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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.

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, humus is sometimes also used to describe mature 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]

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,^[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][10] 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.^[11] Biologically, it allows soil organisms (microbes and animals) to feed and reproduce.^[12][13] 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.^[14] 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^[15]. 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).^[16]

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 saprotrophs initially invade the dead plant organs, while the remaining cellulose and lignin break down more slowly.^[17] 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,^[18] is one of the main precursors of humus,^[19] together with by-products of microbial^[20] and animal^[21] 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 faeces of more or less dark color according to their content in organic matter.^[22] Earthworm humus (vermicompost) is considered by some to be the best organic manure there is.^[23]

Stability of 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. This kind of compost, rich in plant remains and fulvic acids, is an excellent source of plant nutrients, but of little value regarding long-term soil structure and tilth. 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 cannot be penetrated by microbes and therefore are greatly resistant to further decomposition. Thus stable humus adds few readily available nutrients to the soil, but plays an essential part in providing its physical structure. Some very stable humus complexes have survived for thousands of years.^[24] The most stable humus is that formed from the slow oxidation of black carbon, after the incorporation of finely powdered charcoal into the topsoil. This process is at the origin of the formation of the fertile Amazonian Dark Earths or Terra preta de Indio.^[25]

Humus is transformed by soil organisms, which may contribute to increase or decrease its stability according to their enzyme equipment.^[26] The disappearance of humus 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^[27]. 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.^[28] Among other factors, this explains why temperate climates were most favourable to the development of sedentary agriculture in the past millennia, before the advent of mineral fertilizers.^[29]

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.^[30][31]
• 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.^[32]
• 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 (sugars and amino-sugars, aliphatic and phenolic organic acids) which are further transformed into microbial biomass (microbial humus) or are reorganized (and still oxidized) in humic assemblages (fulvic and humic acids, humins) 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 humus as playing a hormonal role rather than a nutritional role in plant physiology.^[33]
• Humus is a colloidal substance, and increases the soil's cation exchange capacity, 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.^[34]
• Humus can hold the equivalent of 80–90% of its weight in moisture, and therefore increases the soil's capacity to withstand drought conditions.^[35][36]
• The biochemical structure of humus enables it to moderate — or buffer — excessive acid or alkaline soil conditions.^[37]
• During the humification process, microbes (bacteria and fungi) secrete sticky gums and mucilages; these contribute to the crumb structure of the soil by holding particles together, allowing greater aeration of the soil.^[38] 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.^[39]
• The dark color of humus (usually black or dark brown) helps to warm up cold soils in the spring.

See also

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

References

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