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In soil science, humus (Origin: 1790–1800; < Latin: earth, ground [1]) 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 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]



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 senses,[6] though some agricultural experts put a greater focus on other features of it, such as disease suppressiveness.[7] It helps the soil retain moisture by increasing microporosity,[8] and encourages the formation of good soil structure.[9][10] 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] 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. Humus can be differentiated from organic matter in that the latter is rough-looking material, with coarse plant remains still visible, whereas fully humified organic matter is 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 that 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]

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