The answer is earthworms.
Earthworms live in the soil where they feed on plant material. By eating and digesting plant material, they mix it with soil mineral components. Now, the organic matter is improved. It is then excreted into the soil becoming high-quality humus. This humus contains many mineral elements, such as nitrogen, phosphorus, calcium, magnesium, etc. Therefore, earthworms help fertilize the soil by shedding digestive material.