Soil and Soil Horizons
Soil is found in the top layers of regolith, the unconsolidated (uncompacted) matter comprised of soil, sediment, and portions of bedrock that form the outer crustal layer of the Earth's surface. Soil includes varying amounts of organic matter mainly derived from plants and animal decay.
Because soil is a superficial layer, it can be highly variable and has a composition that can be readily modified by weather (e.g., rainfall).
Soil is usually found in stratified layers (i.e. a layer of black soil over subsurface layers of sand and/or clay. Although different for each geographic area, geologists use a generic soil model from which to describe unique area differences.
A soil horizon is a coherent layer of soil—similar in characteristics such as composition, texture, and color—that define the horizon from other soil types. Geologists construct soil profiles of an area by describing the various soil horizons (horizontal layers) that exist within a vertical column of soil. Soil profiles, descriptive of the type and relation of soil horizons, are unique to different geologic and climatic areas.
The outermost (most superficial) soil horizon in a soil profile is termed the "A" horizon. Accordingly, "B," "C," and "D" horizons indicate successively deeper layers. A typical soil profile might then consist of vegetation (not strictly a part of the soil horizon), overlying an "A" horizon of humus (zone of leeching), overlying a "B" horizon of regolith (Zone of accumulation) that was superficial to a layer of bedrock.
The zone of leeching is so defined because water is able to percolate through the horizon.
Within the United States, in areas of the Eastern United States with a temperate, humid climate with adequate rainfall (generally defined as greater than 24 inches of rain per year) the soil profile typically consists of forest vegetation growing on the "A" horizon zone of leeching that consists of thick humus and sand. The underlying "B" horizon comprising the zone of accumulation is often clay that is rich in iron or aluminum. The "C" horizon is regolith and the "D" horizon is bedrock. A unique feature of this soil profile is that the mineral calcite (CaCO3) often leeches out of the soil. The loss of this natural buffer allows the soil to become acidic.
In the Western United States—with a more arid climate—a much sparser layer of vegetation (an eventual contributor of organic material) covers an "A" zone of leeching horizon of thin humus and unaltered silicates. The "B" horizon (zone of accumulation) is composed of caliche. As with the Eastern profile the "C" and "D" horizons are regolith and bedrock. Caliche is rich in calcite because the calcium carbonate in the upper soil becomes briefly dissolved in the sparse rains that then wash the calcium carbonate down to the caliche layer.
A tropical soil profile features lush vegetation overlying a "B" horizon layer of accumulation that is rich in bauxites and iron oxide. In the tropical soil profile, the "A" horizon may be missing or just a few centimeters thick. In this thin layer, there is a rapid turnover of organic decay and decomposition. All the quartz or clay elements are generally "weathered out" and the loss of calcite severe.
Leaching; Mass Wasting; Porosity and Permeability; Rate Factors in Geologic Processes; Runoff; Sedimentation; Stratigraphy
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