carbon is an essential element that is distributed between the atmosphere (present as carbon dioxide), plants, animals, and the soil. When plant and animal residues and excreta are returned to the soil, they are colonized by a host of microorganisms, aided by larger soil organisms such as earthworms, which decompose these residues, releasing carbon dioxide and nutrients, and deriving energy for their growth. As the many different carbon compounds in the residues are decomposed, and organisms multiply and then die, the more resistant compounds and newly synthesized carbon compounds gradually accumulate to form a dark brown to black colloidal material called humus (see organic matter). This carbon cycle is important for the nutrition of vines and cover crops, most especially in organic and biodynamic viticulture.
