Final answer:
Humus is primarily composed of dead and decaying plants and animals, along with microorganisms, located in the soil's organic layer or subsoil. It significantly influences soil's structure, nutrient availability, and water retention, and is an integral part of terrestrial nutrient cycles.
Step-by-step explanation:
Humus consists of dead and decaying plants and animals as well as microorganisms in various stages of decomposition, found in the organic layer or subsoil. It is a vital component of soil, contributing to its structure, nutrient content, and water retention capabilities. Humus is not composed of living plants or animals but is instead the resulting material from the breakdown process of their organic matter over time. This rich organic matter is critical to soil fertility and plays a crucial role in the carbon cycle by storing carbon that would otherwise be released into the atmosphere.
Furthermore, it enhances the soil's ability to support plant life by supplying essential minerals and improving water retention. It is found within the topsoil's organic horizon, known as the O horizon, and also present in the A horizon where it mixes with inorganic materials. Humus forms a crucial part of the nutrient cycles and serves as a significant source of available carbon in terrestrial ecosystems.