humus

This term refers specifically to the stable, dark organic matter resulting from the complete decomposition of plant and animal tissues. It is distinct from compost, which is the active process of decomposition, and leaf litter, which consists of raw, undecomposed organic debris.

As a mass noun, humus is treated as uncountable. It describes a substance rather than a discrete object, meaning it does not have a plural form in standard agricultural or geological contexts and requires partitive phrases like a layer of or an amount of when quantification is necessary.