Earth & Space Science: Session 1
A Closer Look: Soil Horizons
What are soil horizons?
As you drive along highways, you are likely to pass places where the roadway cuts through soil. Have you noticed layers in these road cuts? During soil formation, inorganic materials (e.g., minerals) and organic materials (e.g., decaying plants and animals) are transformed and, with the flow of water through the soil, settle downward which creates layers called soil horizons. An established method for describing soil horizons labels them, form the surface down, O, A, E, B, C, and R. There can, however, be subdivisions of the major horizons in some soil profiles. Also, not all of the soil horizons are represented in any given location; the presence or absence of these layers helps to classify soil types. Soils have different and unique profiles depending on a number of factors, including: climate, organisms, parent material, topography, and time.
What are the characteristics of different soil horizons?
O Horizon: The O, or organic, horizon is found in soils formed under forest vegetation. It is composed mostly of vegetation that has fallen to the ground and the remains of animals such as insects, causing it to be dark in color.
A Horizon: Below the O horizon lies the A horizon, which is commonly called topsoil. It is the first soil horizon made mostly of minerals from the weathering of the underlying parent material, but it can also contain decomposed organic material, which gives it a dark color.
E Horizon: In areas that are or once were forested, there can sometimes be an E horizon beneath the A horizon. E stands for “eluviation,” which is the movement of dissolved or suspended material out of a horizon. Water entering the soil moves downward through the O and A horizons, and dissolves various soil materials (iron and aluminum oxides, clay particles, and organic matter) before carrying them to the E horizon, and then on to deeper levels. This process of leaching creates the E horizon’s white or grayish color. It is lighter in color than the layers above or below it.
B Horizon: The B horizon is commonly called subsoil. The B horizon is the illuviated zone, where the soil material that has been leached out of the upper horizons accumulates. In highly weathered soils, the B horizon is commonly rich in clay, iron, or aluminum, and is often colored yellow or red by the iron oxides that are transported down from above. B horizons can be very thick and can be broken down into multiple layers within the horizon.
C Horizon: Directly below the B horizon is the C horizon. It consists of the “raw” or “parent” material from which the soil was created. This includes partially weathered bedrock, materials that were deposited by the wind or water, volcanic material, organic matter, glacial deposits, and other materials.
R Horizon: If unaltered bedrock is within a few feet of the surface, it is called the R horizon.
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