permaculture

[Lawrence London <lfljvenaura@gmail.com>]

Humus - Wikipedia, the free encyclopedia
http://en.wikipedia.org/wiki/Humus

Humus
>From Wikipedia, the free encyclopedia
This article is about the organic matter in soil. For the band, see
Humus (band). For the food, see Hummus.
Humus has a characteristic black or dark brown color, due to an
accumulation of organic carbon

In soil science, humus (coined 1790–1800; < Latin: earth, ground[1])
refers to any organic matter that has reached a point of stability,
where it will break down no further and might, if conditions do not
change, remain as it is for centuries, if not millennia.[2]

In agriculture, humus is sometimes also used to describe mature
compost, or natural compost extracted from a forest or other
spontaneous source for use to amend soil. It is also used to describe
a topsoil horizon that contains organic matter (humus type,[3] humus
form,[4] humus profile).[5]

Contents

    1 Humification
        1.1 Transformation of organic matter into humus
        1.2 Stability of humus
    2 Benefits of soil organic matter and humus
    3 See also
    4 References

[edit] Humification
[edit] Transformation of organic matter into humus

The process of "humification" can occur naturally in soil, or in the
production of compost. The importance of chemically stable humus is
thought by some to be the fertility it provides to soils in both a
physical and chemical senses,[6] though some agricultural experts put
a greater focus on other features of it, such as its ability to
suppress disease.[7] It helps the soil retain moisture[8] by
increasing microporosity,[9] and encourages the formation of good soil
structure.[10][11] 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 by ion exchange.[12] Humus allows soil
organisms to feed and reproduce, and is often described as the
"life-force" of the soil.[13][14] Yet, it is difficult to define humus
precisely; it is a highly complex substance, which is still not fully
understood. Humus should be differentiated from decomposing organic
matter in that the latter is rough-looking material, with the original
plant remains still visible, whereas fully humified organic matter is
uniform in appearance (a dark, spongy, jelly-like substance) and
amorphous in structure, and may remain such for millennia or more.[15]
It has no determinate shape, structure or character. However, humified
organic matter, when examined under the microscope may reveal tiny
plant, animal or microbial remains that have been mechanically, but
not chemically, degraded.[16] This suggests a fuzzy boundary between
humus and organic matter. In most literature, humus is clearly
considered as an integral part of soil organic matter.[17]

Plant remains (including those that passed through an animal gut and
were excreted as feces) 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.[18]
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,[19] is one of the main
precursors of humus,[20] together with by-products of microbial[21]
and animal[22] 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, which has many functions and benefits in
the soil. Earthworm humus (vermicompost) is considered by some to be
the best organic manure there is.[23]
[edit] Stability of humus

Compost that is readily capable of further decomposition is sometimes
referred to as effective or active humus, though scientists would say
that, if it is not stable, it is not humus at all. This kind of
compost, rich in plant remains and fulvic acids, is an excellent
source of plant nutrients, but of little value with respect to
long-term soil structure and tilth. Stable (or passive) humus consists
of humic acids and humins, which are so highly insoluble, or so
tightly bound to clay particles and hydroxides, that they cannot be
penetrated by microbes and are greatly resistant to further
decomposition. Thus stable humus adds few readily available nutrients
to the soil, but plays an essential part in providing its physical
structure. Some very stable humus complexes have survived for
thousands of years.[24] The most stable humus is that formed from the
slow oxidation of black carbon, after the incorporation of finely
powdered charcoal into the topsoil. This process is at the origin of
the formation of the fertile Amazonian dark earths or Terra preta do
Indio.[25]
[edit] 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.[26][27]
    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
physiology.[28]
    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.[29]
    Humus can hold the equivalent of 80–90% of its weight in moisture,
and therefore increases the soil's capacity to withstand drought
conditions.[30][31]
    The biochemical structure of humus enables it to moderate – or
buffer – excessive acid or alkaline soil conditions.[32]
    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.[33] 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
ecosystem.[34]
    The dark color of humus (usually black or dark brown) helps to
warm up cold soils in the spring.

[edit] See also

    Biomass
    Biotic material
    Detritus
    Glomalin
    Humic acid
    Organic matter
    Plant litter

[edit] References

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Categories:

    Composting
    Soil improvers
    Soil science
    Organic gardening
    Organic farming
    Sustainable agriculture