Soil Orders

Alfisols

are a soil order in USDA soil taxonomy. They form in semiarid to humid areas, typically under a hardwood forest cover. They have a clay-enriched subsoil and relatively high native fertility

Spodsols

are acid soils characterized by a subsurface accumulation of humus that is complexed with Al and Fe. These photogenic soils typically form in coarse-textured parent material and have a light-colored E horizon overlying a reddish-brown spodic horizon.

Oxisols

are an order in USDA soil taxonomy, best known for their occurrence in tropical rain forest, 15-25 degrees north and south of the Equator. They are classified as ferralsols in the World Reference Base for Soil Resources; some have been previously classified as laterite soils.

Vertisols

are clay-rich soils that shrink and swell with changes in moisture content. During dry periods, the soil volume shrinks, and deep wide cracks form. The soil volume then expands as it wets up

Ultisols

are strongly leached, acid forest soils with relatively low native fertility. They are found primarily in humid temperate and tropical areas of the world, typically on older, stable landscapes.

Mollisols

are the soils of grassland ecosystems. They are characterized by a thick, dark surface horizon. This fertile surface horizon, known as a mollic epipedon, results from the long-term addition of organic materials derived from plant roots.

Salinization

Process by which water-soluble salts accumulate in the soil

Calcification

Process occurs in warm, semi-arid environments, usually under grassland vegetation. Soil tends to be rich in organic matter and high in soluble bases. The B horizon of the soil is enriched with calcium carbonate precipitated from water moving downward through the soil, or upward by capillary action of water from below.

Podsolization

Process that occurs in cool and moist climates under pine forests. They are typical of the colder portions of the humid continental and subarctic climates. The E horizon is heavily leached and basically composed a of light colored layer of sand.

Elluviation

Refers to the downward movement or loss of dissolved or suspended material within soil by leaching (i.e. salts, nutrients and silicate clays. This leaves the topsoil or A horizon and creates the E horizon. The removal of material from a soil layer

Aridosols

(or desert soils) are a soil order in USA soil taxonomy. They form in an arid or semi-arid climate. They dominate the deserts and xeric shrublands, which occupy about one third of the Earth's land surface.

Laterization

Also known as tropical weathering, is a prolonged process of chemical weathering which produces a wide variety of resulting soils. The deep red to bright orange-red soils of the tropics are a product of this. Occurs in the hot, rainy tropics where chemical weathering proceeds at a rapid rate. Soils subject to this tend toward the acidic and lack much organic matter as decomposition and leaching is extreme. Exposure of the soil to the hot tropic sun by deforestation bakes the soil dry, reducing infiltration, increasing runoff, and reducing fertility.

Illuviation

Materials accumulate in this zone, which is the B horizon underneath the E horizon. Material displaced across a soil profile, from one layer to another one, by the action of rainwater.

Gelisols

They are soils of very cold climates which are defined as containing permafrost within two meters of the soil surface. No B horizon and have an A horizon resting on the permafrost

Inceptisols

They form quickly through alteration of parent material. They are more developed than entisols. They have no accumulation of clays, iron oxide, aluminium oxide or organic matter

Histosols

soil consisting primarily of organic materials. Organic soil material has an organic carbon content (by weight) of 12 to 18 percent, or more, depending on the clay content of the soil. These materials include muck (sapric soil material), mucky peat (hemic soil material), or peat (fibric soil material). Aquic conditions or artificial drainage are required.Typically, they have very low bulk density and are poorly drained because the organic matter holds water very well. Most are acidic and many are very deficient in major plant nutrients which are washed away in the consistently moist soil.

Andisols

soils formed in volcanic ash and defined as soils containing high proportions of glass and amorphous colloidal materials, including allophane, imogolite and ferrihydrite.

Entisols

soils that do not show any profile development other than an A horizon. This soil has no diagnostic horizons, and most are basically unaltered from their parent material, which can be unconsolidated sediment or rock.