As complex as it is, soil can be described simply. It consists of four major components: air, water, organic matter, and mineral matter (Fig. 2.1). In an ideal soil, air and water fill the pore space and compose about 50 percent of the volume; organic matter accounts for about 1-5 percent of the soil volume; and mineral matter accounts for the remaining 45-49 percent. The partitioning of these four components vary considerably. For example, an organic soil in Michigan may be 45 percent organic, while a desert soil from Arizona may be 60 percent mineral.
The mineral and organic matter fractions of the soil are the solids and serve as the storehouse and exchange sites for plant nutrients and other chemicals. They are important from a fertility and environmental standpoint. It is these fractions, along with cultural practices, that influence other physical properties and processes.