Humus ~ Technical1

NOTE: This presentation is not ‘easy-reading’, but it is a very important subject that all compost enthusiasts should be familiar with. Please attempt to read all of it.

The design of this presentation is to show that development of high-humus content in the compost you make is one of the most desirable results, and should be part of your practical management practices of turning compost based on temperature, air and moisture content, and sufficient break-up of material (screening) to maximize CO2 gas exchange. Although an 'extra step', SCREENING really does improve the quality of results, and reduces time 'in the making' of it.

This treatise is a compilation of information from a variety of sources, listed as references in this presentation.

From Wikipedia, the free encyclopedia

"Humus" (Origin: 1790–1800; < Latin: earth, ground [1]) is degraded organic material in soil, which causes some soil layers to be dark brown or black. Humus has a characteristic black or dark brown color, which is due to an abundance of organic carbon. The darker it is, the higher a carbon percentage.

In soil science, humus refers to any organic matter that has reached a point of stability, where it will break down no further and might, if conditions do not change, remain essentially as it is for centuries, if not millennia [2].

In agriculture, the word ‘humus’ is sometimes also used to describe mature (aged) compost, or natural compost extracted from a forest or other spontaneous source for use to amend soil. It is also used to describe a topsoil horizon that contains organic matter (humus type [3], humus form [4], humus profile [5]). And while such products usually may contain a percentage of actual humus, such percentage is usually very small.

The process of "humification" can occur naturally in soil, or (faster and more completely - ed.) 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 [6], though some agricultural experts put a greater focus on other features of it, such as disease suppressiveness [7]. Physically, it helps the soil retain moisture by increasing microporosity [8], and encourages the formation of good soil structure [9][3]. Chemically, 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 by ion exchange (CEC) [10]. Biologically, it allows soil organisms (microbes and animals) to feed and reproduce [11][12].

Humus is often described as the 'life-force' of the soil. Yet it is difficult to define humus in precise terms; it is a highly complex substance, the full nature of which is still not fully understood. Physically, humus can be differentiated from organic matter in that the latter is rough looking material, with coarse plant remains still visible, while once fully humified organic matter becomes more uniform in appearance (a dark, spongy, jelly-like substance) and amorphous in structure, and may remain such for millennia or more [13] .

That is, it has no determinate shape, structure or character. However, humified organic matter, when examined under the microscope without any chemical treatment, may reveal tiny but clearly identifiable plant, animal or microbial remains which have been mechanically, but not chemically degraded [14]. This points to a fuzzy limit between humus and organic matter. In most recent literature, humus is clearly considered as an integral part of soil organic matter (SOM)[4].

Compost that is readily capable of further decomposition is sometimes referred to as effective or active humus, though again scientists would say that if it is not stable, it's not humus at all.

This kind of compost, rich in plant remains and fulvic acids, is an excellent source of plant nutrients, Plant remains (including those that passed through an animal gut and were excreted as feces) 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 saprotrophs initially invade the dead plant organs, while the remaining cellulose and lignin break down more slowly [15].

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 slowly transformed by white-rot fungi [16], is one of the main precursors of humus [17], together with by-products of microbial [18] and animal [19] activity.

The humus that is the end product of this manifold process, is thus a mixture of compounds and complex life chemicals of plant, animal, or microbial origin, which has many functions and benefits in the soil. Most humus in the soil is included in animal feces of more or less dark color according to their content in organic matter [20].

Earthworm humus (vermicompost) is considered by some to be the best organic manure there is[5], and others think that bat guano is best. However, neither are actually humus, based on the fact that the feces of such creatures still contain significant quantities of organic compounds which have not yet been decomposed by microbes. Humus is stable, while such animal excrement is quite unstable, the vast majority of which, will eventually decompose.

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Stability of humus

Transformation of organic matter into humus

Compost that is readily capable of further decomposition is sometimes referred to as ‘effective’ or ‘active’ humus, though again, scientists would say that if it is not stable, it's not humus at all. Compost that continues to decompose, while rich in plant remains and fulvic acids, and is an excellent source of plant nutrients, is usually of little value regarding long-term soil structure and tilth – except that part which becomes actual (stable) humus.

Stable (or passive) humus consisting of humic acids and humins, on the other hand, are so highly insoluble (or so tightly bound to clay particles and hydroxides) that they can no longer be penetrated by microbes and therefore are greatly resistant to further decomposition.

Thus stable humus adds few readily available nutrients to the soil itself, but plays an essential part in capturing/releasing nutrients (CEC), and providing its physical structure to soil. Some very stable humus complexes have survived for thousands of years [21].

The most stable humus found in this century, is that formed from the slow oxidation of black carbon (such as the incorporation of finely powdered charcoal - but not ashes) into the topsoil (but not into compost). Such a process is believed to be at the origin of the formation of the fertile Amazonian Dark Earths or Terra Preta de Indio [22].

Humus is transformed by soil organisms, which may contribute to increase or decrease its stability according to their enzyme equipment [23]. The disappearance of organic matter (OM) is hastened by warm and moist climate, which explains why most tropical soils are so poor in organic matter and suffer from both lack of good structure and available nutrients [24]. In boreal countries and at high altitudes, the lack of active transformation of organic matter into humus, because of harsh climate conditions, leads to a similar decrease in soil fertility, although for opposite reasons [25]. Among other factors, this explains why temperate climates were most favorable to the development of sedentary agriculture in the past millennia, before the advent of mineral fertilizers [6].

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Benefits of soil organic matter and humus

  • The mineralization process that converts raw organic matter to the relatively stable substance that is humus, feeds the soil population of micro-organisms and other creatures, thus maintaining high and healthy levels of soil life [26] [27].
  • 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 terrestrial ecosystems [28].
  • Effective and stable humus (see below) are further sources of nutrients to microbes, the former providing a readily available supply while the latter acts as a more long-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 which are further transformed into microbial biomass (microbial humus) or are reorganized (and still oxidized) in humic assemblages which bind to clay minerals and metal hydroxides. There has been a long debate about the ability of plants to uptake humus from their root systems and to use it for their nutrition. There is now a consensus about humus as playing a hormonal role rather than a nutritional role in plant physiology[29]
  • Humus is a colloidal substance, and increases the soil's cation exchange capacity (CEC), hence its ability to store nutrients by chelation as can clay particles; thus while these nutrient cations are accessible to plants, they are held in the soil safe from leaching away 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 (bacteria and fungi) secrete sticky gums and mucilage’s; these contribute to the crumb structure of the soil by holding particles together, allowing greater aeration (porosity) of the soil [30].
  • 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 prevented from entering the wider ecosystem, thereby detoxifying it [31]..
  • The dark color of humus (usually black or very dark brown) helps to warm up cold soils in the spring.

See also these References

  • Biomass
  • Biotic matter
  • Compost
  • Detritivore
  • Glomalin
  • Humic acid
  • Organic matter
  • Plant litter

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of Food and Agriculture 14:849–857.doi:10.1002/jsfa.2740141201

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Stuttgart, Germany, 145 pp.[1]

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soil fertility and in environmental protection. Landscape and Urban Planning 27:161–167. doi:10.1016/0169-2046(93)90044-E

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  • 8. ^ De Macedo, J.R., Do Amaral Meneguelli, N., Ottoni, T.B., Araujo de Sousa Lima, J., 2002.

Estimation of field capacity and moisture retention based on regression analysis involving chemical and physical properties in Alfisols and Ultisols of the state of Rio de Janeiro. Communications in Soil Science and Plant Analysis, 33: 2037 - 2055.doi:10.1081/CSS-120005747

  • 9. ^ 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 17:275–281.doi:10.1016/0165-2370(90)85016-G

  • 10. ^ 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

  • 11. ^ 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 - 152doi:10.1111/j.1574-6941.2000.tb00679.x

  • 12. ^ 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 30:97–106doi:10.1016/S0038-0717(97)00064-3

  • 13. ^ 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 326:553–559doi:10.1016/S1251-8050(98)80206-1

  • 14. ^ Bernier, N., Ponge, J.F., 1994. Humus form dynamics during the sylvogenetic cycle in a

mountain spruce forest. Soil Biology and Biochemistry 26:183-220doi:10.1016/0038-0717(94)90161-9

  • 15. ^ Berg, B., McClaugherty, C., 2007. Plant litter: decomposition, humus formation, carbon

sequestration, 2nd ed. Springer, 338 pp., ISBN-10: 3540749225

  • 16. ^ 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[2]

  • 17. ^ 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 146:425-433doi:10.1016/j. geoderma.2008.06.018

  • 18. ^ 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 23:1023-1028doi:10.1016/0146-6380(95)00094-1

  • 19. ^ 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 31:1303-1311doi:10.1016/S0038-0717(99)00049-8

  • 20. ^ Ponge, J.F., 1991. Food resources and diets of soil animals in a small area of Scots pine litter.

Geoderma, 49:33–62.doi:10.1016/0016-7061(91)90090-G

  • 21. ^ 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 326:553–559doi:10.1016/S1251-8050(98)80206-1

  • 22. ^ Lehmann, J., Kern, D.C., Glaser, B., Woods, W.I., 2004. Amazonian Dark Earths: origin,

properties, management. Springer, 523 pp. ISBN-13: 978-1402018398

  • 23. ^ Wolters, V., 2000. Invertebrate control of soil organic matter stability. Biology and Fertility of

Soils 31:1–19doi:10.1007/s003740050618

  • 24. ^ Tiessen, H., Cuevas†, E., Chacon, P., 2002. The role of soil organic matter in sustaining soil

fertility. Nature 371:783-785doi:10.1038/371783a0

  • 25. ^ Jerabkova, L., Prescott, C.E., Kishchuk, B.E., 2006. Nitrogen availability in soil and forest floor of

contrasting types of boreal mixedwood forests. Canadian Journal of Forest Research 36:112– 122doi:10.1139/X05-220

  • 26. ^ 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 - 152doi:10.1111/j.1574-6941.2000.tb00679.

  • 27. ^ 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 30:97–106doi:10.1016/S0038-0717(97)00064-3

  • 28. ^ Ponge, J.F., 2003. Humus forms in terrestrial ecosystems: a framework to biodiversity. Soil

Biology and Biochemistry 35:935–945doi:10.1016/S0038-0717(03)00149-4

  • 29. ^ Eyheraguibel, B., Silvestrea, J. Morard, P., 2008. Effects of humic substances derived from

organic waste enhancement on the growth and mineral nutrition of maize. Bioresource Technology 99:4206-4212doi:10.1016/j.biortech.2007.08.082

  • 30. ^ Caesar-Tonthat, T.C., 2002. Soil binding properties of mucilage produced by a basidiomycete

fungus in a model system. Mycological Research 106:930-937doi:10.1017/S0953756202006330

  • 31. ^ Huang, D.L., Zeng, G.M., Feng, C.L., Hu, S., Jiang, X.Y., Tang, L., Su, F.F., Zhang, Y., Zeng, W.,

Liu, H.L., 2008. Degradation of lead-contaminated lignocellulosic waste by Phanerochaete chrysosporium and the reduction of lead toxicity. Environmental Science and Technology 42:4946- 4951doi:10.1021/es800072c

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Other spellings and similar pronunciations:

Hummus (a transliteration of the Arabic: حمّص‎; also spelled hamos, houmous, hommos, hommus, hummos, hummous or humus; see romanization of Arabic) is a Levantine Arab[1] dip or spread made from cooked, mashed chickpeas, blended with tahini, olive oil, lemon juice, salt and garlic. It is a popular food in various local forms throughout the Middle Eastern world.

The spelling 'humus' for the edible dish, is avoided in English due to its having the same spelling as another English word humus, though this is the most common Turkish spelling[3] and the OED indicates the word entered the English language from Turkish.[4] The full Arabic name of the prepared spread is حُمُّص بطحينة (ḥummuṣ bi ṭaḥīna) which means chickpeas with tahina.

Below are some excerpts from my “manuscript in process” which I intend to publish on this website as part of the book I am authoring. If you have comments, suggestions and/or constructive criticism of any – please do not hesitate to make me aware of your thoughts. Even if they are just ‘thoughts’. These words are not written for me – they are written for you. Please send your comments via the Contact link at the bottom of this page.

  • * * * * * * *

Pure humus contains almost no plant nutritive value of itself, being almost pure carbon, but has a high cation exchange capacity (CEC) which binds nutrients to it. It's also impossible to purchase as a pure product, since it can't be manufactured or packaged from decaying organic matter in a reasonably pure form, on any sensible timescale - but high-quality compost can come close, depending on the quality of highly-aged compost used to make it. Remember that "brewed" tea does increase microbe populations, but does not increase plant nutrition or humic content. Humic content will be released into the tea if done a bit differently than the 'normal' "brewing" process of extraction for microbes.

And, soil scientists don't know exactly how, or which microorganisms in combination, actually produce humus – yet. So while you may find products sold as "humus" or "humates" or something similar, "let the buyer beware", because whatever it is – it does not likely contain much humus (usually a very low percentage) and in fact, may contain none at all. When it comes to marketing commercial “organic” products, ‘humus’ is a very “overworked” word to entice uninformed laymen gardeners to purchase a particular product. If the label does not list the percentage of humus content, chances are that you are "buying air".

To a soil scientist, an organic molecule is a molecule that contains carbon. Most life on earth is based on carbon compounds, so living and dead cells are loosely referred to as “organic material”, even though they also contain inorganic molecules like water.

"Organic gardening" is a vague term that means different things to many people. The source of the term is probably that "organic gardeners" tend to avoid adding nutrients in their inorganic, synthetic chemical form. For good reasons.

Actual Humus is the organic, non-cellular, long-lasting component found in natural soils. It is organic because it is composed of natural chemicals containing carbon. It is mostly comprised of extremely stable carbon compounds (with none or very little phosphorus or nitrogen) which resists further breakdown by microorganisms. The term "humus" gets ‘tossed around’ loosely by gardeners, to mean the organic material that makes soil 'brownish', most of which is not humus. In nature, humus accumulates in soil because it lasts for hundreds or even thousands of years. Humus is not a 'solid' substance, but rather is amourphous, having no structure.

Compost may contain some humus at any stage, after feedstock no longer heats above 105F (primarily in the mesophilic stage). Humus is a derivative of microbe activity - it does not occur in sterile soil.

In the strictest sense, humus is made up of humic substances composed of Carbon, Oxygen, and Hydrogen. These include humic acids, fulvic acids, and humins. Some Nitrogen may be present bound to humus, but not present in humus - in any significant quantity. Or it would not be humus, because with nitrogen in it, microbes could eat it. Plants obtain those first three elements by means other than soil, so "pure" humus has no significant nutritive value in and of itself.

So while stable (but immature) compost may contain a very small amount of actual humus, mature (cured or well-finished or fully-aged) compost is not all pure humus either. Mature compost still contains carbohydrates, lipids, and amino acids, as well as living organisms feeding on the remaining partially decomposed material. Not all organic matter in compost will be at the same stage of decomposition at any given time.

Generally a good loam soil contains a roughly equal combination of sand, silt, clay, but for a soil to be classified as loam, it needs NO organic matter in it. For the purposes of gardening, agriculture, and even some soil enthusiasts, humus is considered to be synonymous with soil organic matter - the non-mineral portion naturally occurring in some soils. So when a gardener talks about adding humus to soil they usually mean organic material in a stage of decomposition (such as leaf mold or compost).

"The definitional problem here is actually quite easy to resolve. It is the standard distinction between colloquial usage and scientific usage. Colloquially, if you add mature compost, or even immature compost, to soil, the resulting mix is will contain a percentage of humus over time, through resultant microbial activity.

Peat from an aged bog, is pretty much in its final state, and is often referred to as ‘humus’ – except that while peat is quite stable, and resists further decomposition, it still retains plant cellular structure (true humus has no cellular structure). Peat is generally accepted as an acidic form of ‘humus’ – but in actuality, it is only plant matter that failed to fully decompose because microbes were not able to survive in such acidic conditions – not because it fully decomposed into a primarily carbon state.

The benefit of stable and mature compost is two-fold. *First, it has the immediate effect of adding nutrients and microbial organisms that aid in soil conditioning.

  • Second, it eventually converts a percentage (of the original biomass) to true humus. It is unusual to find soils that contain more than 10% organic material, and it is rare to find mature (aged) compost with more than a 25% component of humus. Sticky stuff, if you can make it.

The word “stable” being used in this text – has TWO different meanings that need clarification:

Stable compost means that the material will no longer reach a significant temperature (usually +110F) and has nothing to do with development of humus by microbial decomposition of organic matter. In this sense, stable compost has completed exponential reproduction of thermophilic microbes which have consumed the majority of carbon in the form of sugars and starches (carbohydrates) – so post-carbohydrate compost is thereafter, mostly decomposed by mesophilic microbe activity in the 75F to 105F range. Basically, two to three weeks after a compost pile has ‘rested’ after being turned twice in a period of 6-8 weeks, the compost is considered safe to amend soil, and will provide a continuous supply of available plant nutrients within the soil, until it is fully decomposed and disappears (except for the actual humus, which remains).

In this sense of the word ‘stable’, the main emphasis is development of a high diversity of microbe populations, particularly in the mesophilic range, because microbes are THE primary means of improving soil. Compost with an initial C:N ratio between 45:1 to 35:1 (depending on the type of feedstock) is the medium in which microbes work best, to accomplish the goal to provide a better environment for plants to grow.

On the other hand, the definition of humus includes the word “stable” to mean organic matter that will no longer readily decompose any further – and has nothing to do with temperature or C:N ratio.

Pure mature compost can often be used by itself for growing plants that appreciate high-moisture root conditions and high nutritive values. Xeriscape plants (low water needs) such as Cacti, will not appreciate a planting media comprised exclusively from compost.

Without significant silicates (sand) or their equivalent (perlite or vermiculite) to provide drainage, cacti may die from too much humidity and nutrition, which creates a fungal condition detrimental to cacti root systems.

For this same reason, most vegetable seeds and cotyledon-only seedlings should not be germinated/planted in pure mature compost, as new growths have not yet developed a sufficient immune system to combat the diversity/ of fungi normally present in compost. Same reason that infant humans are more susceptible to disease/infection in their first months of life.

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The distinction between "organic" as used by science and "organic" as used by gardeners reportedly stemmed from a long-ago disproven belief called ‘Vitalism’ which maintained that organic molecules, carbon compounded with other elements (just having carbon is NOT the sole criterion, else a diamond would be considered organic and it is not, and so would CO2, which also isn't) HAD to be manufactured by living things, and that organic molecules were somehow different from other chemicals by virtue of possessing a force vitae (life force of their own).

This belief was utterly overthrown in 1828 when Friedrich Woehler published a brief paper describing the synthesis of the organic compound urea, formerly isolated from urine, from ammonium cyanate via: (NH4+)(-OHN) ---> O=C(NH2)2. This laid the ground work for the dismantling of the concept of vitalism in chemistry, but not in the popular mind, and the idea still permeates modern society and gardening lore.

Soon thereafter, the chemistry split into two branches, biochemistry which studied the actual chemistry of life, and organic chemistry, which studied the properties and interactions of carbon compounds.

When many people say "organic" they really mean "biologic". This point is important, as many highly toxic substances are "organic" and mistaken beliefs that organic compounds are safe, just because they are biological, is a natural by-product of clinging to the disproven theory of vitalism that marketing 'hypers' want you to believe - which is how the myth is still perpetuated today. The word "organic" sells products. Because people still equate the word "organic" with the word "safe". Marketing BS. But then, some of the issue remains with soil scientists, because they keep calling material 'organic matter' (OM) instead of 'biologic matter' (BM). Maybe because BM is already 'taken' as an acronym?. Not going there...

Organic matter - used in the context of stuff that came from biological sources (diamonds and graphite are pure carbon, but don't decompose!) Such biological organic matter includes mature compost, immature compost, freshly cut plants, live plants, etc. But humus? Depends on who you ask…

Much scientific data is known about the composition of humus, but how microbes ‘make’ humus is still a mystery. Its constituents have been subjected to rigorous scientific investigation, but the fact remains, the we’re still finding out what the ‘sticky stuff’ is, that microbes ‘exude’ - what it consists of, and how it functions in Mother Nature’s “scheme of things”. Humus is a very small fraction of total soil, and it has taken science quite a while to begin serious investigation into this extraordinary component of 'live' soil.

Soil, classified as totally mineral (sand, silt and clay – each alone, or combined (measured as a percentage), is essentially ‘devoid’ of the ability to sustain microbial life. Sterile. It is the organic content that is “alive” in soil.

pH is a major factor. Humus, regardless of the method of its creation, carries basically a neutral (7.0) pH. Because in nature, soil microbes, and their sticky ‘excrement’, seem to have an innate ability to ‘balance’ soil pH – as well as ‘bind’ the soil into larger aggregates – which promote soil aeration; capacity to retain moisture and plant nutrients.

Humus - (soil science definition) is “…the organic portion of the soil that has fully decomposed and is thus stable...” This definition of Humus is important, because it allows the percentage of humus to be considered a separate 'entity' in soil science investigations, to determine the ‘nature’ of a particular soil.

Contrary to general opinion, a large number of the constituents of humus ARE known. Their relative proportions vary according to microbe diversity, however, and thus there exists no simple answer to the question: "what is humus? It appears to be a dynamic pre-mix of organic substances, decomposed by a wide range of microscopic (and larger) creatures.

Colloquially, humus is the ‘permanent’ organic portion of the soil. This definition does not help much to define humus as a certain percentage of the total soil sample either.

And therefore it is difficult to determine. So we temporarily concede that measurement of the humus in soil is purely an estimate. But, there are a number of reliable testing methods to determine the basis for such an “estimate”.

One more important attribute of humus should be noted: In addition to improving soil structure (texture) and water/nutrient retention, humus also has a high cation exchange capacity (CEC), which means it acts as a adsorption ‘storehouse’ for plant nutrients, which is especially important for those with sandy and/or heavy clay soils.

Humus occurs in many types of soils, differentiated by soil morphology and the fractional composition of fulvic and humic acids. In other words, humus development appears to be specifically related to certain morphological forms of naturally-accumulated humic and fulvic substances in a soil profile (under the surface of aerated soil), conditioned by activity of the soil-forming microbe process in proportion to available moisture, which is necessary for the humification of organic matter.

Types of humus found in terrestrial environments are: Mor, Moder and Mull.

Mor occurs largely in North American coniferous forests and European moorland (dominated by dense grasses) soils.
(Moorland or ‘moor’ is a type of habitat found in upland areas, characterized by low growing vegetation on acidic soils. Moor land nowadays generally means uncultivated hill land on which rainfall does not percolate deeply into the mineral soil. This type of humus occurs under more acidic conditions which permit only a low microbiological activity.
Due to the acidic nature of decomposing material, the mineralization of organic matter proceeds slowly, creating dense layers, which maintain a structure of mostly acidophilic fungi along with low levels of active soil invertebrates that also participate in transformations of plant residues into humic acids.
Under these circumstances vegetative litter usually accumulates in thick dense layers which produce a ‘peat-type’ of humus.
The C/N ratio of acidic Mor humus is always more than 20:1 and some has been found in the 30-35:1 range. This type of humus is also characteristic from use of acidic leaf molds such as found on decaying live oak leaves and other such slightly acidic leafy matter.

Moder is a transitional form of humus characteristic of sod-podzolic soils, loesses and mountain grassland soils which receive a moderate annual rainfall. Organic horizons of soil with moder humus consist of thinner, more fully decomposing litter layers (2-3 cm), which gradually form without distinctive bounds, passing on to humus-accumulative horizons.
Moder is a type of medium-humified humus. Acidophilic fungi and arthropodan participates in transformations of plant residues. C/N ratios are usually in the 15-25:1 range. Produced mineral-organic complexes are labile and weakly bounded with mineral portion of soil.

Mull is a type of humus characteristic for ‘chestnut’ soils, phaeozems, rendzinas and others soils. This type of humus accumulates under prairie-type’ grass vegetation.
Mull is a well humified organic matter, which is produced in very biologically active habitat (such as composting). This type of humus is characterized by neutral pH, C/N ratio nearing to 10-15:1 with an ability to create stable mineral-organic complexes (hooray for aerobic compost!!).
Mull is a type of humus which also occurs in soils under cultivation in which plant residues are regularly incorporated back into the planting medium, which is regularly irrigated or has a consistent annual rainfall.

According to the soil scientist D. Kononova, the types of humus are divided as follows:

  • Humus-1 is characteristic for podzolic soils, grey-brown soils and lateric soils under forest communities. Humic acids predominate, thus the humic acid/fulvic acid ratio is usually below 1. Humic acids usually contain only a small extent of aromatic rings development compared to fulvic acids. The considerable hydrophilic properties of humic acids favor creation of chelates with polyvalent cations and the ability for displacement deep into profile of soil. Considerable mobility of this humus favors the process of podsolization.
  • Humus-2 is characteristic for phaeozems, rendzinas, black earths and dark brown soils. The humic acid/fulvic acid ratio is usually greater than 1, with the extent of aromatic ring condensations relatively high in such humic acids, which cause their hydrophobic properties to be lesser in creation of chelates. Humic acids are therefore strongly connected with mineral portion of soil in this type of humus.
  • Humus-3 is characteristic for semi-desert soils. In this type of humus, fulvic acid fractions predominate, whereas arise of humic acids is limited, and are largely bound to the mineral portion of soil.

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