What is humus?

Hint: In agriculture, "humus" can also refer to mature or natural compost that has been taken from a woods or other naturally occurring source for use as a soil conditioner. It's also a term for a dirt horizon that's rich in organic stuff.

Complete answer:
Humus is a term used in soil science to describe the percentage of soil organic matter that is amorphous and lacks the "cellular cake structure characteristic of plants, microorganisms, and animals." Humus was coined in 1790–1800 from the Latin word humus, which means "earth, ground."
Humus has a substantial impact on the bulk density of soil and helps it retain moisture and minerals. Although the terms humus and compost are sometimes used interchangeably, they are two different soil components with different origins: humus is produced by anaerobic fermentation, whereas compost is produced by aerobic decomposition.
More specifically, humus is the black organic matter that accumulates in soil as dead plant and animal debris (including aerobic compost) decomposes further in the presence of anaerobic microbes. Humus contains a variety of elements that benefit soil health, the most significant of which is nitrogen. Humus has a carbon to nitrogen (C:N) ratio of 10:1.
Plant materials are the most important component in the humification process. Humus has a variety of compositions depending on the parent materials and secondary microbial and animal products. The pace at which different chemicals decompose will have an impact on the humus' composition.

Note: Depending on the conditions of decomposition, a portion of the organic matter does not mineralize and is instead changed into concatenations of organic polymers through a process known as "humification." These organic polymers are stable and make up humus because they are resistant to the action of microorganisms.