Editing Humus (section)

==Description==

The primary materials needed for the process of humification are plant detritus and dead animals and microbes, [[Excretion|excreta]] of all soil-dwelling organisms, and also [[black carbon]] resulting from past fires.<ref>{{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 Science+Business Media|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 }}</ref> 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.<ref name=ZFPB1988>{{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 }}</ref>

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.<ref name="Waksman 1936">{{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=[[Lippincott Williams & Wilkins|Williams & Wilkins]] |location=Baltimore, Maryland }}</ref> 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.<ref>{{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 }}</ref> 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 substance]]s'' or ''humification'', proposing the ''Soil Continuum Model'' (SCM).<ref name="Lehmann2015">{{cite journal |last1=Lehmann |first1=Johannes |last2=Kleber |first2=Markus |year=2015 |title=The contentious nature of soil organic matter |journal=[[Nature (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 }}</ref> However, humus can be considered as having distinct properties, mostly linked to its richness in [[functional group]]s, justifying its maintenance as a specific term.<ref>{{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 }}</ref>

Fully formed humus is essentially a collection of very large and complex molecules formed in part from [[lignin]] and other [[polyphenol]]ic molecules of the original plant material (foliage, wood, bark), in part from similar molecules that have been produced by [[Microorganism|microbes]].<ref name="Dou2020">{{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 }}</ref> 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.<ref>{{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 }}</ref>

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.<ref name="Piccolo2002">{{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 }}</ref> 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.<ref>{{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 }}</ref>

The most useful functions of humus are in improving [[soil structure]], all the more when associated with [[cations]] (e.g. [[calcium]]),<ref>{{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 }}</ref> 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.<ref>{{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 }}</ref>

Soil [[carbon sequestration]] is a major property of the soil, also considered as an [[ecosystem service]].<ref>{{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 }}</ref> 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.<ref>{{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 }}</ref>

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.<ref>{{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 }}</ref> Further research has been done in more recent years, though it remains an active field of study.<ref>{{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 }}</ref>