The decayed matter that becomes part of soil is called humus.

Here's a breakdown:

* Humus is the stable, decomposed organic matter in soil. It's formed from the breakdown of dead plants and animals by microorganisms like bacteria and fungi.

* This process of decomposition is called humification.

* Humus is a dark, crumbly substance that's rich in nutrients and improves soil structure, water retention, and aeration.

So, while dead plants and animals are the source material, it's the humus that is the decayed matter that is part of soil.