Term (Latin for soil) for the entirety of the almost exclusively dead organic matter of a soil, which makes up the vast majority of the soil organic matter (OBS or SOM = Soil Organic Matter). However, about 5% of the OBS also consists of living organic matter, i.e. all living animal (such as woodlice, mites, caterpillars, earthworms, snails and spiders) and plant (such as algae, lichens and mosses) organisms, as well as microorganisms (such as bacteria and fungi). One teaspoon of humus contains 100 million bacteria, 60 kilometres of fungal threads and 1,000 threadworms. The living part of the OBS is also called Edaphon (grch. edaphos = soil).
Humus is subject above all to the active activity of living soil organisms, which constantly contribute to the transformation (humification) through their metabolic activities. A distinction is made between a relatively small proportion of as yet unconverted substances from protein and carbohydrates, and the predominant dark-coloured part of organic compounds, the humic substances. In common parlance, both are commonly referred to as humus. Strictly speaking, however, this only refers to the already converted organic fraction. The humification stages are fluid and therefore an exact delimitation is not possible.
The basic materials for the formation of humus are plant residues from dead vegetation, fallen leaves, crop residues, grapes from thinning out, dead root parts, dead microorganisms, excrements of soil animals and green manure (short-term greening), as well as the supply of organic material such as manure, slurry, liquid manure, compost, mulch, vine wood, stable manure and marc for the purpose of fertilization. It is only through humification that a number of nutrients such as mainly nitrogen, but also potassium, phosphorus and others are released from the organic material in a form that is available to plants.
In the vineyard soil, humus is particularly beneficial in the uppermost layer, the A-horizon (see under soil type). It improves warming and activates soil life. Humus can store up to five times its own weight in water, thus improving the water storage capacity. It also reduces the risk of leaching of various nutrients such as potassium, calcium and magnesium. See also under vineyard care.