In soil science, humus is the totality of the dead organic substance of a soil. As a component of the organic soil substance, it is particularly subject to the activity of soil organisms (edaphon), which continually contribute to the formation, reorganization or degradation of humus by means of their metabolism. In specialist literature, the term is narrower and only the decomposed fraction of the organic substance in the soil is referred to as humus. However, since the respective transformation stages are fluent, it is not possible to find an exact definition.