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Humus has a characteristic black or dark brown color, due to an accumulation of organic carbon

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]


Benefits of Humus

  • The mineralisation 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.
  • 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.
  • Humification of dead plant material causes complex organic compounds to break down into simpler forms which are then made available to growing plants for uptake through their root systems.
  • Humus is a colloidal as substance, and increases the soil's cation exchange capacity, hence its ability to store nutrients by chilation 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 secrete sticky gums; these contribute to the crumb structure of the soil by holding particles together, allowing greater aeration of the soil. 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.
  • The dark colour of humus (usually black or dark brown) helps to warm up cold soils in the spring.

Humification of leaf litter and formation of clay-humus complexes

Compost which is readily capable of further decomposition is sometimes referred to as effective or active humus, though again actual scientists would say that if it is not stable, it's not humus at all. This kind of compost is principally derived from sugars, starches, and proteins, and consists of simple organic (fulvic) acids. It 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, or humins, on the other hand, are so highly insoluble (or tightly bound to clay particles that they cannot be penetrated by microbes) that they are greatly resistant to further decomposition. Thus they add few readily available nutrients to the soil, but play an essential part in providing its physical structure. Some very stable humus complexes have survived for thousands of years. Stable humus tends to originate from woodier plant materials, eg, cellulose and lignins.

See also

External links

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