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. When there is lots of air in the soil, microorganisms flourish and speed mineral release. Good tilth allows delicate seedlings to get a good start. Unfortunately, using only chemicals for soil improvement results in a rapid loss of humus, the food source for soil microorganisms.