It is soil.
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Maybe you're speaking of Humus.
Another term for decayed organic matter is "humus".
IIt is called organic matter.
Decayed matter that becomes part of the soil is called organic matter. This includes leaves, plant material, animal remains, and other organic substances that break down and contribute nutrients to the soil. Organic matter is important for soil fertility and structure.
Decayed substances are commonly referred to as decomposed organic matter or simply as decaying matter. Decay occurs when organic material breaks down into simpler compounds through the action of microorganisms like bacteria and fungi.
Decayed organic matter
That would be necromass.
decayed organic matter
humus
Living things that have been decayed
Decayed plant and animal material in the soil is called organic matter. It is a key component for soil fertility as it provides nutrients for plants to grow. Organic matter also improves soil structure and water retention.
The organic matter in soil is called humus.
organic matter important to plant growth is humus.