Soils are composed of solid particles, pore spaces, and organic matter.

The solid particles, or mineral portion of the soil, accounts for about 45% of the soil volume. They are formed from the weathering of the parent material. These particles (not including any gravel, cobbles, or stones) are classified into three different sizes, sand, silt, and clay. Sand is the largest particle, and feels gritty when rubbed between the fingers. Silt is the medium sized particle. It feels smooth like flour. Individual silt particles are barely visible. Clay is the smallest particle and cannot be seen by the unaided eye. Clay feels sticky when wet.

Pores are the voids between the solid particles. They account for about 50% of the soil volume. These spaces are filled with air and water. These pores are extremely important to plants and other organisms that receive oxygen, water, and nutrients from the soil. The size of the pores influences the rate of movement of air and water through the soil. Sandy soils which have large pores transmit air and water more rapidly than clay soils which have very small pores.

Organic matter is comprised of dead plant and animal tissue and the micro-organisms that live in the soil. While organic matter accounts for only about 5% of the volume of the soil, it is a very important part of the soil. Organic matter has many functions including providing energy for micro-organisms, helping to stabilize the solid soil particles, providing for easier penetration of plant roots, increasing infiltration of water into the soil, and reducing runoff and soil erosion.