In soil science, humus (derived in 1790–1800 from the
for earth, ground) denominates the fraction of soil organic matter
that is amorphous and without the "cellular cake structure
characteristic of plants, micro-organisms or animals." Humus
significantly affects the bulk density of soil and contributes to its
retention of moisture and nutrients.
In agriculture, "humus" sometimes also is used to describe mature or
natural compost extracted from a woodland or other spontaneous source
for use as a soil conditioner. It is also used to describe a
topsoil horizon that contains organic matter (humus type, humus
form, humus profile).
Humus is the dark organic matter that forms in soil when dead plant
and animal matter decays.
Humus has many nutrients that improve the
health of soil, nitrogen being the most important. The ratio of carbon
to nitrogen (C:N) of humus is 10:1.
1 Nature (humus)
2.1 Transformation of organic matter into humus
3 Benefits of soil organic matter and humus
4 See also
6 External links
A great part of the organic material that contacts soil is decomposed
by microorganisms, resulting in mineral components that the roots of
plants can absorb. In this way, nitrogen (nitrogen cycle) and the
other nutrients (nutrient cycle) are recycled. This process is
denominated "mineralization". Contingent on the conditions in which
the decomposition occurs, a fraction of the organic matter does not
mineralize, and instead is transformed in the contrary direction into
new organic chains or "polymers". Because these organic polymers are
resistant to the action of microorganisms, they are stable, and
constitute humus. This stability implies that humus integrates into
the permanent structure of soil, thereby improving it.
It is difficult to define humus precisely because it is a very complex
substance which is 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, jelly-like
appearance, and is amorphous; it may remain so for millennia. It
has no determinate shape, structure, or quality. However, when
examined under the microscope, humus may reveal tiny plant, animal, or
microbial remains that have been mechanically, but not chemically,
degraded. This suggests an ambiguous boundary between humus and
soil organic matter. While distinct, humus is an integral part of soil
Transformation of organic matter into humus
The process of humification can occur naturally in soil, or in the
production of compost.
Organic matter is degraded into humus by a
combination of saprotrophic fungi, bacteria, microbes and animals such
as earthworms, nematodes, protozoa and various
arthropods.[better source needed] The importance of
chemically stable humus is thought by some to be the fertility it
provides to soils in both a physical and chemical sense, though
some agricultural experts put a greater focus on other features of it,
such as its ability to suppress disease. It helps the soil retain
moisture by increasing microporosity, and encourages the
formation of good soil structure. The incorporation of oxygen
into large organic molecular assemblages generates many active,
negatively charged sites that bind to positively charged ions
(cations) of plant nutrients, making them more available to the plant
by way of ion exchange.
Humus allows soil organisms to feed and
reproduce, and is often described as the "life-force" of the
Plant remains (including those that passed through an animal gut and
were excreted as faeces) contain organic compounds: sugars, starches,
proteins, carbohydrates, lignins, waxes, resins, and organic acids.
The process of organic matter decay in the soil begins with the
decomposition of sugars and starches from carbohydrates, which break
down easily as detritivores initially invade the dead plant organs,
while the remaining cellulose and lignin break down more slowly.
Simple proteins, organic acids, starches and sugars break down
rapidly, while crude proteins, fats, waxes and resins remain
relatively unchanged for longer periods of time. Lignin, which is
quickly transformed by white-rot fungi, is one of the main
precursors of humus, together with by-products of microbial
and animal activity. The end-product of this process, the humus,
is thus a mixture of compounds and complex life chemicals of plant,
animal, or microbial origin that has many functions and benefits in
Earthworm humus (vermicompost) is considered by some to be
the best organic manure there is.
Much of the humus in most soils has persisted for more than a hundred
years (rather than having been decomposed to CO2), and can be regarded
as stable; this is organic matter that has been protected from
decomposition by microbial or enzyme action because it is hidden
(occluded) inside small aggregates of soil particles or tightly
attached (sorbed or complexed) to clays. Most humus that is not
protected in this way is decomposed within ten years and can be
regarded as less stable or more labile. Thus stable humus contributes
little to the pool of plant-available nutrients in the soil, but it
does play a part in maintaining its physical structure. A very
stable form of humus is formed from the slow oxidation of soil carbon,
after the incorporation of finely powdered charcoal into the topsoil.
This process is thought to have been important in the formation of the
fertile Amazonian dark earths or
Terra preta do Indio.
Humus has a characteristic black or dark brown color and is organic
due to an accumulation of organic carbon.
Soil scientists use the
capital letters O, A, B, C, and E to identify the master horizons, and
lowercase letters for distinctions of these horizons. Most soils have
three major horizons—the surface horizon (A), the subsoil (B), and
the substratum (C). Some soils have an organic horizon (O) on the
surface, but this horizon can also be buried. The master horizon, E,
is used for subsurface horizons that have a significant loss of
minerals (eluviation). Hard bedrock, which is not soil, uses the
Benefits of soil organic matter and humus
The process that converts raw organic matter into humus feeds the soil
population of microorganisms and other creatures, thus maintains high
and healthy levels of soil life.
The rate at which raw organic matter is converted into humus promotes
(when fast) or limits (when slow) the coexistence of plants, animals,
and microbes in soil.
Effective humus and stable humus are further sources of nutrients to
microbes, the former provides a readily available supply, and the
latter acts as a longer-term storage reservoir.
Decomposition of dead plant material causes complex organic compounds
to be slowly oxidized (lignin-like humus) or to break down into
simpler forms (sugars and amino sugars, aliphatic, and phenolic
organic acids), which are further transformed into microbial biomass
(microbial humus) or are reorganized, and further oxidized, into humic
assemblages (fulvic and humic acids), which bind to clay minerals and
metal hydroxides. There has been a long debate about the ability of
plants to uptake humic substances from their root systems and to
metabolize them. There is now a consensus about how humus plays a
hormonal role rather than simply a nutritional role in plant
Humus is a colloidal substance, and increases the soil's cation
exchange capacity, hence its ability to store nutrients by chelation.
While these nutrient cations are accessible to plants, they are held
in the soil safe from being leached by rain or irrigation.
Humus can hold the equivalent of 80–90% of its weight in moisture,
and therefore increases the soil's capacity to withstand drought
The biochemical structure of humus enables it to moderate – or
buffer – excessive acid or alkaline soil conditions.
During the humification process, microbes secrete sticky gum-like
mucilages; these contribute to the crumb structure (tilth) of the soil
by holding particles together, and allowing greater aeration of the
soil. Toxic substances such as heavy metals, as well as excess
nutrients, can be chelated (that is, bound to the complex organic
molecules of humus) and so prevented from entering the wider
The dark color of humus (usually black or dark brown) helps to warm up
cold soils in Spring.
Humus diagnostic horizon
Soil organic matter
Mineralization (soil science)
Mycorrhizal fungi and soil carbon storage
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Find more aboutHumusat's sister projects
Definitions from Wiktionary
Quotations from Wikiquote
Jerzy Weber. "Types of humus in soils". Agricultural University of
Wroclaw, Poland. Retrieved 2013-12-12.
Wershaw, R.L. "Evaluation of conceptual models of natural organic
matter (humus) from a consideration of the chemical and biochemical
processes of humification" (PDF). pubs.usgs.gov. USGS. Retrieved 14
IHSS. "What are Humic Substances?". International Humic Substances
Society. Retrieved 19 Fe