Soil Horizons
If you were to cut a deep slice into the soil, you would see distinct horizontal layers called horizons. A soil profile is a vertical section from surface to parent material and shows the soil horizons. The upper most layer is called the O-horizon. This layer is rich in organic material. In desert soils, this layer may be completely absent. In organically rich soils, this may be the thickest layer. Just beneath the O-layer, is the topsoil or A-horizon. The A-horizon is dark and rich is organic matter and humus. This layer may be somewhat granular and many of its nutrients may have leached out. In some soils, a very heavily leached layer called the E-horizon, develops between the A and B horizon. The B-horizon is the light colored soil beneath the A-horizon. It is a zone of illuviation rich with nutrients that leached down from the A-horizon. It is typically rich in iron and aluminum compounds and clay. The last horizon is the C-horizon. It contains weathered pieces of rock and borders the unweathered parent material. The C-horizon is typically just below the extent of most roots and is often saturated with groundwater.