Humus

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In soil science, humus (derived in 1790-1800 from the Latin humus 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.

Video Humus

Nature (humus)

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 organic matter.

Maps Humus

Humification

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. 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 soil.

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 the soil. Earthworm humus (vermicompost) is considered by some to be the best organic manure there is.

Stability

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.

Horizons

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 letter R.

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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 physiology.
• 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 conditions.
• 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 ecosystem.
• The dark color of humus (usually black or dark brown) helps to warm up cold soils in Spring.

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See also

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References

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External links

• 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 March 2016. 
• IHSS. "What are Humic Substances?". International Humic Substances Society. Retrieved 19 February 2018. 

Source of article : Wikipedia