Humus

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Humus has a characteristic black or dark brown color and is an accumulation of organic carbon. Besides the three major soil horizons of (A) surface/topsoil, (B) subsoil, and (C) substratum, some soils have an organic horizon (O) on the very surface. Hard bedrock (R) is not in a strict sense soil. Soil Horizons.svg
Humus has a characteristic black or dark brown color and is an accumulation of organic carbon. Besides the three major soil horizons of (A) surface/topsoil, (B) subsoil, and (C) substratum, some soils have an organic horizon (O) on the very surface. Hard bedrock (R) is not in a strict sense soil.

In classical [1] soil science, humus is the dark organic matter in soil that is formed by the decomposition of plant and animal matter. It is a kind of soil organic matter. It is rich in nutrients and retains moisture in the soil. Humus is the Latin word for "earth" or "ground". [2]

Contents

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. [3] It is also used to describe a topsoil horizon that contains organic matter (humus type, [4] humus form, [5] or humus profile [6] ).

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 commonly ranges between 8:1 and 15:1 with the median being about 12:1. [7] It also significantly affects the bulk density of soil. Humus is amorphous and lacks the cellular structure characteristic of plants, microorganisms or animals. [8]

Description

The primary materials needed for the process of humification are plant materials. The composition of humus varies dependent on the composition of the primary materials and the secondary microbial and animal products. The decomposition rate of the different compounds will affect the composition of the humus. [9]

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, and jelly-like appearance, and is amorphous; it may gradually decay over several years or persist for millennia. [10] It has no determinate shape, structure, or quality. However, when examined under a microscope, humus may reveal tiny plant, animal, or microbial remains that have been mechanically, but not chemically, degraded. [11] This suggests an ambiguous boundary between humus and soil organic matter. While distinct, humus is an integral part of soil organic matter. [12]


There is little data available on the composition of forest humus because it is a complex mixture that is challenging for researchers to analyze. Researchers in the 1940s and 1960s tried using chemical separation to analyze plant and humic compounds in forest soil, but this proved impossible. Further research has been done in more recent years, though it remains an active field of study. [13] [14] [15]

Humification

Microorganisms decompose a large portion of the soil organic matter into inorganic minerals that the roots of plants can absorb as nutrients. This process is termed "mineralization". In this process, nitrogen (nitrogen cycle) and the other nutrients (nutrient cycle) in the decomposed organic matter are recycled. Depending on the conditions in which the decomposition occurs, a fraction of the organic matter does not mineralize and instead is transformed by a process called "humification". Prior to modern analytical methods, early evidence led scientists to believe that humification resulted in concatenations of organic polymer resistant to the action of microorganisms, [16] however recent research has demonstrated that microorganisms are capable of digesting humus. [17]

Humification can occur naturally in soil or artificially in the production of compost. Organic matter is humified by a combination of saprotrophic fungi, bacteria, microbes and animals such as earthworms, nematodes, protozoa, and arthropods. [18] [ circular reference ] Plant remains, including those that animals digested and excreted, contain organic compounds: sugars, starches, proteins, carbohydrates, lignins, waxes, resins, and organic acids. Decay in the soil begins with the decomposition of sugars and starches from carbohydrates, which decompose easily as detritivores initially invade the dead plant organs, while the remaining cellulose and lignin decompose more slowly. [19] [ page needed ] Simple proteins, organic acids, starches, and sugars decompose 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, [20] is one of the primary precursors of humus, [21] together with by-products of microbial [22] and animal [23] activity. The humus produced by humification is thus a mixture of compounds and complex biological chemicals of plant, animal, or microbial origin that has many functions and benefits in soil. Some judge earthworm humus (vermicompost) to be the optimal organic manure. [24]

Stability

Much of the humus in most soils has persisted for more than 100 years, rather than having been decomposed into CO2, and can be regarded as stable; this organic matter has been protected from decomposition by microbial or enzyme action because it is hidden (occluded) inside small aggregates of soil particles, or tightly sorbed or complexed to clays. [25] Most humus that is not protected in this way is decomposed within 10 years and can be regarded as less stable or more labile.

Stable humus contributes few plant-available nutrients in soil, but it helps maintain its physical structure. [26] A very stable form of humus is formed from the slow oxidation (redox) of soil carbon after the incorporation of finely powdered charcoal into the topsoil. This process is speculated to have been important in the formation of the unusually fertile Amazonian terra preta do Indio . [27] [ page needed ] However, recent work [28] suggests that complex soil organic molecules may be much less stable than previously thought: “the available evidence does not support the formation of large-molecular-size and persistent ‘humic substances’ in soils. Instead, soil organic matter is a continuum of progressively decomposing organic compounds.″

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 significantly lost minerals (eluviation). Bedrock, which is not soil, uses the letter R.

Benefits of soil organic matter and humus

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, [29] though some agricultural experts put a greater focus on other features of it, such as its ability to suppress disease. [30] It helps the soil retain moisture [31] by increasing microporosity [32] and encourages the formation of good soil structure. [33] [34] 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. [35] Humus allows soil organisms to feed and reproduce and is often described as the "life-force" of the soil. [36] [37]

See also

Related Research Articles

<span class="mw-page-title-main">Compost</span> Mixture used to improve soil fertility

Compost is a mixture of ingredients used as plant fertilizer and to improve soil's physical, chemical, and biological properties. It is commonly prepared by decomposing plant and food waste, recycling organic materials, and manure. The resulting mixture is rich in plant nutrients and beneficial organisms, such as bacteria, protozoa, nematodes, and fungi. Compost improves soil fertility in gardens, landscaping, horticulture, urban agriculture, and organic farming, reducing dependency on commercial chemical fertilizers. The benefits of compost include providing nutrients to crops as fertilizer, acting as a soil conditioner, increasing the humus or humic acid contents of the soil, and introducing beneficial microbes that help to suppress pathogens in the soil and reduce soil-borne diseases.

<span class="mw-page-title-main">Soil</span> Mixture of organic matter, minerals, gases, liquids, and organisms that together support life

Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support the life of plants and soil organisms. Some scientific definitions distinguish dirt from soil by restricting the former term specifically to displaced soil.

Soil formation, also known as pedogenesis, is the process of soil genesis as regulated by the effects of place, environment, and history. Biogeochemical processes act to both create and destroy order (anisotropy) within soils. These alterations lead to the development of layers, termed soil horizons, distinguished by differences in color, structure, texture, and chemistry. These features occur in patterns of soil type distribution, forming in response to differences in soil forming factors.

<span class="mw-page-title-main">Decomposition</span> Process in which organic substances are broken down into simpler organic matter

Decomposition or rot is the process by which dead organic substances are broken down into simpler organic or inorganic matter such as carbon dioxide, water, simple sugars and mineral salts. The process is a part of the nutrient cycle and is essential for recycling the finite matter that occupies physical space in the biosphere. Bodies of living organisms begin to decompose shortly after death. Animals, such as earthworms, also help decompose the organic materials. Organisms that do this are known as decomposers or detritivores. Although no two organisms decompose in the same way, they all undergo the same sequential stages of decomposition. The science which studies decomposition is generally referred to as taphonomy from the Greek word taphos, meaning tomb. Decomposition can also be a gradual process for organisms that have extended periods of dormancy.

<span class="mw-page-title-main">Decomposer</span> Organism that breaks down dead or decaying organisms

Decomposers are organisms that break down dead or decaying organisms; they carry out decomposition, a process possible by only certain kingdoms, such as fungi. Like herbivores and predators, decomposers are heterotrophic, meaning that they use organic substrates to get their energy, carbon and nutrients for growth and development. While the terms decomposer and detritivore are often interchangeably used, detritivores ingest and digest dead matter internally, while decomposers directly absorb nutrients through external chemical and biological processes. Thus, invertebrates such as earthworms, woodlice, and sea cucumbers are technically detritivores, not decomposers, since they are unable to absorb nutrients without ingesting them.

<span class="mw-page-title-main">Humic substance</span> Major component of natural organic matter

Humic substances (HS) are coloured recalcitrant organic compounds naturally formed during long-term decomposition and transformation of biomass residues. The colour of humic substances varies from yellow to brown to black. Humic substances represent the major part of organic matter in soil, peat, coal and sediments and are important components of dissolved natural organic matter (NOM) in lakes, rivers and sea water.

Organic matter, organic material, or natural organic matter refers to the large source of carbon-based compounds found within natural and engineered, terrestrial, and aquatic environments. It is matter composed of organic compounds that have come from the feces and remains of organisms such as plants and animals. Organic molecules can also be made by chemical reactions that do not involve life. Basic structures are created from cellulose, tannin, cutin, and lignin, along with other various proteins, lipids, and carbohydrates. Organic matter is very important in the movement of nutrients in the environment and plays a role in water retention on the surface of the planet.

<span class="mw-page-title-main">Podzol</span> Typical soils of coniferous or boreal forests

In soil science, podzols are the typical soils of coniferous or boreal forests and also the typical soils of eucalypt forests and heathlands in southern Australia. In Western Europe, podzols develop on heathland, which is often a construct of human interference through grazing and burning. In some British moorlands with podzolic soils, cambisols are preserved under Bronze Age barrows.

<i>Terra preta</i> Very dark, fertile Amazonian anthropogenic soil

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<span class="mw-page-title-main">Soil biology</span> Study of living things in soil

Soil biology is the study of microbial and faunal activity and ecology in soil. Soil life, soil biota, soil fauna, or edaphon is a collective term that encompasses all organisms that spend a significant portion of their life cycle within a soil profile, or at the soil-litter interface. These organisms include earthworms, nematodes, protozoa, fungi, bacteria, different arthropods, as well as some reptiles, and species of burrowing mammals like gophers, moles and prairie dogs. Soil biology plays a vital role in determining many soil characteristics. The decomposition of organic matter by soil organisms has an immense influence on soil fertility, plant growth, soil structure, and carbon storage. As a relatively new science, much remains unknown about soil biology and its effect on soil ecosystems.

Soil chemistry is the study of the chemical characteristics of soil. Soil chemistry is affected by mineral composition, organic matter and environmental factors. In the early 1870s a consulting chemist to the Royal Agricultural Society in England, named J. Thomas Way, performed many experiments on how soils exchange ions, and is considered the father of soil chemistry. Other scientists who contributed to this branch of ecology include Edmund Ruffin, and Linus Pauling.

Mineralization in soil science is the decomposition of the chemical compounds in organic matter, by which the nutrients in those compounds are released in soluble inorganic forms that may be available to plants. Mineralization is the opposite of immobilization.

<span class="mw-page-title-main">Phosphorus cycle</span> Biogeochemical movement

The phosphorus cycle is the biogeochemical cycle that describes the movement of phosphorus through the lithosphere, hydrosphere, and biosphere. Unlike many other biogeochemical cycles, the atmosphere does not play a significant role in the movement of phosphorus, because phosphorus and phosphorus-based compounds are usually solids at the typical ranges of temperature and pressure found on Earth. The production of phosphine gas occurs in only specialized, local conditions. Therefore, the phosphorus cycle should be viewed from whole Earth system and then specifically focused on the cycle in terrestrial and aquatic systems.

Soil organic matter (SOM) is the organic matter component of soil, consisting of plant and animal detritus at various stages of decomposition, cells and tissues of soil microbes, and substances that soil microbes synthesize. SOM provides numerous benefits to the physical and chemical properties of soil and its capacity to provide regulatory ecosystem services. SOM is especially critical for soil functions and quality.

Soil biodiversity refers to the relationship of soil to biodiversity and to aspects of the soil that can be managed in relative to biodiversity. Soil biodiversity relates to some catchment management considerations.

<span class="mw-page-title-main">Plant litter</span> Dead plant material that has fallen to the ground

Plant litter is dead plant material that have fallen to the ground. This detritus or dead organic material and its constituent nutrients are added to the top layer of soil, commonly known as the litter layer or O horizon. Litter is an important factor in ecosystem dynamics, as it is indicative of ecological productivity and may be useful in predicting regional nutrient cycling and soil fertility.

<span class="mw-page-title-main">Fungal extracellular enzyme activity</span> Enzymes produced by fungi and secreted outside their cells

Extracellular enzymes or exoenzymes are synthesized inside the cell and then secreted outside the cell, where their function is to break down complex macromolecules into smaller units to be taken up by the cell for growth and assimilation. These enzymes degrade complex organic matter such as cellulose and hemicellulose into simple sugars that enzyme-producing organisms use as a source of carbon, energy, and nutrients. Grouped as hydrolases, lyases, oxidoreductases and transferases, these extracellular enzymes control soil enzyme activity through efficient degradation of biopolymers.

<span class="mw-page-title-main">Particulate organic matter</span>

Particulate organic matter (POM) is a fraction of total organic matter operationally defined as that which does not pass through a filter pore size that typically ranges in size from 0.053 millimeters (53 μm) to 2 millimeters.

The term humus form is not the same as the term humus. Forest humus form describes the various arrangement of organic and mineral horizons at the top of soil profiles. It can be composed entirely of organic horizons, meaning an absence of the mineral horizon. Experts worldwide have developed different types of classifications over time, and humus forms are mainly categorized into mull, mor, and moder orders in the ecosystems of British Columbia. Mull humus form is distinguishable from the other two forms in formation, nutrient cycling, productivity, etc.

References

  1. ↑ Popkin, Gabriel (27 July 2021), A Soil-Science Revolution Upends Plans to Fight Climate Change, Quanta Magazine, "The latest edition of The Nature and Properties of Soils, published in 2016, cites Lehmann's 2015 paper and acknowledges that "our understanding of the nature and genesis of soil humus has advanced greatly since the turn of the century, requiring that some long-accepted concepts be revised or abandoned."
  2. ↑ "Humus" . Retrieved 23 September 2008– via Dictionary.com Random House Dictionary Unabridged.
  3. ↑ "Humus". Encyclopaedia Britannica Online. 2011. Retrieved 24 November 2011.
  4. ↑ Chertov, O. G.; Kornarov, A. S.; Crocker, G.; Grace, P.; Klir, J.; Körschens, M.; Poulton, P. R.; Richter, D. (1997). "Simulating trends of soil organic carbon in seven long-term experiments using the SOMM model of the humus types". Geoderma. 81 (1–2): 121–135. Bibcode:1997Geode..81..121C. doi:10.1016/S0016-7061(97)00085-2.
  5. ↑ Baritz, R. (2003). Humus Forms in Forests of the Northern German Lowlands. Stuttgart: Schweizerbart.
  6. ↑ Bunting, B. T.; Lundberg, J. (1995). "The humus profile-concept, class and reality". Geoderma. 40 (1–2): 17–36. Bibcode:1987Geode..40...17B. doi:10.1016/0016-7061(87)90011-5.
  7. ↑ Weil, Ray R.; Brady, Nyle C. (2017). The Nature and Properties of Soils (15th ed.). Columbus, Ohio: Pearson Education (published April 2017). p. 536. ISBN   978-0-13-325448-8. LCCN   2016008568. OCLC   936004363.
  8. ↑ Whitehead, D. C.; Tinsley, J. (1963). "The biochemistry of humus formation". Journal of the Science of Food and Agriculture. 14 (12): 849–857. Bibcode:1963JSFA...14..849W. doi:10.1002/jsfa.2740141201.
  9. ↑ Kögel-Knabner, Ingrid; Zech, Wolfgang; Hatcher, Patrick G. (1988). "Chemical composition of the organic matter in forest soils: The humus layer". Zeitschrift für Pflanzenernährung und Bodenkunde (in German). 151 (5): 331–340. doi:10.1002/jpln.19881510512.
  10. ↑ Di Giovanni, C.; Disnar, J. R.; Bichet, V.; Campy, M. (1998). "Sur la présence de matières organiques mésocénozoïques dans des humus actuels (bassin de Chaillexon, Doubs, France)". Comptes Rendus de l'Académie des Sciences, Série IIA (in French). 326 (8): 553–559. Bibcode:1998CRASE.326..553D. doi:10.1016/S1251-8050(98)80206-1.
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  16. ↑ Weil, Ray R.; Brady, Nyle C. (2017). The Nature and Properties of Soils (15th ed.). Columbus, Ohio: Pearson Education (published April 2017). p. 549. ISBN   978-0-13-325448-8. LCCN   2016008568. OCLC   936004363. It is now thought that humic substances in soil extracts do not represent the nature of most of the organic matter as it exists in soil.
  17. ↑ Popkin, G. (2021). "A Soil-Science Revolution Upends Plans to Fight Climate Change". Quanta magazine. Soil researchers have concluded that even the largest, most complex molecules can be quickly devoured by soil's abundant and voracious microbes.
  18. ↑ Soil biology
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