What is primarily made up of decomposing organic matter including dead leaves and excrement?

Detritus refers specifically to organic matter that is in the process of decomposition, which includes materials such as dead leaves and animal excrement. This definition highlights its role in the ecosystem as a crucial component of nutrient cycling. Detritus serves as an important food source for various organisms, including detritivores like earthworms and fungus, which break it down further, enriching the soil and contributing to the overall health of the ecosystem.

Humus, while related to decomposing organic matter, specifically refers to the stable, dark organic material resulting from the breakdown of detritus. It is important for soil structure and fertility but does not encompass the initial state of decomposition. Soil includes a variety of components such as minerals, water, and air in addition to organic matter, and is not solely focused on decomposing materials. Biomass refers to the total mass of living matter in a given area or volume, which can include plants, animals, and microorganisms, but does not specifically pertain to dead organic material in the process of decomposition. Thus, the focus on decomposing organic matter makes detritus the most accurate choice.