What is humus primarily made up of?

Humus is primarily composed of decomposing plant and animal materials. This organic matter results from the breakdown of various biological substances, including leaves, stems, and other plant materials, as well as animal remains. The decomposition process, which is facilitated by microorganisms, transforms these materials into a dark, nutrient-rich component that enhances soil fertility and improves its structure. Humus plays a crucial role in soil health, providing essential nutrients to plants and improving the soil's ability to retain moisture.

The other options represent components not typically associated with humus. Mineral particles from rocks, for instance, are more relevant to the formation of the soil's inorganic framework rather than its organic content. Sandy soil components refer specifically to the texture and grain size of the soil, which do not contribute to the organic matter characteristics that define humus. Water and air mixtures pertain more to the soil's physical properties rather than its organic makeup, which highlights the significance of the decomposition process in the formation of humus.