Compost supplies organic matter to soil, which attracts earthworms and stimulates beneficial soil microorganisms. Compost also performs the following:
Increases soil water holding capacity
Increases soil nutrient retention
Improves soil tilth and friability
Improves soil drainage
Loosens heavy clay soils
Suppresses soil-borne plant pathogens (diseases)
Although compost is not a fertilizer, it does contain many trace nutrients that are essential for plant growth. Nitrogen and phosphorus, mostly in organic forms, are released slowly to plants and not readily leached from the topsoil.