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Structured Geological Glossary: Soils

soil In a geologic sense of the term, a weathered horizon at the earth's surface. During periods of landscape stability, various physical and chemical processes modify the upper few meters of the surface. Typically, new mineral assemblages such as clay are introduced, as is organic matter, and calcium carbonate if the climate is dry. Once a soil is buried by other sediment, it is then referred to as a paleosol.
catena Sequence of proximately located soil of about the same age and derived from similar parent material but having different characteristics due to variation in relief and/or drainage.
regolith Any solid material lying on top of bedrock. Includes soil, alluvium, and rock fragments weathered from the bedrock.
regolith reduction Diminishment in volume and mass of parent material to a smaller volume and mass of weathered soil residue as mobile constituents are carried away, in solution or by aeolian transport.
soil profile Vertical section of the soil through all of its distinct layer (horizons) produced by the soil-forming process and extending into the parent material.
grenz A soil horizon, frequently marked by a bed of clay that results from a temporary halt in the accumulation of vegetal material.
A horizon The top layer of soil. It is usually dark and contains organic material formed do to the decay of vegetable matter.
micromorphology Texture of a soil as viewed in petrographic thin section.
B horizon The subsoil. This layer lies below the topsoil and contains more clay and iron oxides than either the A-horizon or the C-horizon.
B-horizon The intermediate layer in a soil, situated below the A-horizon and consisting of clay and oxides. Also called the zone of accumulation.
A-horizon The top layer of soil. Plant and other organic debris builds up in this layer. This is the part of the soil generally referred to as 'top soil'.
C-horizon The lowest layer of soil, consisting of fragments of rock and their chemically weathered products.
calcic horizon In regions that receive about 1.3 m of rain or less, calcium carbonate will often accumulate in the lower portion of a soil profile. In fine-grained soils, such as under the prairies of the mid-western United States, the carbonate often takes the form of discrete nodules up to several centimeters in diameter. In gravelly soils such as are common in the western U.S., carbonate will begin by coating clast, and with time it will entirely plug soil voids, forming what is commonly called a caliche or more formally a calcic horizon.
caliche Nodular calcium carbonate (opaque reddish-brown-to-white) that accumulates in the B-horizon ot soil in warm climates that are dry part of the year. Also called calcrete.
translocation crossover Soil depth at which mass accumulation of an element by downward translocation changes from positive to zero and then to negative, thereby separating an upper soil system dominated by invasion of foreign detrital mineral from a lower, less contaminated, soil system referred to as saprolite.
pedocal A common soil type of arid regions, characterized by accumulation of calcium carbonate in the A-horizon.
leaching The removal of elements from a soil by dissolution in water moving downward in the ground.
pedalfer A common soil type in humid regions, characterized by an abundance of iron oxides and clay mineral deposited in the B-horizon by leaching.
podzolization Soil-forming process typified by formation of humic acids and leaching of acids and sesquioxides. Continuum of soil developed range from peat, podzol soils (upper mineral soil bleached nearly white by organic acids) to podzolic soils where leaching of sesquioxides is insufficient to bleach mineral soil.
duricrust Indurated soil crust or hardcap occurring on or near an eroded planation surface. May be indicative of climatic change from tropical to drier conditions with alternating wet and dry seasons. Contains tubular voids acting as avenues of translocation.
durable crust An outer rind or crust formed on a rock. Durable crusts form when rock chemically reacts with water and possibly atmospheric dust, producing a hard outer surface that resists weathering.
pisolites Concretionary spherical nodules formed by transformation of duricrust by accretionary growth and dilation in the uppermost laterite zone.
bauxite " A rock composed primarily of hydrous aluminum oxides and formed by weathering in tropical areas with good drainage; a major ore of aluminum. "
lateritization Soil-forming process typical of warm humid climates where in mature landscapes, primary mineral are essentially completely weathered away to form red soil (laterites) of high aggregate stability composed primarily of sesquioxides (oxides and hydroxides of iron and aluminum).
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