About Soil Horizons
Soil horizons describe layers of soil material that run parallel to the surface beginning with top soil and moving downward to bedrock. You can see soil horizons in areas where several feet of dirt have been removed for construction. Soil horizons differ by color and texture with each layer having an alphabet letter designation. In addition to the effect of organic decomposition, soil horizons develop through the action of weather, water and time on parent materials.
Upper Layers
The O horizon contains organic material and exists in areas that have not been cultivated. The O Horizon layer exists in areas that receive regular contributions of leaves or other organic materials. A dark layer, the A Horizon has both humus and minerals. The A Horizon has the greatest soil fertility of all soil horizons and typically ranges in depth from 2 to 12 inches, although in some areas the A Horizon may reach 2 to 3 feet. The top soil in areas under cultivation forms the A Horizon.
Lower Layers
The E horizon has a lighter color because minerals including carbonate, humus, iron and oxides have been filtered out through a process called eluviation. Materials remaining in the E Horizon include sand, silt and sometimes quartz. The color of the E Horizon may be white or light gray. The B Horizon, also called subsoil, contains material transported in from higher layers. The B Horizon usually controls the ability of water to move through the lower horizons. The B Horizon may be red, yellow, brown or gray in color. The C and R Horizons contain unweathered parent materials. C Horizon materials may be dug when moist, whereas R Horizon materials may not be dug.
With this knowledge of soil horizons, you will be able to identify them in your own backyard.
