The terms of the field, defined once.

Dark organic material found in soils, natural waters and organic deposits such as peat and brown coal, defined by how it behaves in an alkaline extraction. It is a mixture of many compounds, not one molecule.^8,9

Example. Whether it exists in soil as a separate natural class is argued over. Lehmann and Kleber argue that it does not, and that the extract is made by the procedure; Olk and colleagues reply that the extracted fractions still track real environmental and agricultural behaviour.^1,9,10

The part of an alkaline extract that settles out as a dark solid when the extract is acidified to about pH 1 to 2.^8,11

Example. In a glass, the brown material that gathers and sinks once acid is added.¹¹

The part that stays dissolved at that pH, purified further on a resin. On average it is more oxygen-rich and more acidic than the humic acid fraction.^11,12,13

Example. IHSS counts 13.24 milliequivalents of carboxyl groups per gram of carbon in its Elliott soil fulvic acid, against 8.28 in the humic acid from the same soil.¹²

The part of the organic matter that alkali never dissolves.^8,11

Example. What is left of the organic matter after both the alkali and the acid steps.⁸

Oxidised lignite.¹⁶

Example. IHSS keeps a leonardite humic acid among its standard samples, beside river, peat and soil samples. It carries 7.46 milliequivalents of carboxyl groups per gram of carbon.^12,14

Brown coal, one of the organic deposits that hold humic substances.⁸

Example. Lignite and peat are both among the sources of the humic materials tested on plants.¹⁵

An organic deposit, one of those that hold humic substances.⁸

Example. IHSS draws one of its standard humic acids from Pahokee peat.^12,14

A specific material made from a source by specific processing steps.¹⁷

Example. Two products sold as humic acid can differ widely in composition, and some differ from their own labels, so a result travels only with the preparation that produced it.¹⁷

The holding and swapping of positively charged nutrients on negatively charged surfaces of clay and organic matter, including the acidic groups of humic material.^12,18

Example. A calcium ion (Ca²⁺) can take the place of two hydrogen ions (H⁺) on a surface, so the total charge stays balanced.¹⁸

A measure of how much negative charge a soil carries, and so how many positive ions it can hold for exchange. It is reported in milliequivalents per 100 grams, the same as centimoles of charge per kilogram.¹⁸

Example. The part contributed by organic matter rises with pH, because more of its acidic groups let go of their hydrogen ions. The part from clay is fixed by the mineral’s structure.¹⁸

A general word for one substance holding on to another. In soil and water it covers several different kinds of association, including attraction between opposite charges, coordination to a metal ion (complexation) and weaker forces such as hydrogen bonding.⁹

Example. Binding does not by itself mean uptake or benefit. Humic material binds some persistent organic pollutants, which makes them less bioavailable to organisms.⁹

The holding of a substance at a surface, rather than throughout a material.^9,18

Example. Water chemists isolate humic and fulvic fractions by passing acidified river water through a resin, where they adsorb. In soil, positive ions are held close to the surfaces of clay.^9,18

The release of an adsorbed substance from a surface back into the water around it. Together with adsorption, it is what makes holding at a surface reversible.^9,18

Example. Humic and fulvic fractions held on a resin are desorbed with an alkaline solution. In a soil test, ammonium displaces the cations held by the soil into the extracting solution, where they are measured.^9,18

The bonding of a metal ion to a ligand, a molecule or ion that supplies a pair of electrons to bond with it, to form a complex. In soils, much of the hold on plant-available metal ions comes from the weakly acidic groups of humic material.^9,20

Example. Iron held by a water-soluble humic fraction from peat: cucumber roots reduced the iron at their surface and took it up, more at pH 6.0 than at 7.5. Being complexed and being taken up are separate steps.²¹

A particular form of complexation in which one binding partner holds a metal ion at more than one site, the way a claw holds a marble.²⁰

Example. Heme holds an iron ion at four sites, and EDTA, used in detergents, holds a metal ion at six. Models of humic material allow a metal ion with two positive charges to be held at one site or at two.^9,20

How much of a substance a living thing can take up and use, in a given setting. It depends on the substance’s form, the conditions and the organism, so it is measured in a named system rather than assumed from being dissolved.^8,9

Example. Binding can lower it as well as raise it: humic material makes some persistent organic pollutants less bioavailable. Iron from one peat humate reached cucumber and barley by different routes.^9,21

The passing of electrons from one substance to another. In respiration, a cell takes electrons from its food and passes them on to an acceptor.^22,24

Example. In laboratory cultures, the iron-reducing bacterium Geobacter metallireducens used humic substances in place of iron as the acceptor. Here humic material takes the electrons; it is not the food.^22,23

Electron transfer by way of a go-between. In laboratory cultures, some bacteria pass electrons from their respiration to humic material, which can pass them on to iron minerals. A different role from supplying carbon.^22,24

Example. Microbes also reduced humic material in solid form, not only dissolved, so the go-between need not be in solution.²⁴

Microbial breakdown of plant fibre and other feed in part of the gut, such as the rumen or the horse’s hindgut.^29,30

The largest compartment of a cattle stomach, a major microbial fermentation environment.³⁰

The caecum and colon. In the horse, where forage is fermented by microbes.²⁹

A blind pouch at the start of the large intestine. In the horse it holds about three times the volume of the stomach.²⁹

A microbe’s share of a sequenced community. It can change while the absolute number does not.^31,32

Example. If every microbe in a sample doubles, each one’s share stays the same; if one group falls, every other group’s share rises without any of them growing.³¹