Why is humus important to soil?

Humus is the end result of organic matter breaking down in the soil or in a compost pile. Humus is "fluffy", it soaks up water like a sponge and has a tremendous surface area packed into a small space. The surface area allows for more chemical reactions with the soil solution, increasing the soil's nutrient-exchange capacity and allowing it to support more complex plant communities on the surface. Humus increases the water-holding and nutrient-exchanging capacity of the soil, and its gradual breakdown releases a slow, steady supply of nutrients to plants.