The old saying "Dynamite comes in small packages" could have been coined for soil microorganisms. They are microscopic, but in their living and dying process they play an indispensable role: Without them the soil is "dead"; with them the soil is alive itself and is capable of giving life to plants. Although bacteria, fungi and all the others perform innumerable soil services, the crucial one is conversion of nitrogen into usable form for plants.
Bacteria that live on the roots of clovers and other legumes, for example, add significantly to a soil's supply of nitrogen. Fungi serve a different but highly important function in breaking down the soil's organic and mineral materials into plant nutrients.