Tutorial:USDA
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[edit] USDA Soil Classification
The USDA soil classification scheme uses a heirarchical system of taxonomy based on:
- Soil Orders
- Soil Suborders
- Soil Great Groups
- Soil Subgroups
- Soil Families: Family names are descriptive and relating to the property used to differentiate between families in the same sub-group.
- Soil Series: This divides families according to properties that may affect the use of the soil.
The first 4 levels relate to the dominant soil properties and the presence of diagnostic horizons.
The properties of the 12 soil orders are outlined in the table below.
| Alfisols | Found in semi-arid to moist areas, alfisols are the result of weathering processes that leach clay and other constituents out of the surface layer and depositing them in the sub-soil. |
| Andisols | These parent materials for these soils are volcanic, particularly tephra. |
| Aridisols | These soils are common in desert areas and are too dry for normal plant growth. They often contain accumulations of salt, gypsum and calcium carbonate. |
| Entisols | These occur in areas where there has been recent sediment deposition, e.g. dunes or floodplains, where there has been insufficient time for the development of soil horizons. Entisols are young soils. |
| Gelisols | These are found at high latitudes or elevations and are soils affected by permafrost and/or cryoturbation within a metres depth of the surface. |
| Histosols | These are organic or organic rich soils formed from partially decomposed plant remains. |
| Inceptisols | These soils are found in a variety of climates and have a wide-range of possible characteristics and hence have few diagnostic features. Almost all soil processes, such as weathering and illuviation, may be operating but not to such an extent that diagnostic horizons develop. |
| Mollisols | These are typically found under grass in areas that have a pronounced dry season. Mollisols have a deep (typically 60-80 cm), dark, fertile, organic rich topsoil. |
| Oxisols | These are found in tropical and subtropical regions. They are highly weathered soils having formed in stable landsurfaces and are typically red or yellowish in colour. |
| Spodosols | They are often found in sandy parent materials under coniferous forest in humid areas. They are acid soils formed by the downprofile movement of iron, aluminium and organic matter. Typically they have grey horizon from which organic matter and iron have been leached which overlies a black/brown and/or reddish brown spodic horizon where the organic matter and iron accumulates. |
| Ultisols | They are found in humid temperate areas and have a sub-surface clay rich horizon derived from weathering or from material washed down the profile. |
| Vertisols | These soils contain a high proportion of clay minerals that sweel when wet causing the soils to heave and crack periodically. |
The USDA provide free, on-line access to all the literarture and keys necessary to identify soil types. Use the external links below to access these resources.
[edit] External Links
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