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 sense,
[7][8][9] though some agricultural experts put a greater focus on other features of it, such as its ability to suppress disease.
[10] It helps the soil retain
moisture[11] by increasing
microporosity,
[12] and encourages the formation of good
soil structure.
[13][14] 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.
[15] Humus allows soil organisms to feed and reproduce, and is often described as the "life-force" of the soil.
[16][17]
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. The latter is rough-looking material
[8][9] and remains of the original plant are still visible. Fully humified organic matter, on the other hand, has a uniform dark, spongy, jelly-like appearance, and is amorphous. It may remain like this for millennia or more.
[18] 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.
[19] This suggests a fuzzy boundary between humus and organic matter. In most literature, humus is considered an integral part of
soil organic matter.
[20]
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.
[21] 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,
[22] is one of the main precursors of humus,
[23] together with by-products of microbial
[24] and animal
[25] 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 the soil.
Earthworm humus (
vermicompost) is considered by some to be the best organic
manure there is.
[26]