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. 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.
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In classical
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 Soil horizon that contains organic matter ( humus type, humus form, or humus profile).
Humus has many that improve the health of soil, nitrogen being the most important. The ratio of carbon to nitrogen () of humus commonly ranges between 8:1 and 15:1 with the median being about 12:1. It also significantly improves (decreases) the bulk density of soil. Humus is Amorphous solid and lacks the cellular structure characteristic of .
The Biosolids residue of sewage sludge treatment, which is a secondary phase in the wastewater treatment process, is also called humus. When not judged Contamination by , toxic heavy metals, or persistent organic pollutants according to standard tolerance levels, it is sometimes and used as a fertilizer.
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. 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. 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 or humification, proposing the Soil Continuum Model (SCM). However, humus can be considered as having distinct properties, mostly linked to its richness in , justifying its maintenance as a specific term.
Fully formed humus is essentially a collection of very large and complex molecules formed in part from lignin and other molecules of the original plant material (foliage, wood, bark), in part from similar molecules that have been produced by Microorganism. 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.
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. 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.
The most useful functions of humus are in improving soil structure, all the more when associated with cations (e.g. calcium), 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.
Soil carbon sequestration is a major property of the soil, also considered as an ecosystem service. 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.
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. Further research has been done in more recent years, though it remains an active field of study.
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 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.
Lignin, which is quickly transformed by white-rot fungi, is one of the primary precursors of humus, together with by-products of microbial and animal 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. Some judge earthworm humus (vermicompost) to be the optimal organic manure.
Stable humus contributes few plant-available nutrients in soil, but it helps maintain its physical structure. A very stable form of humus is formed from the slow oxidation (redox) of soil carbon after the incorporation of finely powdered Biochar into the topsoil, suggested to result from the grinding and mixing activity of a tropical earthworm. This process is speculated to have been important in the formation of the unusually fertile Amazonian Terra preta]]. However, some authors 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.″
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