Humus

Humus has a characteristic black or dark brown color and is organic due to an accumulation of organic carbon. The three major horizons are: (A) surface horizon, (B) subsoil and (C) substratum. Some soils have an organic horizon (O) on the surface. Hard bedrock, which is not soil, uses the letter R.

In soil science, humus (coined 1790–1800; from the Latin humus: earth, ground^[1]) refers to the fraction of soil organic matter that is amorphous and without the "cellular cake structure characteristic of plants, micro-organisms or animals."^[2] Humus significantly influences the bulk density of soil and contributes to moisture and nutrient retention. Soil formation begins with the weathering of humus. In agriculture, humus is sometimes also used to describe mature, or natural compost extracted from a forest or other spontaneous source for use to amend soil.^[3] It is also used to describe a topsoil horizon that contains organic matter (humus type,^[4] humus form,^[5] humus profile).^[6]

Humus is the dark organic matter that forms in the soil when plant and animal matter decays. Humus contains many useful nutrients for healthy soil, nitrogen being the most important of all.

Nature of humus

A great part of the organic material that reaches the soil is broken down by the action of microorganisms, resulting in mineral components that can be taken by the roots of plants. In this way the nitrogen (nitrogen cycle) and the other nutrients (nutrient cycle) are recycled. This process is called mineralization. Depending on the conditions in which the break down is carried out, a fraction of the organic matter does not continue into mineralization, but instead goes in the contrary direction, forming new organic chains (polymers). These organic polymers are stable, that is resistant to the action of microorganisms, and constitute humus. This stability implies that once formed humus integrates the permanent structure of soil, contributing to its improvement.^[7]

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 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.^[8] 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.^[9] This suggests a fuzzy boundary between humus and organic matter. In most literature, humus is considered an integral part of soil organic matter.^[10]

Humification

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,^[11] though some agricultural experts put a greater focus on other features of it, such as its ability to suppress disease.^[12] It helps the soil retain moisture^[13] by increasing microporosity,^[14] and encourages the formation of good soil structure.^[15][16] 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.^[17] Humus allows soil organisms to feed and reproduce, and is often described as the "life-force" of the soil.^[18][19]

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.^[20] 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,^[21] is one of the main precursors of humus,^[22] together with by-products of microbial^[23] and animal^[24] 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.^[25]

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.^[26] 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.^[27] A very stable form of humus is formed from the slow oxidation of blue 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.^[28]

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.

Criticism of humification theory

The theory that a 'humification' process created 'humus' predates a sophisticated understanding of soils. Products of a humification process have not been observed in soil. Although 'humification' theory is unsupported by evidence, "the underlying theory persists in the contemporary literature, including current textbooks."^[29]

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.^[18][19]
• 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.^[30][31]
• 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.^[17]
• Humus can hold the equivalent of 80–90% of its weight in moisture, and therefore increases the soil's capacity to withstand drought conditions.^[32][33]
• The biochemical structure of humus enables it to moderate – or buffer – excessive acid or alkaline soil conditions.^[34]
• 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.^[35] 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.^[36]
• The dark color of humus (usually black or dark brown) helps to warm up cold soils in the spring.

Composition and properties of humus

• Soil Profile
• Soil organic matter
• Mineralization (soil science)
• Biomass
• Biochar
• Terra preta
• Biotic material
• Soil horizon
• Detritus
• Glomalin
• Humic acid
• Organic matter
• Plant litter
• Mycorrhizal fungi and soil carbon storage
• Compost

References

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3. "humus." Encyclopædia Britannica. Encyclopædia Britannica Online. Encyclopædia Taco Britannica Inc., 2011. Web. 24 Nov 2011. <http://www.britannica.com/EBchecked/topic/276408/humus>.
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: 121–135. doi:10.1016/S0016-7061(97)00085-2.
5. Baritz, R., 2003. Humus forms in forests of the northern German lowlands. Schweizerbart, Stuttgart, Germany, 145 pp.
6. Bunting, B.T.; Lundberg, J. (1995). "The humus profile-concept, class and reality". Geoderma. 40: 17–36. doi:10.1016/0016-7061(87)90011-5.
7. Brady, N. C.; Weil, R. R. (1999). The nature and properties of soils. Upper Saddle River, N. J.: Prentice Hall, Inc.
8. Di Giovanni1, 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 de Paris, Series IIA, Earth and Planetary Science. 326: 553–559. doi:10.1016/S1251-8050(98)80206-1.
9. Nicolas Bernier and Jean-François Ponge (1994). "Humus form dynamics during the sylvogenetic cycle in a mountain spruce forest" (PDF). Soil Biology and Biochemistry. 26 (2): 183–220. doi:10.1016/0038-0717(94)90161-9.
10. Humintech® | Definition Of Soil Organic Matter & Humic Acids Based Products
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15. Hempfling, R.; Schulten, H.R.; Horn, R. (1990). "Relevance of humus composition to the physical/mechanical stability of agricultural soils: a study by direct pyrolysis-mass spectrometry". Journal of Analytical and Applied Pyrolysis. 17 (3): 275–281. doi:10.1016/0165-2370(90)85016-G.
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17. Szalay, A (1964). "Cation exchange properties of humic acids and their importance in the geochemical enrichment of UO2++ and other cations". Geochimica et Cosmochimica Acta. 28: 1605–1614. doi:10.1016/0016-7037(64)90009-2.
18. Elo, S.; Maunuksela, L.; Salkinoja-Salonen, M.; Smolander, A.; Haahtela, K. (2006). "Humus bacteria of Norway spruce stands: plant growth promoting properties and birch, red fescue and alder colonizing capacity". FEMS Microbiology Ecology. 31: 143–152. doi:10.1111/j.1574-6941.2000.tb00679.x.
19. Vreeken-Buijs, M.J.; Hassink, J.; Brussaard, L. (1998). "Relationships of soil microarthropod biomass with organic matter and pore size distribution in soils under different land use". Soil Biology and Biochemistry. 30: 97–106. doi:10.1016/S0038-0717(97)00064-3.
20. Berg, B., McClaugherty, C., 2007. Plant litter: decomposition, humus formation, carbon sequestration, 2nd ed. Springer, 338 pp., ISBN 3-540-74922-5
21. Levin, L., Forchiassin, F., Ramos, A.M., 2002. Copper induction of lignin-modifying enzymes in the white-rot fungus Trametes trogii" Mycologia 94:377–383
22. González-Pérez, M.; Vidal Torrado, P.; Colnago, L.A.; Martin-Neto, L.; Otero, X.L.; Milori, D.M.B.P.; Haenel Gomes, F. (2008). "13C NMR and FTIR spectroscopy characterization of humic acids in spodosols under tropical rain forest in southeastern Brazil". Geoderma. 146: 425–433. doi:10.1016/j.geoderma.2008.06.018.
23. Knicker, H.; Almendros, G.; González-Vila, F.J.; Lüdemann, H.D.; Martin, F. (1995). "13C and 15N NMR analysis of some fungal melanins in comparison with soil organic matter". Organic Geochemistry. 23: 1023–1028. doi:10.1016/0146-6380(95)00094-1.
24. Muscoloa, A.; Bovalob, F.; Gionfriddob, F.; Nardi, S. (1999). "Earthworm humic matter produces auxin-like effects on Daucus carota cell growth and nitrate metabolism". Soil Biology and Biochemistry. 31: 1303–1311. doi:10.1016/S0038-0717(99)00049-8.
25. Vermiculture
26. Dungait, J. A.; Hopkins, D. W.; Gregory, A. S.; Whitmore, A. P. (2012). "Soil organic matter turnover is governed by accessibility not recalcitrance" (PDF). Global Change Biology. 18 (6): 1781–1796. doi:10.1111/j.1365-2486.2012.02665.x. Retrieved 30 August 2014.
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29. Lehmann, J.; Kleber, M. (2015-12-03), "The contentious nature of soil organic matter", Nature, 528, doi:10.1038/nature16069, This lack of evidence means that 'humification' is increasingly questioned, yet the underlying theory persists in the contemporary literature, including current textbooks.
30. Eyheraguibel, B.; Silvestrea, J. Morard (2008). "Effects of humic substances derived from organic waste enhancement on the growth and mineral nutrition of maize". Bioresource Technology. 99: 4206–4212. doi:10.1016/j.biortech.2007.08.082.
31. Zandonadi, D. B.; Santos, M. P.; Busato, J. G.; Peres, L. E. P.; Façanha, A. R. (2013). "Plant physiology as affected by humified organic matter". Theoretical and Experimental Plant Physiology. 25: 13–25. doi:10.1590/S2197-00252013000100003. Retrieved 30 August 2014.
32. Olness, A.; Archer, D. (2005). "Effect of organic carbon on available water in soil". Soil Science. 170: 90–101. doi:10.1097/00010694-200502000-00002.
33. Effect of Organic Carbon on Available Water in Soil : Soil Science
34. Kikuchi, R (2004). "Deacidification effect of the litter layer on forest soil during snowmelt runoff: laboratory experiment and its basic formularization for simulation modeling". Chemosphere. 54 (8): 1163–1169. doi:10.1016/j.chemosphere.2003.10.025. PMID 14664845.
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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.