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 improves (decreases) the bulk density of soil. [8] Humus is amorphous and lacks the cellular structure characteristic of organisms. [9]

The solid residue of sewage sludge treatment, which is a secondary phase in the wastewater treatment process, is also called humus. [10] When not judged contaminated by pathogens, toxic heavy metals, or persistent organic pollutants according to standard tolerance levels, it is sometimes composted and used as a soil amendment. [11]

Description

The primary materials needed for the process of humification are plant detritus and dead animals and microbes, excreta of all soil-dwelling organisms, and also black carbon resulting from past fires. [12] The composition of humus varies with that of primary (plant) materials and secondary microbial and animal products. The decomposition rate of the different compounds will affect the composition of the humus. [13]

It is difficult to define humus precisely because it is a very complex substance which is still 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. [14] 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. [15] This suggests an ambiguous boundary between humus and soil organic matter, leading some authors to contest the use of the term humus and derived terms such as humic substances or humification, proposing the Soil Continuum Model (SCM). [16] However, humus can be considered as having distinct properties, mostly linked to its richness in functional groups, justifying its maintenance as a specific term. [17]

Fully formed humus is essentially a collection of very large and complex molecules formed in part from lignin and other polyphenolic molecules of the original plant material (foliage, wood, bark), in part from similar molecules that have been produced by microbes. [18] During decomposition processes these polyphenols are modified chemically so that they are able to join up with one another to form very large molecules. Some parts of these molecules are modified in such a way that protein molecules, amino acids, and amino sugars are able to attach themselves to the polyphenol “base” molecule. As protein contains both nitrogen and sulfur, this attachment gives humus a moderate content of these two important plant nutrients. [19]

Radiocarbon and other dating techniques have shown that the polyphenolic base of humus (mostly lignin and black carbon) can be very old, but the protein and carbohydrate attachments much younger, while to the light of modern concepts and methods the situation appears much more complex and unpredictable than previously thought. [20] It seems that microbes are able to pull protein off humus molecules rather more readily than they are able to break the polyphenolic base molecule itself. As protein is removed its place may be taken by younger protein, or this younger protein may attach itself to another part of the humus molecule. [21]

The most useful functions of humus are in improving soil structure, all the more when associated with cations (e.g. calcium), [22] and in providing a very large surface area that can hold nutrient elements until required by plants, an ion exchange function comparable to that of clay particles. [23]

Soil carbon sequestration is a major property of the soil, also considered as an ecosystem service. [24] Only when it becomes stable and acquires its multi-century permanence, mostly via multiple interactions with the soil matrix, molecular soil humus should be considered to be of significance in removing the atmosphere's current carbon dioxide overload. [25]

There is little data available on the composition of 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 and agricultural soils, but this proved impossible because extractants interacted with the analysed organic matter and created many artefacts. [26] Further research has been done in more recent years, though it remains an active field of study. [27]

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 polymers resistant to the action of microorganisms, [28] however recent research has demonstrated that microorganisms are capable of digesting humus. [29]

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 (see Soil biology). 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. Simple proteins, organic acids, starches, and sugars decompose rapidly, while crude proteins, fats, waxes, and resins remain relatively unchanged for longer periods of time. [30]

Lignin, which is quickly transformed by white-rot fungi, [31] is one of the primary precursors of humus, [32] together with by-products of microbial [33] and animal [34] activity. The humus produced by humification is thus a mixture of compounds and complex biological chemicals of plant, animal, and microbial origin that has many functions and benefits in soil. [18] Some judge earthworm humus (vermicompost) to be the optimal organic manure. [35]

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. [36] Most humus that is not protected in this way is decomposed within 10 years and can be regarded as less stable or more labile. [37] The mixing activity of soil-consuming invertebrates (e.g. earthworms, termites, some millipedes) contribute to the stability of humus by favouring the formation of organo-mineral complexes with clay at the inside of their guts, [38] [39] hence more carbon sequestration in humus forms such as mull and amphi, with well-developed mineral-organic horizons, when compared with moder where most organic matter accumulates at the soil surface. [40]

Stable humus contributes few plant-available nutrients in soil, but it helps maintain its physical structure. [41] 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, suggested to result from the grinding and mixing activity of a tropical earthworm. [42] This process is speculated to have been important in the formation of the unusually fertile Amazonian terra preta do Indio . [43] However, some authors [16] suggest 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 soil 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. [44] The master horizon (E) is used for subsurface horizons that have significantly lost minerals (eluviation). Bedrock, which is not soil, uses the letter R. The richness of soil horizons in humus determines their more or less dark color, generally decreasing from O to E, to the exception of deep horizons of podzolic soils enriched with colloidal humic substances which have been leached down the soil profile. [45]

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, [46] though some agricultural experts put a greater focus on other features of it, such as its ability to suppress disease. [47] It helps the soil retain moisture [48] by increasing microporosity [49] and encourages the formation of good soil structure. [50] [51] 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. [52] Humus allows soil organisms to feed and reproduce and is often described as the "life-force" of the soil. [53] [54]

See also

Related Research Articles

<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, 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.

<span class="mw-page-title-main">Soil pH</span> Measure of how acidic or alkaline the soil is

Soil pH is a measure of the acidity or basicity (alkalinity) of a soil. Soil pH is a key characteristic that can be used to make informative analysis both qualitative and quantitatively regarding soil characteristics. pH is defined as the negative logarithm (base 10) of the activity of hydronium ions in a solution. In soils, it is measured in a slurry of soil mixed with water, and normally falls between 3 and 10, with 7 being neutral. Acid soils have a pH below 7 and alkaline soils have a pH above 7. Ultra-acidic soils and very strongly alkaline soils are rare.

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 organic matter and release the nutrients into the environment around them. Decomposition is a chemical process similar to digestion, and many sources use the words digestion and decomposition interchangeably. In both processes, complex molecules are chemically broken down by enzymes into simpler, smaller ones. The term "digestion," however, is most commonly used to refer to food breakdown that occurs within animal bodies, and results in the absorption of nutrients from the gut into the animal's bloodstream. Decomposition happens outside of an organism's body, in the environment. Decomposition is also referred to as external digestion; the decomposer works not by swallowing the dead tissue and then digesting it, but by releasing enzymes directly onto it. After allowing the enzymes time to digest the material, the decomposer then absorbs the nutrients released by the chemical reaction into its cells.

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

Humic substances (HS) are colored relatively recalcitrant organic compounds naturally formed during long-term decomposition and transformation of biomass residues. The color of humic substances varies from bright yellow to light or dark brown leading to black. The term comes from humus, which in turn comes from the Latin word humus, meaning "soil, earth". 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. Humic substances account for 50 – 90% of cation exchange capacity in soils.

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

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

Terra preta, also known as Amazonian dark earth or Indian black earth, is a type of very dark, fertile anthropogenic soil (anthrosol) found in the Amazon Basin. In Portuguese its full name is terra preta do índio or terra preta de índio. Terra mulata is lighter or brownish in color.

<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.

In soil science, mineralization 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.

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 soil's physical and chemical properties and its capacity to provide regulatory ecosystem services. SOM is especially critical for soil functions and quality.

<span class="mw-page-title-main">Forest floor</span> Part of the forest ecosystem

The forest floor, also called detritus or duff, is the part of a forest ecosystem that mediates between the living, aboveground portion of the forest and the mineral soil, principally composed of dead and decaying plant matter such as rotting wood and shed leaves. In some countries, like Canada, forest floor refers to L, F and H organic horizons. It hosts a wide variety of decomposers and predators, including invertebrates, fungi, algae, bacteria, and archaea.

<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.

Seventeen elements or nutrients are essential for plant growth and reproduction. They are carbon (C), hydrogen (H), oxygen (O), nitrogen (N), phosphorus (P), potassium (K), sulfur (S), calcium (Ca), magnesium (Mg), iron (Fe), boron (B), manganese (Mn), copper (Cu), zinc (Zn), molybdenum (Mo), nickel (Ni) and chlorine (Cl). Nutrients required for plants to complete their life cycle are considered essential nutrients. Nutrients that enhance the growth of plants but are not necessary to complete the plant's life cycle are considered non-essential, although some of them, such as silicon (Si), have been shown to improve nutrent availability, hence the use of stinging nettle and horsetail macerations in Biodynamic agriculture. With the exception of carbon, hydrogen and oxygen, which are supplied by carbon dioxide and water, and nitrogen, provided through nitrogen fixation, the nutrients derive originally from the mineral component of the soil. The Law of the Minimum expresses that when the available form of a nutrient is not in enough proportion in the soil solution, then other nutrients cannot be taken up at an optimum rate by a plant. A particular nutrient ratio of the soil solution is thus mandatory for optimizing plant growth, a value which might differ from nutrient ratios calculated from plant composition.

<span class="mw-page-title-main">Mor humus</span> Humus formed in coniferous forests

Mor humus is a form of forest floor humus occurring mostly in coniferous forests. Mor humus consists of evergreen needles and woody debris that litter the forest floor. This litter is slow to decompose, in part due to their chemical composition, but also because of the generally cool and wet conditions where mor humus is found. This results in low bacterial activity and an absence of earthworms and other soil fauna. Because of this, most of the organic matter decomposition in mor humus is carried out by fungi.

Moder is a forest floor type formed under mixed-wood and pure deciduous forests. Moder is a kind of humus whose properties are the transition between mor humus and mull humus types. Moders are similar to mors as they are made up of partially to fully humified organic components accumulated on the mineral soil. Compared to mulls, moders are zoologically active. In addition, moders present as in the middle of mors and mulls with a higher decomposition capacity than mull but lower than mor. Moders are characterized by a slow rate of litter decomposition by litter-dwelling organisms and fungi, leading to the accumulation of organic residues. Moder humus forms share the features of the mull and mor humus forms.

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

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