Over time, distinctive layers develop in undisturbed soils. Between the parent layer and the topsoil is the subsoil. Subsoil is finely weathered like the topsoil, but it lacks organic matter in the quantity found in the topsoil layer. The roots of green plants flourish in the topsoil, richest in organic matter and shallowest in depth, and rely on it for nutrients, support, water, and air.
The organic matter in soil comes from the decomposition of plant and animal tissue. When green plants are plowed into the soil they are immediately acted on by soil organisms that rapidly break the plant tissue down into a form usable for their own growth. Organic compounds that do not decompose quickly eventually succumb to enzymatic action, forming a complex mixture called humus. Humus as well as green manure (plowed under green plants) are important to the soil’s structure. This organic matter increases both the water-holding and mineral-holding capacity of the soil.
Water and Air
Water and air exist around and between the soil particles. As much as 50 percent of the topsoil may be air and water in liquid or vapor form. The ratio of air to water depends on the texture of the soil and how wet it is. A wet soil leaves less space for the air to occupy than a dry soil.
