Physical Properties of the Soil

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 Illustration shows a cross-section of soil layers, or horizons. The top layer, from zero to two inches, is the O horizon. The O horizon is a rich, deep brown color. From two to ten inches is the A horizon. This layer is slightly lighter in color than the O horizon, and extensive root systems are visible. From ten to thirty inches is the B horizon. The B horizon is reddish brown. Longer roots extend to the bottom of this layer. The C  horizon extends from 30 to 48 inches. This layer is rocky and devoid of roots.
This soil profile shows the different soil layers (O horizon, A horizon, B horizon, and C horizon) found in typical soils. (credit: modification of work by USDA)

OpenStax Biology 2e

Soils are named and classified based on their horizons. The soil profile has four distinct layers: 1) O horizon; 2) A horizon; 3) B horizon, or subsoil; and 4) C horizon, or soil base. The O horizon has freshly decomposing organic matter—humus—at its surface, with decomposed vegetation at its base. Humus enriches the soil with nutrients and enhances soil moisture retention. Topsoil—the top layer of soil—is usually two to three inches deep, but this depth can vary considerably. For instance, river deltas like the Mississippi River delta have deep layers of topsoil. Topsoil is rich in organic material; microbial processes occur there, and it is the “workhorse” of plant production. The A horizon consists of a mixture of organic material with inorganic products of weathering, and it is therefore the beginning of true mineral soil. This horizon is typically darkly colored because of the presence of organic matter. In this area, rainwater percolates through the soil and carries materials from the surface. The B horizon is an accumulation of mostly fine material that has moved downward, resulting in a dense layer in the soil. In some soils, the B horizon contains nodules or a layer of calcium carbonate. The C horizon, or soil base, includes the parent material, plus the organic and inorganic material that is broken down to form soil. The parent material may be either created in its natural place, or transported from elsewhere to its present location. Beneath the C horizon lies bedrock.

Some soils may have additional layers, or lack one of these layers. The thickness of the layers is also variable, and depends on the factors that influence soil formation. In general, immature soils may have O, A, and C horizons, whereas mature soils may display all of these, plus additional layers.

 In the photo, soil has been cut away to reveal the soil profile. The O horizon is at the soil surface and is a rich black color. The brown A horizon starts beneath the O horizon and extends to about two-and-a-half feet beneath the surface. The B horizon is reddish brown and extends from the bottom of the A horizon to about two feet deep. The C horizon extends from the bottom of the B horizon to the bottom of the photo at a depth of four feet. The C horizon is light brown and has a coarser consistency than the A or B horizons.
The San Joaquin soil profile has an O horizon, A horizon, B horizon, and C horizon. (credit: modification of work by USDA)

Source:

Clark, M., Douglas, M., Choi, J. Biology 2e. Houston, Texas: OpenStax. Access for free at: https://openstax.org/details/books/biology-2e

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