* Decomposition: When plants and animals die, they are broken down by microorganisms like bacteria and fungi. This process is called decomposition.

* Humus: The end product of decomposition is humus. It's a dark, stable, and rich organic matter that is essential for healthy soil.

* Importance: Humus improves soil in many ways:

* Water retention: Humus helps soil hold onto water, making it available to plants.

* Nutrient content: It releases nutrients slowly, providing a steady supply for plant growth.

* Structure: Humus improves soil structure, making it easier for roots to grow and for air and water to circulate.

* Biodiversity: It supports a healthy ecosystem of soil organisms.

So, while "decayed plant and animal matter" is a good starting point, the term humus is more accurate and highlights the important role this material plays in the environment.