The microorganisms release carbon dioxide gas into the film of water surrounding soil particles, making the water slightly acid. This causes it to react with the rock particles and dissolve them, providing a slow, steady release of nutrients. All microorganisms decompose decayed vegetation or other microorganisms. The decayed vegetation, called humus, is the basis for soil life. Without humus, we get no naturally formed nitrogen, and plants become weak and more susceptible to insect attack.
Humus has other beneficial effects besides feeding soil microorganisms.
It mechanically loosens the soil itself and creates a texture called good tilth. When soil has good tilth, it is soft, porous and easy to work and crumbles easily into fine pieces. Water penetrates the soil better and the soil also holds more water. Tilth prevents the formation of hard crusts on the soil's surface. It allows plants to put out larger root systems easily.