Bacteria and fungi are examples of microorganisms. Those we are most interested in in agriculture are those who do something beneficial for the soil. Bacteria break down plant material and build up organic material in the soil. Fungi can have symbiotic relationships with plants, where the fungus provides the plant with nutrients (like N) in exchange for sugar. There are many aspects of the relationship between fungi, bacteria and plants which we do not understand fully. In undisturbed soil there are many more bacteria and fungi than tilled soil.