A soil horizon is a layer of soil, approximately parallel to the soil surface, with distinct characteristics produced by soil-forming processes. The physical and chemical characteristics observed within the soil profile are the basis for differentiating one soil from another.

Over time, soil forms layers (horizons) in the following broad categories:

• TOPSOIL (O): dark color, nicely textured, high organic matter content, very important to plant growth. 
• SUBSOIL (A): below topsoil horizon, has accumulated elements from topsoil, but only minor amounts of organic matter. 
• PARENT MATERIAL (B): unconsolidated rock material below the subsoil, the material directly broken down into soil. 
• BEDROCK (C): solid rock below parent material.