agricultural Experiment Station |volume=145 |access-date=24 November 2024 }} There is now a consensus that humus functions hormonally rather than simply nutritionally in plant physiology,{{cite journal |last1=Eyheraguibel |first1=Boris |last2=Silvestre |first2=Jérôme |last3=Morard |first3=Philippe |date=July 2008 |title=Effects of humic substances derived from organic waste enhancement on the growth and mineral nutrition of maize |url=https://hal.science/hal-00940093/file/Eyheraguibel_10804.pdf |journal=Bioresource Technology |volume=99 |issue=10 |pages=4206–12 |doi=10.1016/j.biortech.2007.08.082 |pmid=17962015 |bibcode=2008BiTec..99.4206E |access-date=17 November 2024 }}{{cite journal |last1=Zandonadi |first1=Daniel Basilio |last2=Santos |first2=Mirella Pupo |last3=Busato |first3=Jader Galba |last4=Peres |first4=Lázaro Eustáquio Pereira |last5=Façanha |first5=Arnoldo Rocha |title=Plant physiology as affected by humified organic matter |journal=Theoretical and Experimental Plant Physiology |date=2013 |volume=25 |issue=1 |pages=12–25 |doi=10.1590/S2197-00252013000100003 |doi-access=free }} and that organic sunstances exuded by roots and transformed in humus by soil organisms are an evolved strategy by which plants "talk" to the soil.{{cite journal |last1=Nardi |first1=Serenella |last2=Ertani |first2=Andrea |last3=Francioso |first3=Ornella |date=February 2017 |title=Soil–root cross-talking: the role of humic substances |journal=Journal of Plant Nutrition and Soil Science |volume=180 |issue=1 |pages=5–13 |doi=10.1002/jpln.201600348 |url=https://www.academia.edu/102119488 |access-date=17 November 2024 |hdl=2318/1731194 |hdl-access=free }}
• Humus is a negatively charged colloidal substance which increases the cation-exchange capacity of soil, hence its ability to store nutrients by chelation.{{cite journal |last1=Shoba |first1=V. N. |last2=Chudnenko |first2=Konstantin V. |date=August 2014 |title=Ion exchange properties of humus acids |journal=Eurasian Soil Science |volume=47 |issue=8 |pages=761–71 |doi=10.1134/S1064229314080110 |url=https://www.researchgate.net/publication/269385340 |access-date=24 November 2024 }} While these nutrient cations are available to plants, they are held in the soil and prevented from being leached by rain or irrigation.
• Humus can hold the equivalent of 80–90% of its weight in moisture and therefore increases the soil's capacity to withstand drought.{{cite journal |last1=Olness |first1=Alan |last2=Archer |first2=David |date=February 2005 |title=Effect of organic carbon on available water in soil |journal=Soil Science |volume=170 |issue=2 |pages=90–101 |doi=10.1097/00010694-200502000-00002 |bibcode=2005SoilS.170...90O |s2cid=95336837 |url=https://fr.articles.sk/book/54264627/14eda9 |access-date=24 November 2024 }}
• The biochemical structure of humus enables it to moderate, i.e. buffer, excessive acidic or alkaline soil conditions.{{cite journal |last1=Kikuchi |first1=Ryunosuke |date=February 2004 |title=Deacidification effect of the litter layer on forest soil during snowmelt runoff: laboratory experiment and its basic formularization for simulation modeling |journal=Chemosphere |volume=54 |issue=8 |pages=1163–69 |doi=10.1016/j.chemosphere.2003.10.025 |pmid=14664845 |bibcode=2004Chmsp..54.1163K |url=https://fr.articles.sk/book/16655436/acdc92 |access-date=24 November 2024 }}
• During humification, microbes secrete sticky, gum-like mucilages; these contribute to the crumby structure (tilth) of the soil by adhering particles together and allowing greater aeration of the soil.{{cite journal |last1=Caesar-Tonthat |first1=Thecan C. |date=August 2002 |title=Soil binding properties of mucilage produced by a basidiomycete fungus in a model system |url=https://fr.articles.sk/book/20537646/09bc94 |journal=Mycological Research |volume=106 |issue=8 |pages=930–37 |doi=10.1017/S0953756202006330 |access-date=24 November 2024 }} Toxic substances such as heavy metals and excess nutrients can be chelated, i.e., bound to the organic molecules of humus, and so prevented from leaching away.{{cite journal |last1=Zhu |first1=Rui |last2=Wu |first2=Min |last3=Yang |first3=Jian |date=February 2011 |title=Mobilities and leachabilities of heavy metals in sludge with humus soil |journal=Journal of Environmental Sciences |volume=23 |issue=2 |pages=247–54 |doi=10.1016/S1001-0742(10)60399-3 |url=https://fr.articles.sk/book/14270592/0155c3 |access-date=24 November 2024 }}
• The dark, usually brown or black, color of humus helps to warm cold soils in spring.{{cite journal |last1=Ludwig |first1=J. W. |last2=Harper |first2=John L. |date=July 1958 |title=The influence of the environment on seed and seedling mortality. VIII. The influence of soil colour |journal=Journal of Ecology |volume=46 |issue=2 |pages=381–89 |doi=10.2307/2257402 |url=https://fr.articles.sk/book/57099612/546f22 |access-date=1 December 2024 }}
• Humus can contribute to climate change mitigation through its carbon sequestration potential.{{cite journal |last1=Amelung |first1=Wulf| last2=Bossio |first2=Deborah |last3=De Vries |first3=Wim |last4=Kögel-Knabner |first4=Ingrid |author4-link=Ingrid Kögel-Knabner|last5=Lehmann |first5=Johannes |last6=Amundson |first6=Ronald |last7=Bol |first7=Roland |last8=Collins |first8=Chris |last9=Lal |first9=Rattan |last10=Leifeld |first10=Jens |last11=Minasny |first11=Budiman |last12=Pan |first12=Gen-Xing |last13=Paustian |first13=Keith |last14=Rumpel |first14=Cornelia |last15=Sanderman |first15=Jonathan |last16=Van Groenigen |first16=Jan Willem |last17=Mooney |first17=Sacha |last18=Van Wesemael |first18=Bas |last19=Wander |first19=Michelle |last20=Chabbi |first20=Abbad |date=27 October 2020 |title=Towards a global-scale soil climate mitigation strategy |journal=Nature Communications |language=en |volume=11 |issue=1 |pages=5427 |doi=10.1038/s41467-020-18887-7 |pmid=33110065 |pmc=7591914 |bibcode=2020NatCo..11.5427A |issn=2041-1723 |doi-access=free }} 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.{{cite journal |last1=Tang |first1=Chunyu |last2=Li |first2=Yuelei |last3=Song |first3=Jingpeng |last4=Antonietti |first4=Markus |last5=Yang |first5=Fan |date=25 June 2021 |title=Artificial humic substances improve microbial activity for binding CO2 |journal=iScience |language=en |volume=24 |issue=6 |pages=102647 |doi=10.1016/j.isci.2021.102647 |pmid=34466779 |pmc=8387571 |bibcode=2021iSci...24j2647T |issn=2589-0042 |doi-access=free }}

{{div col|colwidth=30em}}

• Biochar
• Biomass
• Biotic material
• Detritus
• Glomalin
• Humic acid
• Immobilization (soil science)
• Mineralization (soil science)
• Mycorrhizal fungi and soil carbon storage
• Organic matter
• Plant litter
• Soil horizon
• Soil science
• Terra preta

{{div col end}}

{{Reflist}}

{{sister project links |wikt=humus |b=no |q=humus |s=no |commons=no|n=no|v=no |species=no}}

• {{cite web |url=http://karnet.up.wroc.pl/~weber/typy2.htm |first=Jerzy |last=Weber |title=Types of humus in soils |publisher=Agricultural University of Wroclaw, Poland |access-date=8 December 2024 }}
• {{cite web |last1=Wershaw |first1=Robert L. |title=Evaluation of conceptual models of natural organic matter (humus) from a consideration of the chemical and biochemical processes of humification |url=https://pubs.usgs.gov/sir/2004/5121/pdf/sir2004-5121.pdf |work=Pubs.USGU.gov |publisher=United States Geological Survey |access-date=8 December 2024 }}
• {{cite web |url=https://humic-substances.org/ |title=What are humic substances? |publisher=International Humic Substances Society |access-date=8 December 2024 }}

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