Overgrazing also reduces the root mass. Each soil organism has a role to keep the system functioning. Bacteria are the prime colonizers of dead plants and roots. The sticky secretions that come from bacteria as they move along bind soil particles. They transform ammonia to nitrogen so plants can take it up.
Bacteria also produce growth-promoting compounds that stimulate plant growth from the roots.
Fungi are often thought of as pathogens, but most have a valuable use in decomposing litter and binding soil particles together. When mycorrhizal fungi colonize a plant, they increase the amount of sugar that moves to the roots. This increases photosynthesis and amino acids, which makes soil phosphorus more available to the plant. Fungi and bacteria are also a food source for other organisms living in the soil. Earthworms are ecosystem engineers and can dramatically change soil.