Organic matter, humic and fulvic

What the living world leaves behind.

Soil organic matter is what living things leave in the soil, at every stage of decomposition. This page follows it from a fallen leaf to the fractions named humic and fulvic, what they do, and where the measurements stop.1, 2

Start with a leafSee the open questions

  1. 1 Shredded by earthworms, mites and springtails

  2. 2 Threaded by fungi and dotted with bacteria

  3. 3 Dissolved compounds settle onto clay

  4. 4 Microbial remains attach to a mineral grain

Illustration. A leaf becoming soil organic matter, in four stages. Organisms enlarged; not to scale.

Origins

The story begins with organic matter.

Decomposers keep breaking organic fragments into smaller pieces. Microbes can take up only small fragments, typically under about 600 daltons, so larger material is broken down outside the cell first.1

Decomposing litter feeds soil organic matter by two routes. Early on, soluble compounds processed by microbes end up attached to minerals. Later, physical fragments of the litter become particulate organic matter.3

Microbes build as well as consume. In laboratory model soils given simple substrates and no plant litter, microbes produced organic matter resembling soil organic matter, and a microbe’s remains are part of what persists on mineral surfaces.4

What persists is what minerals, aggregates and inaccessible pores protect from microbes. Persistence is set by the surroundings more than by the chemistry of the material.1

In soils, waters and deposits

Humic material runs through soils, waters and buried ground.

Humic substances occur in soils, in natural waters and in organic deposits.5 Groundwater carries dissolved organic matter, including humic and fulvic fractions.6 Peat, soil, brown coal, composts and manures have all served as sources of the humic materials tested on plants.7 Leonardite, the source in several studies in the library, is oxidised lignite.

The International Humic Substances Society keeps standard samples from a river, a peat, a soil and leonardite, so laboratories can compare like with like.8

Four sample jars: river water, peat, soil and leonardite
Illustration. The four kinds of source the International Humic Substances Society keeps standard samples from: a river, a peat, a soil and leonardite. Not to scale.
The acidification step of the separationA tube of extract after acidification. Dark flecks of the humic acid fraction have settled in a layer at the bottom. The liquid above, the fulvic acid fraction, stays in solution.Fulvic acid fractionstays in solutionHumic acid fractionsettles out
Illustration. A dark alkaline extract, acidified: the humic acid fraction settles, and the fulvic acid fraction stays dissolved above it. This step is where the names come from.

Definitions

Names for fractions, not single molecules.

Extract with alkali. Acidify the extract to about pH 1 to 2. The dark material that settles is the humic acid fraction. The amber material that stays dissolved, purified further on a resin, is the fulvic acid fraction. What alkali never dissolves is humin.9

TermWhat it describes
Humic substancesA broad category of heterogeneous organic materials.
Humic acid fractionMaterial extracted under alkaline conditions that precipitates when sufficiently acidified in the separation procedure.
Fulvic acid fractionMaterial that remains in solution during that acidification step and is isolated through further separation.
HuminThe material alkali never dissolves.

Extraction is never complete. It leaves 50 to 70 percent of soil organic carbon behind, and at pH 13 it ionises groups that would never dissociate between soil pH 3.5 and 8.5, which exaggerates the extract’s reactivity.1 Fulvic fractions are, on average, more oxygen-rich and more acidic than humic fractions, which fits their staying dissolved at low pH.10, 11

What scientists argue about

Whether soil holds these fractions in the form the glass shows is an open argument among leading soil scientists. Markus Kleber and Johannes Lehmann argue that alkaline extraction cannot separate humic from non-humic material and may create products not found in nature, so the fractions are poor stand-ins for organic matter in the field.12 Dan Olk and colleagues answer that the fractions remain useful for real questions, such as tracking nutrients, metals and pesticides, without knowing their exact structure.13 The magazine of the agronomy, crop and soil science societies set out both sides for a general reader.14 This site uses the names for what a laboratory isolates, and keeps the argument in view.

Read the debate in CSA News

Why charge matters

Humic material holds nutrients, binds metals and, in laboratory cultures, carries electrons.

Humic material is chemically reactive and affects how elements and organic compounds become available and move in soil and water.5 Measured at pH 7, soil organic matter holds more exchangeable positive charge per gram than the common clay minerals: 200 to 400 meq/100 g, against 80 to 100 for montmorillonite and less for illite and kaolinite.15 Three of the ways it does so are drawn here, one scene at a time: charge and exchange at a surface, a metal held in a complex, and electrons passed on by bacteria.

Clay and organic matter carry negatively charged sites that hold positively charged ions and swap them for others in the soil water: the soil’s cation exchange.

Step 1 of 4

A surface in soil water

Clay minerals and organic matter both carry negatively charged sites. Humic material is extracted from that organic matter; its acidic groups stand in here for the organic matter’s charged sites.16, 17, 10

Step 2 of 4

Acidic groups, magnified

Humic material carries acidic groups, counted by titration as carboxyl and phenolic. Whether a group keeps its hydrogen ion or carries a negative charge depends on the pH of the water: the lower the pH, the more groups keep theirs.

Step 3 of 4

Ions held at the charged sites

Positively charged ions such as potassium are held at the charged sites. So is ammonium, a form of nitrogen; nitrate, which carries a negative charge, is not held at these sites and generally stays in the soil water. In more acidic water, more groups keep their hydrogen ion and carry no charge, so fewer sites are left to hold a nutrient.

Step 4 of 4

Ions keep moving between surface and water

Held ions are not fixed. A calcium ion, carrying two charges, can take the place of two ions carrying one charge each, and the two it replaces return to the water. Exchange keeps ions moving between the surface and the water; whether a plant takes one up is a separate question, measured in the plant.

Sources16, 17, 10

NDSU Extension on cation exchange

pH of the water Illustrative

Drawn from how IHSS counts the groups: carboxyl by pH 8, phenolic between pH 8 and 10. The Alkaline setting reaches that range, above the pH of most soils. Not a prediction of nutrient availability.1, 10

Illustration. Charge and exchange at a surface, step 1: A surface in soil water.a: Clay mineral surface, negatively charged. b: Humic material, standing in for the organic matter it is extracted from. c: Positively charged ions in the water. d: Magnified next: a few of its acidic groups.K+Ca2+K+abcd
Illustration. Charge and exchange at a surface, step 2: Acidic groups, magnified.a: Part of the humic material, magnified. A schematic: it is a mixture with no single structure.. b: Carboxyl group. c: Phenolic group. d: Hydrogen ion in the water.H+H+H+H+H+H+H+H+H+H+H+H+H+H+abcd
Illustration. Charge and exchange at a surface, step 3: Ions held at the charged sites.a: Humic material, magnified (schematic). b: Potassium ion held at a charged site. c: Calcium ion in the water. d: A group keeping its hydrogen ion, so it carries no charge.H+H+H+H+H+H+H+H+K+K+Ca2+K+H+H+H+H+K+K+K+K+Ca2+K+H+H+K+K+K+K+K+Ca2+K+abcd
Illustration. Charge and exchange at a surface, step 4: Ions keep moving between surface and water.a: Humic material, magnified (schematic). b: A calcium ion held across two charged sites. c: The two ions it replaced, back in the water.H+H+H+H+Ca2+H+H+H+H+K+K+K+H+H+Ca2+H+H+K+K+K+K+K+H+Ca2+H+K+K+K+K+K+K+abc

Humic material can hold a metal ion at more than one group. In one laboratory study, cucumber and barley took up iron from a soluble iron humate; the authors propose a different route for each.

Step 1 of 4

A peat humic fraction and iron

Cesco and colleagues used a water-soluble humic fraction extracted from peat and combined it with iron carrying a radioactive label, iron-59, so the iron could be traced.18, 19

Step 2 of 4

Held at more than one group

In a complex, humic material can hold an iron ion at more than one group; with a water-soluble fraction such as this one, the iron stays dissolved with it. Which groups bind, and how many, this source does not show: the three drawn are a schematic.

Step 3 of 4

At the cucumber root

Cucumber roots took up iron from the complex, more at pH 6.0 than at pH 7.5, and reduced the iron in it from iron(III) to iron(II), faster at pH 6.0. The authors propose that uptake runs through this reduction, by a reductase in the root’s plasma membrane.

Step 4 of 4

Iron from the complex was measured in the plants

Holding iron and supplying it to a plant are different findings. Here the iron was given to the plants already held by the humic fraction, and iron-59 was measured in the plants, in the laboratory. In barley, uptake followed the daily rhythm of the iron-binding compounds its roots release, and the authors propose the iron passes to them.

Sources18, 19

Read Cesco et al. 2002
Illustration. Iron held in a complex, step 1: A peat humic fraction and iron.a: Water-soluble humic fraction, from peat. b: Iron(III), labelled as iron-59.Fe3+Fe3+Fe3+ab
Illustration. Iron held in a complex, step 2: Held at more than one group.a: Humic material, magnified (schematic). b: Iron(III) ion. c: Groups bonded to the one ion: a complex.Fe3+abc
Illustration. Iron held in a complex, step 3: At the cucumber root.a: The complex, in the uptake solution. b: Root reductase, in the plasma membrane (its role here proposed by the authors). c: Iron(II) entering the root, as the authors propose. d: Cucumber root.Fe3+Fe2+abcd
Illustration. Iron held in a complex, step 4: Iron from the complex was measured in the plants.a: Association: iron held by the humic fraction, as supplied. b: Uptake: iron-59 measured in the plant (its form inside not shown).Fe3+FeFeFeab

In laboratory cultures, some iron-reducing bacteria pass the electrons from their respiration to humic material, which can pass them on to iron minerals. A different role from supplying carbon.

Step 1 of 4

A laboratory culture without oxygen

Some bacteria from soils and sediments oxidise organic compounds or hydrogen for energy when there is no oxygen. Among those studied in laboratory culture is the iron-reducing Geobacter metallireducens.20, 21

Step 2 of 4

Electrons to humic material

The cell passes the electrons from that oxidation to humic substances, which stand in for iron(III) as the acceptor at the end of its respiration. The authors report that this electron transport yields energy to support growth.

Step 3 of 4

A shuttle to iron minerals

The humic material can pass the electrons on to insoluble iron(III) oxides, which the bacteria reach less easily. At the mineral, iron(III) is reduced to iron(II).

Step 4 of 4

Electrons, carbon and ions each take their own path

Here humic material is an electron acceptor: it takes electrons, a different role from supplying carbon. The carbon the cell uses is in its food; the ions are the iron at the mineral. This is a laboratory result; how much of it happens in a given soil or sediment has to be measured there.

Sources20, 21

Read Lovley et al. 1996
Illustration. Electrons passed on by bacteria, step 1: A laboratory culture without oxygen.a: The bacterium, in laboratory culture. b: Organic compounds it oxidises for energy: its food.ab
Illustration. Electrons passed on by bacteria, step 2: Electrons to humic material.a: The bacterium. b: Its food. c: Electrons, passed out of the cell. d: Humic material, accepting the electrons.e−abcd
Illustration. Electrons passed on by bacteria, step 3: A shuttle to iron minerals.a: Humic material. b: Electrons, passed on. c: Insoluble iron(III) oxide. d: Iron(II).e−Fe2+abcd
Illustration. Electrons passed on by bacteria, step 4: Electrons, carbon and ions each take their own path.a: Carbon: the organic compounds the cell oxidises. b: Electrons: from the cell, through the humic material, to the mineral. c: Ions: iron, reduced from iron(III) to iron(II). d: Humic material: here an electron acceptor.e−Fe2+abcd
  • Humic material: a mixture, drawn as fragments
  • Mineral surface
  • Negatively charged site
  • Hydrogen ion
  • Other positive ions: nutrients, metals
  • Electrons
  • Carbon compound: food for the cell
  • Bacterium
  • Root
Illustrations, not to scale, and the number of ions drawn is not a count. Three scenes: choose one, then step through it. Humic material is drawn as a cloud of fragments because it is a mixture, not one molecule; magnified views show one part of it, and their groups are schematic.
The three scenes as plain text

1. Charge and exchange at a surface

Clay and organic matter carry negatively charged sites that hold positively charged ions and swap them for others in the soil water: the soil’s cation exchange.16, 17, 10

  1. A surface in soil water. Clay minerals and organic matter both carry negatively charged sites. Humic material is extracted from that organic matter; its acidic groups stand in here for the organic matter’s charged sites.
  2. Acidic groups, magnified. Humic material carries acidic groups, counted by titration as carboxyl and phenolic. Whether a group keeps its hydrogen ion or carries a negative charge depends on the pH of the water: the lower the pH, the more groups keep theirs.
  3. Ions held at the charged sites. Positively charged ions such as potassium are held at the charged sites. So is ammonium, a form of nitrogen; nitrate, which carries a negative charge, is not held at these sites and generally stays in the soil water. In more acidic water, more groups keep their hydrogen ion and carry no charge, so fewer sites are left to hold a nutrient.
  4. Ions keep moving between surface and water. Held ions are not fixed. A calcium ion, carrying two charges, can take the place of two ions carrying one charge each, and the two it replaces return to the water. Exchange keeps ions moving between the surface and the water; whether a plant takes one up is a separate question, measured in the plant.

2. Iron held in a complex

Humic material can hold a metal ion at more than one group. In one laboratory study, cucumber and barley took up iron from a soluble iron humate; the authors propose a different route for each.18, 19

  1. A peat humic fraction and iron. Cesco and colleagues used a water-soluble humic fraction extracted from peat and combined it with iron carrying a radioactive label, iron-59, so the iron could be traced.
  2. Held at more than one group. In a complex, humic material can hold an iron ion at more than one group; with a water-soluble fraction such as this one, the iron stays dissolved with it. Which groups bind, and how many, this source does not show: the three drawn are a schematic.
  3. At the cucumber root. Cucumber roots took up iron from the complex, more at pH 6.0 than at pH 7.5, and reduced the iron in it from iron(III) to iron(II), faster at pH 6.0. The authors propose that uptake runs through this reduction, by a reductase in the root’s plasma membrane.
  4. Iron from the complex was measured in the plants. Holding iron and supplying it to a plant are different findings. Here the iron was given to the plants already held by the humic fraction, and iron-59 was measured in the plants, in the laboratory. In barley, uptake followed the daily rhythm of the iron-binding compounds its roots release, and the authors propose the iron passes to them.

3. Electrons passed on by bacteria

In laboratory cultures, some iron-reducing bacteria pass the electrons from their respiration to humic material, which can pass them on to iron minerals. A different role from supplying carbon.20, 21

  1. A laboratory culture without oxygen. Some bacteria from soils and sediments oxidise organic compounds or hydrogen for energy when there is no oxygen. Among those studied in laboratory culture is the iron-reducing Geobacter metallireducens.
  2. Electrons to humic material. The cell passes the electrons from that oxidation to humic substances, which stand in for iron(III) as the acceptor at the end of its respiration. The authors report that this electron transport yields energy to support growth.
  3. A shuttle to iron minerals. The humic material can pass the electrons on to insoluble iron(III) oxides, which the bacteria reach less easily. At the mineral, iron(III) is reduced to iron(II).
  4. Electrons, carbon and ions each take their own path. Here humic material is an electron acceptor: it takes electrons, a different role from supplying carbon. The carbon the cell uses is in its food; the ions are the iron at the mineral. This is a laboratory result; how much of it happens in a given soil or sediment has to be measured there.
Adsorption, complexation and chelation in the Glossary

Carbon and microbial use

Roots feed the soil’s microbes. Added material is a separate question.

Roots, residues and the remains of other microbes are their food. Living roots release carbon compounds that feed the microbes around them, directly or through mycorrhizal fungi. Which parts of an added humic or fulvic material microbes use is measured one material and one soil at a time.22, 23, 4, 24

Magnified view of the root zone, beside an added humic materialA root along the top with two root hairs reaching down into the soil, the carbon compounds the root releases (small dots), and bacteria clustered in the soil just below its surface. Fungal hyphae thread through the soil, one into a soil aggregate. A green tint marks the soil closest to the root, where microbial numbers and activity are highest. To the right, outside the view, a cloud of varied fragments stands for an added humic material. A dotted line runs from it toward the nearest bacteria and stops short of them.abcdeWhich parts are used:measured one materialand one soil at a time.

Dotted line: which parts are used, measured one material and one soil at a time.

  • a Root
  • b Root exudates
  • c Bacteria
  • d Hyphae
  • e Aggregate
Illustration. Roots feeding the soil, and a fungal thread trading carbon for nitrogen, as shown in root-organ culture. The fragments are a mixture, not one molecule. Not to scale.23, 25

Four questions for a carbon-containing preparation

  • Carbon content

    Question. How much carbon was measured, using which analytical method?

  • Solubility

    Question. What portion remains in solution under the specified conditions?

  • Microbial use

    Question. What measurements show that particular organisms use the material or its components?26

  • Functional response

    Question. What changed in the community, host or wider system, and how was that change measured?

A claim that a preparation supplies microbial fuel needs evidence of use, such as a labelled substrate traced into the organisms. A claim that it favours particular organisms needs evidence of which organisms responded and what they did.26

A change in community composition alone does not establish that the added material was directly consumed as a carbon source.27, 28

Humic substances have long been described as resistant to biodegradation. Lehmann and Kleber report that extracts can decompose surprisingly fast, and that an old radiocarbon age does not by itself show persistence.1

From chemistry to observation

What have researchers measured?

Across 81 published studies, humic substances raised shoot dry weight by an average of 22 percent, and about half the shoot studies found less than a 5 percent gain.7 In one Iowa maize study, a micronized leonardite product had no field-wide yield effect in any of four site-years, and raised yield only on eroded upland ground, mostly in drier seasons.29 Each study below is read in its own setting.

Early plant growth, shoot dry weight, mostly pots and seedlings (81 studies)

051015202530About half the shoot studies: under 5Peat-derived: 4, not significantUnstressed plants: 18Average across studies: 22Stressed plants: 28Compost-derived: 28 to 29
  • About half the shoot studies: under 5
  • Peat-derived: 4, not significant
  • Unstressed plants: 18
  • Average across studies: 22
  • Stressed plants: 28
  • Compost-derived: 28 to 29

Maize grain yield, Iowa fields, one study (four site-years)

051015202530No field-wide effect: 0Eroded upland ground only: 7 to 19
  • No field-wide effect: 0
  • Eroded upland ground only: 7 to 19

Percent change against untreated controls

Drawn to the reported percentages. Each row is a different measure, and the rows are not comparable to each other. Rose et al. 2014; Olk et al. 2021.

  • Plant research · Meta-analysis

    Across 81 studies, early growth rose on average, with wide spread.

    Question. How consistently do humic substances increase plant growth, and what changes the size of the response?

    Rose and colleagues pooled 81 published papers. Shoot dry weight rose by 22 percent on average and root dry weight by 21 percent, but about half the shoot studies found less than a 5 percent gain. Source and conditions changed the response: peat-derived materials did not significantly raise shoot growth, compost-derived materials did, and stressed plants responded more than unstressed ones.

    Interpretation

    An average across very different experiments, mostly of early growth rather than field yield. It does not predict the response of a particular material, crop or soil.

    Source
    Rose et al., 2014. Advances in Agronomy 124:37 to 89.7
    Preparation
    Humic substances from brown coal (including leonardite), peat, soil, composts and manures
    Route
    Root, foliar, soil and combined applications
    Design
    Meta-analysis with narrative review
    Read the evidence summaryRead the published study

  • Plant research · Humic preparation

    Application route changed the response being examined.

    Question. Do root and foliar applications of a sedimentary humic acid produce the same physiological responses?

    De Hita and colleagues reported improved shoot and root growth with both application routes in their cucumber experiment, alongside differences in some physiological responses. Foliar application did not reproduce all of the short-term root responses observed with root application.

    Interpretation

    The study illustrates why similar growth outcomes do not necessarily establish identical mechanisms. Its findings concern the preparation, plant system and experimental conditions tested.

    Source
    De Hita et al., 2020. Frontiers in Plant Science 11:493.30
    Preparation
    Sedimentary humic acid extracted from leonardite
    Route
    Root (nutrient solution) and foliar spray, compared
    Design
    Controlled experiment in hydroponic culture
    Read the evidence summaryRead the published study

  • Poultry research · Fulvic preparation

    A study examined production measures and microbial composition together.

    Question. What changed when older laying hens received the studied fulvic preparation?

    Xiao and colleagues studied 252 hens beginning at 55 weeks of age over eight weeks. They reported improvements in egg weight, shell-breaking strength and egg breakage, alongside changes in the relative abundance of some microbial groups. Laying rate and measures of microbial alpha diversity did not differ significantly.

    Interpretation

    The results concern the tested preparation and conditions. Observing microbial and production changes together does not establish that one caused the other or that microbes directly consumed the preparation.

    Source
    Xiao et al., 2022. Frontiers in Veterinary Science 9:953564.31
    Preparation
    Fulvic acid from a commercial supplier
    Route
    Feed
    Design
    Randomised controlled feeding trial
    Read the evidence summaryRead the published study
Open the evidence library

Reading a preparation

What needs to match before a finding can travel?

A useful comparison examines the material, the application and the system. Sharing the words “humic” or “fulvic” is a starting point for investigation, not proof of equivalence. Commercial products vary widely in composition, sometimes against their own labels.32

  1. Source

    Where the material originated.

  2. Preparation

    How it was extracted, separated or formulated.

  3. Characterisation

    What analysis tells us about the resulting material.

A material’s origin is part of its identity. Research also needs to describe the preparation that was tested.
Information to examineQuestion to ask
Source materialWhere did the preparation originate?
Extraction and processingHow was the material obtained and modified?
Composition and analytical methodWhat was measured, and how were the reported fractions defined?
Concentration and formulationWhat was present in the tested preparation, including other ingredients?
Application routeWas it applied to roots, leaves, water, feed or another environment?
Exposure and durationHow much was used, how often and for how long?
Study systemWhich species, life stage or growing conditions were involved?
OutcomeWas the study measuring chemistry, microbial responses, physiology or practical performance?

The planet’s future

The world’s soils hold more carbon than the air and all the world’s plants together.

Down to two metres, the world’s soils hold about 2,060 billion tonnes of organic carbon, more than the air, about 870, and all the world’s vegetation, about 450, together.33, 34

The two figures count soil differently: 2,060 is one estimate for all soils to two metres; the IPCC figure drawn here gives 1,700 for soils outside the permafrost region and 1,200 for permafrost, and states no depth. That carbon is soil organic carbon, the carbon contained in soil organic matter. Humic and fulvic are the fractions a laboratory extracts from part of it, so the question is how much of this ledger they help hold.

Groundwater carries dissolved organic matter, including humic and fulvic fractions.6

Carbon held, in billion tonnes

  1. Atmosphere about 870279 of it added since 1750
  2. Vegetation about 450
  3. Soils outside the permafrost region 1,700
  4. Permafrost 1,200
Illustration drawn to scale. Areas are proportional to carbon held on land, in billion tonnes: soils outside the permafrost region 1,700; permafrost 1,200; vegetation 450; atmosphere about 870. IPCC AR6 WG1, figure 5.12, which counts permafrost separately and does not state a soil depth. The oceans, not drawn, hold far more.

Carbon

What keeps soil carbon in place is its surroundings more than its chemistry: minerals and soil crumbs that shelter it from decomposers, and cold or waterlogged ground where decomposers work slowly. An old radiocarbon age does not by itself show that carbon will stay.1, 34

Fertility

Its acidic groups hold positively charged nutrients, and it binds metals such as iron; cucumber and barley plants took up iron from a soluble iron humate.10, 16, 18

Whether adding humic or fulvic material leaves soil holding more carbon than the material itself brought, no study in this library has measured. The leonardite itself is old carbon moved from one place to another, not carbon taken from the air. An application record is not sequestration.

The working reading

What the project holds, and what remains open.

These are the project’s positions today, and the questions they leave for the evidence to answer.

How the project reads these materials today

Across its publications, Carbon Biome Project reads these materials through three working functions: what microbes do with carbon, what humic material binds, and what fulvic material does at a root. Each is an interpretation, stated with the evidence behind it and the question it leaves open.

Carbon and microbes

Soil bacteria feed on carbon: roots, residues and the remains of other microbes are their food.22, 23, 4 Which parts of an added humic or fulvic material they use is measured one material and one soil at a time.24 Separately, in laboratory studies some bacteria pass electrons to humic material during respiration, which is a different role from supplying carbon.20, 35

Binding

Humic substances are chemically reactive and affect the availability and movement of elements and organic compounds.5 In one soil incubation without plants, a humic acid kept pH higher and buffered the effect of urea on ammonia-oxidising microbes, until the differences converged by week 12.36 Binding measured in solution does not by itself show removal from a body or a change in nutrient supply.

At the root

In rice seedlings grown short of phosphate, a fulvic preparation raised whole-plant phosphorus uptake by about 20 percent; when the solution’s pH was held steady, the gain disappeared.37 In winter wheat pots, fulvic fractions raised grain nitrogen and iron.38 Tritium from a labelled leonardite humic acid entered wheat seedling roots within 24 hours; most stayed in the roots, and the label may travel on breakdown products.39

  • Does the carbon in fulvic material feed the microbes living inside the plant?

    Treatments shifted those communities in stevia leaves and tomato roots. No study in this library has traced fulvic carbon into one.40, 41

  • Which parts of a humic or fulvic material do soil microbes use as carbon, and how much?

    Humic acid raised microbial respiration in one low-organic-matter soil only. Stable-isotope probing is one method that could answer it.24, 26

  • What explains the plant growth response?

    Several mechanisms are proposed; none explains every result.1, 42, 37

  • Do alkali extracts represent a natural soil fraction?

    Contested.1, 13

  • Do humic or fulvic materials act in the gut of any animal or person the way they act at a root?

    No record in this collection shows it.

  • Do the microbial changes seen in hens, broilers, trout or plants cause the production changes measured beside them?

    Not shown in any record in this collection.27

  • Does adding humic or fulvic material leave soil holding more carbon than it brought?

    Not measured in any study in this library.

Read the frameworkContribute expertise

Sources and further reading

Origins

  1. Lehmann J, Kleber M. The contentious nature of soil organic matter. Nature 528:60 to 68. 2015. doi.org/10.1038/nature16069 Full text.What soil organic matter is, the continuum view, persistence set by the surroundings, and why fraction names are operational; alkali extraction at pH 13 ionises groups that would not dissociate between soil pH 3.5 and 8.5.
  2. USDA Natural Resources Conservation Service. Soil Biology Primer. www.nrcs.usda.gov/resources/education-and-teaching-materials/soil-biology-primerSoil organic matter as what living things leave behind; the soil community.
  3. Cotrufo MF, et al. Formation of soil organic matter via biochemical and physical pathways of litter mass loss. Nature Geoscience 8:776 to 779. 2015. doi.org/10.1038/ngeo2520 Publisher summary only.Two routes from litter to soil organic matter.
  4. Kallenbach CM, Frey SD, Grandy AS. Direct evidence for microbial-derived soil organic matter formation and its ecophysiological controls. Nature Communications 7:13630. 2016. doi.org/10.1038/ncomms13630 Search text only.Microbes build soil organic matter as well as consume it.
  5. International Humic Substances Society. What are humic substances. humic-substances.org/what-are-humic-substances-2/Where humic substances occur, fraction definitions and chemical reactivity.
  6. U.S. Geological Survey. Organic matter in ground water. Search text only.Groundwater carries dissolved organic matter, including humic and fulvic fractions.
  7. Rose MT, Patti AF, Little KR, Brown AL, Jackson WR, Cavagnaro TR. A meta-analysis and review of plant-growth response to humic substances: practical implications for agriculture. Advances in Agronomy 124:37 to 89. 2014. doi.org/10.1016/B978-0-12-800138-7.00002-4 Full text.The sources of the humic materials tested on plants, and the 81-study meta-analysis.
  8. International Humic Substances Society. Source materials for IHSS samples. Read 22 October 2026. humic-substances.org/source-materials-for-ihss-samples/ Full page.Standard samples from a river, a peat, a soil and leonardite.

Names

  1. International Humic Substances Society. Isolation of IHSS samples. humic-substances.org/isolation-of-ihss-samples/ Full page.The separation procedure that defines the humic and fulvic acid fractions.
  2. International Humic Substances Society. Acidic functional groups of IHSS samples. Read 22 October 2026. humic-substances.org/acidic-functional-groups-of-ihss-samples/ Full page.Carboxyl and phenolic groups, counted by titration (carboxyl at pH 8, phenolic between pH 8 and 10); fulvic fractions are more acidic.
  3. International Humic Substances Society. Elemental compositions and stable isotopic ratios of IHSS samples. Full page.Fulvic fractions are, on average, more oxygen-rich than humic fractions.
  4. Kleber M, Lehmann J. Humic substances extracted by alkali are invalid proxies for the dynamics and functions of organic matter in terrestrial and aquatic ecosystems. Journal of Environmental Quality 48(2):207 to 216. 2019. doi.org/10.2134/jeq2019.01.0036 Publisher summary only.The case that extracted fractions do not represent organic matter as it exists in soil.
  5. Olk DC, et al. Environmental and agricultural relevance of humic fractions extracted by alkali from soils and natural waters. Journal of Environmental Quality 48:217 to 232. 2019. doi.org/10.2134/jeq2019.02.0041 Agency summary and search text.The reply: alkali-extracted fractions still track real behaviour.
  6. McCauley DJ. Soil organic matter black box: examining alkaline extraction and humic substances research. CSA News 65(1). 28 January 2020. www.sciencesocieties.org/publications/csa-news/2020/january/soil-organic-matter-black-box Full page.Both sides of the debate, for a general reader.

Chemistry

  1. Saha UK, Sonon LS, Kissel DE. Cation exchange capacity and base saturation. University of Georgia Cooperative Extension Circular 1040, full review 8 September 2022. Read 24 October 2026. fieldreport.caes.uga.edu/publications/C1040/cation-exchange-capacity-and-base-saturation/ Full page.Table 1, at pH 7: organic matter 200 to 400 meq/100 g; kaolinite 3 to 15, illite 15 to 40, montmorillonite 80 to 100. Per unit weight, not per soil: clay is usually most of a soil's weight.
  2. Franzen D. Cation exchange and cation exchange capacity. Chapter 7 of the North Dakota State University Extension soil fertility ebook. Read 22 October 2026. www.ndsu.edu/agriculture/sites/default/files/ebook/Franzen/Chp_7.html Full page.Clay and organic matter carry negative charge that holds positive ions, exchangeable with others in the soil water; organic matter groups keep their protons at lower pH.
  3. Keena M, Meehan M, Scherer T. Nitrogen behavior in the environment. North Dakota State University Extension NM1299, reviewed August 2022. Read 24 October 2026. www.ndsu.edu/agriculture/extension/publications/nitrogen-behavior-environment Full page.Soil's negatively charged mineral and organic particles hold ammonium; nitrate, negatively charged, is generally repelled and remains in the soil solution.
  4. Cesco S, Nikolic M, Römheld V, Varanini Z, Pinton R. Uptake of 59Fe from soluble 59Fe-humate complexes by cucumber and barley plants. Plant and Soil 241:121 to 128. 2002. doi.org/10.1023/A:1016061003397 Abstract only.Iron held by a water-soluble peat humic fraction: cucumber took it up, more at pH 6.0 than 7.5, and its roots reduced it; the authors propose uptake runs through that reduction by a root plasma-membrane reductase, and for barley a route through the iron-binding compounds its roots release.
  5. OpenStax. Chemistry 2e, section 19.2, Coordination chemistry of transition metals. Read 22 October 2026. openstax.org/books/chemistry-2e/pages/19-2-coordination-chemistry-of-transition-metals Full page.A ligand that bonds to a metal ion by more than one donor atom; complexes and chelates.
  6. Lovley DR, Coates JD, Blunt-Harris EL, Phillips EJP, Woodward JC. Humic substances as electron acceptors for microbial respiration. Nature 382:445 to 448. 1996. doi.org/10.1038/382445a0 Abstract only.In laboratory cultures, some microorganisms from soils and sediments pass electrons to humic substances, which can shuttle them to insoluble iron(III) oxides; the electron transport supports growth.
  7. Coates JD, Ellis DJ, Blunt-Harris EL, Gaw CV, Roden EE, Lovley DR. Recovery of humic-reducing bacteria from a diversity of environments. Applied and Environmental Microbiology 64:1504 to 1509. 1998. doi.org/10.1128/AEM.64.4.1504-1509.1998 Full text.Names organisms from this line of work: the iron-reducing Geobacter metallireducens and Shewanella alga can use humic substances in place of iron(III) as the terminal electron acceptor.

Carbon

  1. USDA Natural Resources Conservation Service. Soil Health Management. www.nrcs.usda.gov/conservation-basics/soil/soil-health/soil-health-managementLiving roots release carbon compounds that feed the microbes around them.
  2. Kaiser C, et al. Exploring the transfer of recent plant photosynthates to soil microbes: mycorrhizal pathway vs direct root exudation. New Phytologist. 2014 online, 2015 in print. doi.org/10.1111/nph.13138 Search text only.Recently fixed carbon reaches soil microbes through roots and through mycorrhizal fungi.
  3. Hartz TK, Bottoms TG. Humic substances generally ineffective in improving vegetable crop nutrient uptake or productivity. HortScience 45:906 to 910. 2010. doi.org/10.21273/HORTSCI.45.6.906 Full text.Humic acid raised microbial respiration in one low-organic-matter soil only.
  4. Fellbaum CR, et al. Carbon availability triggers fungal nitrogen uptake and transport in arbuscular mycorrhizal symbiosis. Proceedings of the National Academy of Sciences 109:2666 to 2671. 2012. doi.org/10.1073/pnas.1118650109 Search text only.In root-organ culture, the fungus took carbon from the root and passed nitrogen back.
  5. Radajewski S, Ineson P, Parekh NR, Murrell JC. Stable-isotope probing as a tool in microbial ecology. Nature 403:646 to 649. 2000. doi.org/10.1038/35001054A method that links a labelled substrate to the organisms that use it.
  6. Gloor GB, Macklaim JM, Pawlowsky-Glahn V, Egozcue JJ. Microbiome datasets are compositional: and this is not optional. Frontiers in Microbiology 8:2224. 2017. doi.org/10.3389/fmicb.2017.02224Limits of interpreting relative changes in community composition.
  7. Zhang Z, Qu Y, Li S, et al. Soil bacterial quantification approaches coupling with relative abundances reflecting the changes of taxa. Scientific Reports 7:4837. 2017. doi.org/10.1038/s41598-017-05260-wRelative and absolute abundance can diverge.

Research

  1. Olk DC, Dinnes DL, Scoresby JR, Darlington JW, Hurburgh CR, Rippke GR. Maize growth and grain yield responses to a micronized humic product across soil types and annual weather patterns in central Iowa, United States. Frontiers in Plant Science 12:672078. 2021. doi.org/10.3389/fpls.2021.672078 Full text.Iowa maize: no field-wide effect in four site-years; gains on eroded upland ground.
  2. De Hita D, Fuentes M, Fernández V, Zamarreño AM, Olaetxea M, García-Mina JM. Discriminating the short-term action of root and foliar application of humic acids on plant growth: emerging role of jasmonic acid. Frontiers in Plant Science 11:493. 2020. doi.org/10.3389/fpls.2020.00493 Full text.Plant application-route example.
  3. Xiao G, Liu S, Yan X, et al. Effects of fulvic acid addition on laying performance, biochemical indices, and gut microbiota of aged hens. Frontiers in Veterinary Science 9:953564. 2022. doi.org/10.3389/fvets.2022.953564 Full text.Laying-hen example.
  4. Lyons G, Genc Y. Commercial humates in agriculture: real substance or smoke and mirrors? Agronomy 6:50. 2016. doi.org/10.3390/agronomy6040050Commercial products vary widely in composition, including against their labels.

The planet

  1. Batjes NH. Harmonized soil property values for broad-scale modelling (WISE30sec) with estimates of global soil carbon stocks. Geoderma 269:61 to 68. 2016. doi.org/10.1016/j.geoderma.2016.01.034 Abstract only.Soil organic carbon to two metres, about 2,060 billion tonnes.
  2. Canadell JG, Monteiro PMS, Costa MH, et al. Global carbon and other biogeochemical cycles and feedbacks. In: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the IPCC, pp. 673 to 816. Cambridge University Press. 2021. doi.org/10.1017/9781009157896.007 Figure and chapter text.Figure 5.12, depth not stated: carbon in soils outside the permafrost region, permafrost, vegetation and the atmosphere. Box 5.1: slow decomposition in frozen and waterlogged soil.
  3. Roden EE, Kappler A, Bauer I, et al. Extracellular electron transfer through microbial reduction of solid-phase humic substances. Nature Geoscience 3:417 to 421. 2010. doi.org/10.1038/ngeo870 Abstract only.Microbial reduction of solid-phase humic material.

Open questions

  1. Dong L, Cordova-Kreylos AL, Yang J, Yuan H, Scow KM. Humic acids buffer the effects of urea on soil ammonia oxidizers and potential nitrification. Soil Biology and Biochemistry 41:1612 to 1621. 2009. doi.org/10.1016/j.soilbio.2009.04.023 Full text.A soil incubation without plants: pH held higher, urea effects buffered until week 12.
  2. Lv X, Li Q, Deng X, et al. Fulvic acid application increases rice seedlings performance under low phosphorus stress. BMC Plant Biology 24:703. 2024. doi.org/10.1186/s12870-024-05435-4 Full text.Increased phosphorus uptake with a fulvic acid preparation, and a pH buffer that removed the effect.
  3. Liang Y, Wang Z, Shi Q, et al. Effects of fulvic acid with different molecular weights on growth and nutrient utilization of winter wheat. Journal of Plant Nutrition and Fertilizers 29:308 to 322. 2023. In Chinese. doi.org/10.11674/zwyf.2022307 English abstract only.Winter wheat pots: fulvic fractions raised grain nitrogen and iron.
  4. Kulikova NA, Abroskin DP, Badun GA, et al. Label distribution in tissues of wheat seedlings cultivated with tritium-labeled leonardite humic acid. Scientific Reports 6:28869. 2016. doi.org/10.1038/srep28869Tritium from a labelled humic acid traced mainly in roots, with a small portion in shoots.
  5. Yu X, Yang J, Wang E, Li B, Yuan H. Effects of growth stage and fulvic acid on the diversity and dynamics of endophytic bacterial community in Stevia rebaudiana Bertoni leaves. Frontiers in Microbiology 6:867. 2015. doi.org/10.3389/fmicb.2015.00867 Full text.Endophyte communities in stevia leaves shifted with fulvic acid.
  6. Lengrand S, Dubois B, Pesenti L, Debode F, Legrève A. Humic substances increase tomato tolerance to osmotic stress while modulating vertically transmitted endophytic bacterial communities. Frontiers in Plant Science 15:1488671. 2024. doi.org/10.3389/fpls.2024.1488671 Full text.Endophyte communities in tomato roots shifted with a humic product.
  7. Olaetxea M, et al. Hypothetical framework integrating the main mechanisms involved in the promoting action of rhizospheric humic substances on plant root- and shoot-growth. Applied Soil Ecology 123:521 to 537. 2018. doi.org/10.1016/j.apsoil.2017.06.007 Abstract only.The mechanisms proposed for the plant growth response.

Citing the International Humic Substances Society does not imply its endorsement of Carbon Biome Project. Published 19 October 2026 by Carbon Biome Project. External scientific review of this page has not been completed. Revised 22 October 2026. Cite as: Carbon Biome Project. Organic Matter, Humic and Fulvic. 22 October 2026. carbonbiomeproject.com/science/

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