Humification

In soil science, humus (coined 1790–1800; from the Latin humus: earth, ground) refers to the fraction of soil organic matter that is amorphous and without the "cellular cake structure characteristic of plants, micro-organisms or animals." 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. It is also used to describe a topsoil horizon that contains organic matter (humus type, humus form, humus profile).

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.

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.

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. 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. This suggests a fuzzy boundary between humus and organic matter. In most literature, humus is considered an integral part of soil organic matter.

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Wikipedia