The organic component of soil, formed by the decomposition of leaves and other plant materials that adds to the fertility of the soil is called :

Humus is the dark, organic material in soil formed by the decomposition of plant and animal matter by microorganisms. It is a vital component of soil fertility as it provides essential nutrients, improves soil structure, and enhances the soil's ability to retain moisture and air.

• Topsoil (A-horizon) is the uppermost layer of soil that contains a high concentration of humus and microorganisms, but it refers to the layer itself rather than the specific organic component.
• B-horizon (subsoil) is the layer below the topsoil, which is typically rich in minerals leached from the upper layers but contains significantly less organic matter.
• Mineralization is the biological process where organic substances are converted into inorganic forms (minerals) that plants can absorb; it is a process, not a substance.