The humus is subject above all to the active activity of the living soil organisms, which through their metabolic activities constantly contribute to the transformation (humification). 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 also soil type). It improves the warmth and thus 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.