Humic substance

The formation of HS in nature is one of the least understood aspects of humus chemistry and one of the most intriguing. Historically, there have been three main theories to explain it: the lignin theory of Waksman (1932), the polyphenol theory, and the sugar-amine condensation theory of Maillard (1911).Stevenson, F.J. (1994). Humus Chemistry: Genesis, Composition, Reactions, Wiley & Sons, New York, 1994, pp. 188-210. {{ISBN|0471594741}}.Tan, K. H. (2014). Humic matter in soil and the environment: principles and controversies. 2nd ed. Boca Ranton: CRC Press. {{ISBN|1482234459}}. Humic substances are formed by the microbial degradation of dead biota matter, such as lignin, cellulose. ligno-cellulose and charcoal.{{citation |last1=Ponomarenko |first1=E.V. |last2=Anderson |first2=D.W. |title=Importance of charred organic matter in Black Chernozem soils of Saskatchewan |year=2001 |journal=Canadian Journal of Soil Science |volume=81 |issue=3 |pages=285–297 |quote= The present paradigm views humus as a system of heteropolycondensates, largely produced by the soil microflora, in varying associations with clay (Anderson 1979). Because this conceptual model and the simulation models rooted within the concept do not accommodate a large char component, a considerable change in conceptual understanding (a paradigm shift) appears imminent. |doi=10.4141/s00-075|bibcode=2001CaJSS..81..285P }}{{cite journal|year=2012|title=Abundant and stable char residues in soils: implications for soil fertility and carbon sequestration|journal=Environmental Science and Technology|volume=46|issue=17|pages= 9571–9576 |doi=10.1021/es301107c|pmid=22834642|url=https://www.researchgate.net/publication/230571591

|last1=Mao|first1=J.-D.|last2=Johnson|first2=R. L.|last3=Lehmann|first3=J.|last4=Olk|first4=D. C.|last5=Neves|first5=E. G.|last6=Thompson|first6=M. L.|last7=Schmidt-Rohr|first7=K.|bibcode=2012EnST...46.9571M|citeseerx=10.1.1.698.270}} Humic substances in the lab are resistant to further biodegradation. Their structure, elemental composition and content of functional groups of a given sample depend on the water or soil source and the specific procedures and conditions of extraction. Nevertheless, the average properties of lab extractes HS from different sources are remarkably similar.

Historically, scientists have used variations of similar methods for extracting HS from NOM and separating the extracts into HA and FA. The International Humic Substances Society advocates using standard laboratory methods to prepare humic and fulvic acids. Humic substances are extracted from soil and other solid sources using 0.1 M NaOH, under a nitrogen atmosphere, to prevent abiotic oxidation of some of the components of HS. The HA is then precipitated at pH 1. The soluble fraction is treated on a resin column to separate fulvic acid components from other acid soluble compounds.{{Cite web |title=Isolation of IHSS Soil Fulvic and Humic Acids {{!}} IHSS |url=https://humic-substances.org/isolation-of-ihss-soil-fulvic-and-humic-acids/ |access-date=2024-10-24 |language=en-US}} The fraction of NOM not extracted by 0.1 NaOH is humin. Humic and fulvic acid are extracted from natural waters using a resin column after microfiltration and acidification to pH 2. The humic materials are eluted from the column with NaOH, and humic acid is precipitated at pH 1. After adjusting the pH to 2, fulvic acid is separated from other acid soluble compounds, using a resin column as with solid phase sources.{{Cite web |title=Isolation of IHSS Aquatic Humic and Fulvic Acids {{!}} IHSS |url=https://humic-substances.org/isolation-of-ihss-aquatic-humic-and-fulvic-acids/ |access-date=2024-10-24 |language=en-US}} An analytical method for quantifying humic acid and fulvic acid in commercial ores and humic products, has been developed based on the IHSS humic acid and fulvic acid preparation methods.{{Cite journal |last1=Lamar |first1=Richard T |last2=Olk |first2=Daniel C |last3=Mayhew |first3=Lawrence |last4=Bloom |first4=Paul R |date=2014-05-01 |title=A New Standardized Method for Quantification of Humic and Fulvic Acids in Humic Ores and Commercial Products |url=https://academic.oup.com/jaoac/article/97/3/721-730/5654784 |journal=Journal of AOAC International |language=en |volume=97 |issue=3 |pages=721–730 |doi=10.5740/jaoacint.13-393 |pmid=25051616 |issn=1060-3271}}

Scientists associated with the IHSS have also isolated the entire NOM from blackwater rivers using reverse osmosis. The retentate from this process contains both humic and fulvic acids, predominately humic acid. The NOM from hard water streams has been isolated using reverse osmosis and electrodialysis in tandem.{{Cite web |title=Isolation of NOM by Reverse Osmosis {{!}} IHSS |url=https://humic-substances.org/isolation-of-nom-by-reverse-osmosis/ |access-date=2024-10-24 |language=en-US}}

Extracted humic acid not a single acid; instead, it is a complex mixture of many different acids containing carboxyl and phenolate groups so that the mixture behaves functionally as a dibasic acid or, occasionally, as a tribasic acid. Commercial humic acid used to amend soil is manufactured using these well-established procedures. Humic acids can form complexes with ions that are commonly found in the environment creating humic colloids.{{cite web|url=http://www.vetservis.sk/media/object/433/effects_of_humic_acid_on_animals_and_humans.pdf|title=Effects of Humic Acid on Animals and Humans: An Overview of Literature and a Review of Current Research|website=vet servis}}

A sequential chemical fractionation can isolate more homogeneous humic fractions and determine their molecular structures by advanced spectroscopic and chromatographic methods.{{cite journal |author= Nebbioso A. and Piccolo A.|doi=10.1016/j.aca.2012.01.027|pmid=22365124|title=Advances in humeomics: Enhanced structural identification of humic molecules after size fractionation of a soil humic acid|journal=Analytica Chimica Acta|volume=720|pages=77–90|year=2012|bibcode=2012AcAC..720...77N }} Substances identified in humic extracts and directly in soil include mono-, di-, and tri-hydroxycarboxylic acids, fatty acids, dicarboxylic acids, linear alcohols, phenolic acids, terpenoids, carbohydrates, and amino acids.{{cite journal |author= Drosos M. and Piccolo A.|doi=10.1002/ldr.2989|title=The molecular dynamics of soil humus as a function of tillage|journal=Land Degradation & Development|volume=29|issue=6|pages=1792–1805|year=2018|bibcode=2018LDeDe..29.1792D |s2cid=135445097 }} This suggests humic molecules may form a supramolecular structures held together by non-covalent forces, such as van der Waals force, π-π, and CH-π bonds.{{cite book |author=Piccolo, A. |title=The Supramolecular structure of humic substances. A novel understanding of humus chemistry and implications in soil science |year=2002 |isbn=978-0-12-000793-6 |series=Advances in Agronomy |volume=75 |pages=57–134 |doi=10.1016/S0065-2113(02)75003-7}}

Since the dawn of modern chemistry, humic substances are among the most studied among natural materials. Despite long study, their molecular structure remains debatable. The traditional view has been that humic substances are hetero- poly-condensates, in varying associations with clay.{{citation |last1=Ponomarenko |first1=E.V. |title=Importance of charred organic matter in Black Chernozem soils of Saskatchewan |journal=Canadian Journal of Soil Science |volume=81 |issue=3 |pages=285–297 |year=2001 |doi=10.4141/s00-075 |last2=Anderson |first2=D.W.|bibcode=2001CaJSS..81..285P }} A more recent view is that relatively small molecules also play major a role.

A typical humic substance is a mixture of many molecules, some of which are based on a motif of aromatic nuclei with phenolic and carboxylic substituents, linked together; The functional groups that contribute most to surface charge and reactivity of humic substances are phenolic and carboxylic groups. Humic substances commonly behave as mixtures of dibasic acids, with a pK₁ value around 4 for protonation of carboxyl groups and around 8 for protonation of phenolate groups in HA. Fulvic acids are more acidic than HA. There is considerable overall similarity among individual humic acids. For this reason, measured pK values for a given sample are average values relating to the constituent species. The other important characteristic is charge density.File:Humic acid.svg, phenol, catechol, and sugar moieties{{cite book |author=Stevenson F.J. |publisher=John Wiley & Sons|place= New York |title=Humus Chemistry: Genesis, Composition, Reactions |year=1994}}]]

File:Fulvic_acid.pngThe more recent determinations of molecular weights of HS show that the molecular weights are not as great as once thought. Reported number average molecular weights of soil HA are < 6000 but they are highly poly disperse with some components with much larger measure molecular weights and much lower.{{Cite journal |last1=Olk |first1=D. C. |last2=Bloom |first2=P. R. |last3=Perdue |first3=E. M. |last4=McKnight |first4=D. M. |last5=Chen |first5=Y. |last6=Farenhorst |first6=A. |last7=Senesi |first7=N. |last8=Chin |first8=Y.-P. |last9=Schmitt-Kopplin |first9=P. |last10=Hertkorn |first10=N. |last11=Harir |first11=M. |date=2019 |title=Environmental and Agricultural Relevance of Humic Fractions Extracted by Alkali from Soils and Natural Waters |journal=Journal of Environmental Quality |language=en |volume=48 |issue=2 |pages=217–232 |doi=10.2134/jeq2019.02.0041 |issn=0047-2425|doi-access=free |pmid=30951132 |bibcode=2019JEnvQ..48..217O }} Measured number average molecular weights of aquatic HS with HA ≤ 1700 and FA < 900. The aquatic HA and FA are also highly poly disperse. The number of individually distinct components in HS, as measured by mass spectroscopy is in the thousands. The average composition of HA and FA can be represented by model structures.

The presence of carboxylate and phenolate groups gives the humic acids the ability to form complexes with ions such as Mg²⁺, Ca²⁺, Fe²⁺, and Fe³⁺ creating humic colloids. Many humic acids have two or more of these groups arranged so as to enable the formation of chelate complexes.{{cite journal |last=Tipping |first=E |year=1994 |title='WHAM – a chemical equilibrium model and computer code for waters, sediments, and soils incorporating a discrete site/electrostatic model of ion-binding by humic substances |journal=Computers and Geosciences |volume=20 |issue=6 |pages=973–1023 |bibcode=1994CG.....20..973T |doi=10.1016/0098-3004(94)90038-8}} The formation of (chelate) complexes is an important aspect of the biological role of humic acids in regulating bioavailability of metal ions.{{cite book |editor-last=Ghabbour |editor-first=E.A. |title=Humic Substances: Structures, Models and Functions |editor-last2=Davies |editor-first2=G. |publisher=RSC publishing |year=2001 |isbn=978-0-85404-811-3 |location=Cambridge, U.K.}}

----Criticism{{One source| date = October 2021}}

Decomposition products of dead plant materials form intimate associations with minerals, making it difficult to isolate and characterize soil organic constituents. 18th century soil chemists successfully used alkaline extraction to isolate a portion of the organic constituents in soil. This led to the theory that a 'humification' process created distinct 'humic substances' like 'humic acid', 'fulvic acid', and 'humin'. However, modern chemical analysis methods applied to unprocessed mineral soil have not directly observed large humic molecules. This suggests that the extraction and fractionation techniques used to isolate humic substances alter the original chemical composition of the organic matter. Since the definition of humic substances like humic and fulvic acids relies on their separation through these methods, it raises the question of whether the distinction between these compounds accurately reflects the natural state of organic matter in soil.{{cite book| last1=Weil | first1=Ray R. | last2=Brady | first2=Nyle C.| title=The Nature and Properties of Soils| url=https://www.pearson.com/us/higher-education/program/Weil-Nature-and-Properties-of-Soils-The-15th-Edition/PGM219427.html| date=2017 | edition=15th | publication-date=April 2017 |publisher=Pearson Education | location=Columbus, Ohio | language=en| isbn=978-0-13-325448-8 |lccn=2016008568 |oclc=936004363 | page=549 | quote=[new analytical techniques have] found very little in the way of humic macromolecules in mineral soils. Instead, evidence suggests that the alkali extraction process itself actually creates giant polymers from smaller biomolecules.}} Despite these concerns, the 'humification' theory persists in the field and in even textbooks, and attempts to redefine 'humic substances' in soil have resulted in a proliferation of conflicting definitions. This lack of consensus makes it difficult to communicate scientific understanding of soil processes and properties accurately."

The presence of humic acid in water intended for potable or industrial use can have a significant impact on the treatability of that water and the success of chemical disinfection processes. For instance, humic and fulvic acids can react with the chemicals used in the chlorination process to form disinfection byproducts such as dihaloacetonitriles, which are toxic to humans.{{cite journal|doi=10.1021/es00108a003|pmid=22295957|title=Dihaloacetonitriles in drinking water: Algae and fulvic acid as precursors|journal=Environmental Science & Technology|volume=17|issue=2|pages=80–83|year=1983|last1=Oliver|first1=Barry G.|bibcode=1983EnST...17...80O}}{{cite journal|doi=10.1016/0043-1354(90)90038-8|title=Dihaloacetonitriles in Dutch drinking waters|journal=Water Research|volume=24|issue=6|pages=797|year=1990|last1=Peters|first1=Ruud J.B.|last2=De Leer|first2=Ed W.B.|last3=De Galan|first3=Leo|bibcode=1990WatRe..24..797P }} Accurate methods of establishing humic acid concentrations are therefore essential in maintaining water supplies, especially from upland peaty catchments in temperate climates.

As a lot of different bio-organic molecules in very diverse physical associations are mixed together in natural environments, it is cumbersome to measure their exact concentrations in the humic superstructure. For this reason, concentrations of humic acid are traditionally estimated out of concentrations of organic matter, typically from concentrations of total organic carbon (TOC) or dissolved organic carbon (DOC).

Extraction procedures are bound to alter some of the chemical linkages present in the soil humic substances (mainly ester bonds in biopolyesters such as cutins and suberins). The humic extracts are composed of large numbers of different bio-organic molecules that have not yet been totally separated and identified. However, single classes of residual biomolecules have been identified by selective extractions and chemical fractionation, and are represented by alkanoic and hydroxy alkanoic acids, resins, waxes, lignin residues, sugars, and peptides.

Organic matter soil amendments have been known by farmers to be beneficial to plant growth for longer than recorded history.{{cite book |url=https://books.google.com/books?id=VFzVAAAAMAAJ|title=McGraw-Hill encyclopedia of science and technology: an international reference work, Volume 12 |editor= Lapedes, Daniel N. |publisher=McGraw-Hill |year=1966 |page=428 |isbn=978-0070452657 |quote=The value of adding organic matter to the soil in the form of animal manures, green manures, and crop residues for producing favorable soil tilth has been known since ancient times}} However, the chemistry and function of the organic matter have been a subject of controversy since humans began postulating about it in the 18th century. Until the time of Liebig, it was supposed that humus was used directly by plants, but, after Liebig showed that plant growth depends upon inorganic compounds, many soil scientists held the view that organic matter was useful for fertility only as it was broken down with the release of its constituent nutrient elements into inorganic forms.

At the present time, soil scientists hold a more holistic view and at least recognize that humus influences soil fertility through its effect on the water-holding capacity of the soil. Also, since plants have been shown to absorb and translocate the complex organic molecules of systemic insecticides, they can no longer discredit the idea that plants may be able to absorb the soluble forms of humus;{{cite book |url=https://books.google.com/books?id=sG4oAAAAMAAJ |title=Ciencia interamericana: Volumes 24–27 |year=1984 |author=Pan American Union. Dept. of Cultural Affairs. División de Fomento Científico, Pan American Union. Dept. of Scientific Affairs, Organization of American States. Dept. of Scientific Affairs |quote=And since plants have shown their ability to absorb and translocate the complex molecules of systemic insecticides, they can no longer discredit the idea that plants are able to absorb the soluble humic nutrients, containing by far ...}} this may in fact be an essential process for the uptake of otherwise insoluble iron oxides.

A study on the effects of humic acid on plant growth was conducted at Ohio State University which said in part "humic acids increased plant growth" and that there were "relatively large responses at low application rates".{{cite journal|last1=Arancon|first1=Norman Q.|last2=Edwards|first2=Clive. A.|last3=Lee|first3=Stephen|last4=Byrne|first4=Robert|year=2006|title=Effects of humic acids from vermicomposts on plant growth|journal=European Journal of Soil Biology|volume=42|pages=S65–S69|doi=10.1016/j.ejsobi.2006.06.004|bibcode=2006EJSB...42..S65A |citeseerx=10.1.1.486.2522}}

A 1998 study by scientists at the North Carolina State University College of Agriculture and Life Sciences showed that addition of humate to soil significantly increased root mass in creeping bentgrass turf.{{cite journal|doi=10.2135/cropsci1998.0011183X003800060037x |title=Influence of Humic Substances on Rooting and Nutrient Content of Creeping Bentgrass |journal=Crop Science |volume=38 |issue=6 |pages=1639 |year=1998 |last1=Cooper |first1=R. J. |last2=Liu |first2=Chunhua |last3=Fisher |first3=D. S.}}{{cite journal|url=http://archive.lib.msu.edu/tic/tgtre/article/1999aug6.pdf|title=Humic Substances Their Influence on Creeping Bentgrass Growth and Stress Tolerance|date=August 1999|journal=TurfGrass Trends|page=6|author=Liu, Chunhua|author2=Cooper, R. J.}}

A 2018 study by scientists at the University of Alberta showed that humic acids can reduce prion infectivity in laboratory experiments, but that this effect may be uncertain in the environment due to minerals in the soil that buffer the effect.{{cite journal|title=Soil humic acids degrade CWD prions and reduce infectivity|date=November 2018|journal=PLOS Pathogens|author=Kuznetsova, Alsu|author2=Cullingham, Catherine|author3=McKenzie, Debbie|author4=Aiken, Judd M.|volume=14 |issue=11 |pages=e1007414 |doi=10.1371/journal.ppat.1007414 |pmid=30496301 |pmc=6264147 |doi-access=free }}

Humans can affect the production of humic substances via a variety of ways: by making use of natural processes by composting lignin or adding biochar (see soil rehabilitation), or by industrial synthesis of artificial humic substances from organic feedstocks directly. These artificial substances may be similarly divided into artificial humic acid (A-HA) and artificial fulvic acid (A-FA).{{cite journal|first=Kyoichi|last=Kumada|year=1956|title=On artificial humic acids|journal=Soil Science and Plant Nutrition|volume=2|issue=1|pages=106–111|doi= 10.1080/00380768.1956.10431868|bibcode=1956SSPN....2..106K |quote=the most stable artificial humic acid was obtained from charcoal… These stable humic acids showed absorption spectra similar to those of soil humic acids}}

A more recent process known as hydrothermal humification and fulvification {{Cite journal |last=Marzban |first=Nader |date=2024 |title=From hydrothermal carbonization to hydrothermal humification of biomass: the role of process conditions |url=https://depositonce.tu-berlin.de/handle/11303/22182 |language=en |doi=10.14279/depositonce-20983}}, allows the conversion of a wide range of biomass and biogenic residues into artificial humin, A-HA, and A-FA under controlled temperature (180°C–250°C) and autogenic pressure, similar to hydrothermal carbonization but in an alkaline solution, which results in the autoneutralization of the reaction medium through the conversion of biomass components—cellulose, hemicellulose, and lignin—within minutes to hours, compared to the natural process which takes years in nature.{{Cite journal |last1=Marzban |first1=Nader |last2=Libra |first2=Judy A. |last3=Rotter |first3=Vera Susanne |last4=Herrmann |first4=Christiane |last5=Ro |first5=Kyoung S. |last6=Filonenko |first6=Svitlana |last7=Hoffmann |first7=Thomas |last8=Antonietti |first8=Markus |date=2024-05-09 |title=Maximizing the value of liquid products and minimizing carbon loss in hydrothermal processing of biomass: an evolution from carbonization to humification |journal=Biochar |language=en |volume=6 |issue=1 |pages=44 |doi=10.1007/s42773-024-00334-1 |issn=2524-7867|doi-access=free |bibcode=2024Bioch...6...44M }}{{Cite journal |last1=Tkachenko |first1=Vitalii |last2=Marzban |first2=Nader |last3=Vogl |first3=Sarah |last4=Filonenko |first4=Svitlana |last5=Antonietti |first5=Markus |date=2023-01-31 |title=Chemical insights into the base-tuned hydrothermal treatment of side stream biomasses |journal=Sustainable Energy & Fuels |language=en |volume=7 |issue=3 |pages=769–777 |doi=10.1039/D2SE01513G |issn=2398-4902|doi-access=free }} This method enables rapid and tunable production of artificial humic substances while retaining critical functional groups important for soil health, carbon sequestration, and plant growth stimulation.{{Cite journal |last1=Marzban |first1=Nader |last2=Psarianos |first2=Marios |last3=Herrmann |first3=Christiane |last4=Schulz-Nielsen |first4=Lisa |last5=Olszewska-Widdrat |first5=Agata |last6=Arefi |first6=Arman |last7=Pecenka |first7=Ralf |last8=Grundmann |first8=Philipp |last9=Schlüter |first9=Oliver K. |last10=Hoffmann |first10=Thomas |last11=Rotter |first11=Vera Susanne |last12=Nikoloski |first12=Zoran |last13=Sturm |first13=Barbara |date=2025-03-01 |title=Smart integrated biorefineries in bioeconomy: A concept toward zero-waste, emission reduction, and self-sufficient energy production |url=https://www.biofueljournal.com/article_216411.html |journal=Biofuel Research Journal |volume=12 |issue=1 |pages=2319–2349 |doi=10.18331/BRJ2025.12.1.4|doi-access=free }} Artificial humic acids have also been shown to mitigate the negative effects of drought on soil microbial communities, supporting microbial diversity and functionality under stress conditions.{{Cite journal |last1=Hoefle |first1=Daniel |last2=Sperber |first2=Sebastian |last3=Marzban |first3=Nader |last4=Antonietti |first4=Markus |last5=Hoffmann |first5=Thomas |last6=Wicaksono |first6=Wisnu Adi |last7=Abdelfattah |first7=Ahmed |last8=Berg |first8=Gabriele |date=2024 |title=Artificial Humic Acid Diminishes the Effect of Drought on the Soil Microbiome |journal=Journal of Sustainable Agriculture and Environment |volume=3 |issue=4 |pages=e70034 |doi=10.1002/sae2.70034 |issn=2767-035X|doi-access=free |bibcode=2024JSAE....3E0034H }} The synthesized humic substances, produced within a few hours, were successfully applied to save a 160-year-old beech tree in Park Sanssouci, Potsdam, Germany, which was under stress due to water scarcity and the sandy soil conditions typical of Brandenburg.{{Cite web |date=2024-12-10 |title=ATB in Potsdam does pioneering work: Artificially produced humic substances for agriculture |url=https://www.atb-potsdam.de/en/news-and-press/news/news-details/artificially-produced-humic-substances-for-agriculture |access-date=2025-03-21 |website=www.atb-potsdam.de |language=en}}

Lignosulfonates, a by-product from the sulfite pulping of wood, are valorized in the industrial fabrication of concrete where they serve as water reducer, or concrete superplasticizer, to decrease the water-cement ratio (w/c) of fresh concrete while preserving its workability. The w/c ratio of concrete is one of the main parameter controlling the mechanical strength of hardened concrete and its durability. The same wood pulping process can also be applied to obtain humus-like substances by hydrolysis and oxidation. A kind of artificial "lignohumate" can be directly produced from wood in this way.{{cite journal |last1=Yakimenko |first1=Olga |last2=Stepanov |first2=Andrey |last3=Patsaeva |first3=Svetlana |last4=Khundzhua |first4=Daria |last5=Osipova |first5=Olesya |last6=Gladkov |first6=Oleg |title=Formation of Humic-Like Substances during the Technological Process of Lignohumate Synthesis as a Function of Time |journal=Separations |date=3 July 2021 |volume=8 |issue=7 |pages=96 |doi=10.3390/separations8070096 |doi-access=free}}

Agricultural litter can be turned into an artificial humic substance by a hydrothermal reaction. The resulting mixture can increase the content of dissolved organic matter (DOM) and total organic carbon (TOC) 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=2021-06-25|title=Artificial humic substances improve microbial activity for binding CO2|journal=iScience|language=en|volume=24|issue=6|pages=102647|pmid=34466779|doi=10.1016/j.isci.2021.102647|pmc=8387571|bibcode=2021iSci...24j2647T |issn=2589-0042}}

Lignite (brown coal) may also be oxidized to produce humic substances, reversing the natural process of coal formation under anoxic and reducing conditions. This form of "mineral-derived fulvic acid" is widely used in China.{{cite journal |last1=Gong |first1=Guanqun |last2=Xu |first2=Liangwei |last3=Zhang |first3=Yingjie |last4=Liu |first4=Weixin |last5=Wang |first5=Ming |last6=Zhao |first6=Yufeng |last7=Yuan |first7=Xin |last8=Li |first8=Yajun |title=Extraction of Fulvic Acid from Lignite and Characterization of Its Functional Groups |journal=ACS Omega |date=3 November 2020 |volume=5 |issue=43 |pages=27953–27961 |doi=10.1021/acsomega.0c03388|pmid=33163778 |pmc=7643152 }} This process also occurs in nature, producing leonardite.{{Cite web |url=http://www.anl.gov/PCS/acsfuel/preprint%20archive/Files/07_1_CINCINNATI_01-63_0012.pdf |title=Youngs, R.W. & Frost, C.M. 1963. Humic acids from leonardite – a soil conditioner and organic fertilizer. Ind. Eng. Chem., 55, 95–99 |access-date=2022-03-18 |archive-date=2010-06-01 |archive-url=https://web.archive.org/web/20100601171311/http://www.anl.gov/PCS/acsfuel/preprint%20archive/Files/07_1_CINCINNATI_01-63_0012.pdf |url-status=dead }}

In economic geology, the term humate refers to geological materials, such as weathered coal beds (leonardite), mudrock, or pore material in sandstones, that are rich in humic acids. Humate has been mined from the Fruitland Formation of New Mexico for use as a soil amendment since the 1970s, with nearly 60,000 metric tons produced by 2016.{{cite journal |last1=Newcomer |first1=Robert W. |last2=Nybo |first2=John P. |last3=Newcomer |first3=Jacob K. |title=Humate in the upper Cretaceous Fruitland Formation in northwestern New Mexico |journal=New Mexico Geological Society Special Publication |date=2020 |volume=14 |pages=41–46 |url=https://nmgs.nmt.edu/publications/special/14/NMGS_SP-14.pdf |access-date=26 October 2020}} Humate deposits may also play an important role in the genesis of uranium ore bodies.{{cite journal |last1=McLemore |first1=Virginia T. |title=Uranium deposits in the Poison Canyon trend, Ambrosia Lake Subdistrict, Grants Uranium District, McKinley and Cibola Counties, New Mexico |journal=New Mexico Geological Society Special Publication |date=2020 |volume=14 |pages=53–63 |url=https://nmgs.nmt.edu/publications/special/14/NMGS_SP-14.pdf |access-date=26 October 2020}}

The heavy-metal binding abilities of humic acids have been exploited to develop remediation technologies for removing lead from wastewater. To this end, Yurishcheva et al. coated magnetic nanoparticles with humic acids. After capturing lead ions, the nanoparticles can then be captured using a magnet.{{cite journal |url=https://www.amsi.ge/jbpc/21313/13-2-abs-2.htm |title=Sorption of Pb2+ by magnetite coated with humic acids|journal=J. Biol. Phys. Chem.|volume=13|issue=2|pages=61–68 |year=2013 |last1=Yurishcheva |first1=A.A. |last2=Kydralieva |first2=K.A. |last3=Zaripova |first3=A.A. |last4=Dzhardimalieva |first4=G.I. |last5=Pomogaylo |first5=A.D. |last6=Jorobekova |first6=S.J.|doi=10.4024/04YU13A.jbpc.13.02 }}

Archeology finds that ancient Egypt used mudbricks reinforced with straw and humic acids.{{cite book |author=Lucas, A. |author2=Harris, J.R. |title=Ancient Egyptian Materials and Industries |publisher=Dover Publications |location=New York |year=1998 |page=62 |url=https://archive.org/details/in.ernet.dli.2015.69404/page/n75/mode/2up| isbn=978-0-486-40446-2}}

• Black water (drink)
• Humin
• Humus
• Polycyclic aromatic hydrocarbon
• Soil

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• {{cite book | last1=Schnitzer | first1=M. | last2=Khan | first2=S.U. | title=Soil Organic Matter | publisher=Elsevier Science | series=Developments in Soil Science | year=1975 | isbn=978-0-08-086975-9 | url=https://books.google.com/books?id=nUmMTWcaPkMC&pg=PP1 | access-date=2025-03-10 | page=1}}

• [https://humic-substances.org/ International Humic Substances Society]

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Category:Composting

Category:Organic acids

Category:Soil chemistry