Soils of the United States
The US soil taxonomic hierarchy includes orders, suborders, great groups, subgroups, families and series, with each series representing a unique kind of soil. In the United States, over 19,000 soil series have been identified. The percentages of land area (in the US and associated territories, etc.) occupied by soils of the twelve orders have been estimated as:
Alfisols and Inceptisols occur widely in the US, but the circumstances of their occurrence would be difficult to characterize briefly. The Alfisols have a subsurface ("B") horizon characterized by phyllosilicate clay accumulation (suggesting illuviation of such clay from above). The Inceptisols have a weakly developed B horizon as a consequence of weathering and/or other processes.
Andisols are found in areas where soils have formed in certain kinds of volcanic ejecta (usually pumice and/or volcanic ash).
Aridisols occur in parts of the western United States which are too dry for the growth of mesophytic plants.
Entisols, which exhibit little soil profile development, are characteristic of areas where soil parent materials have quite recently been deposited, e.g. on recent river alluvium.
In the US, Gelisols occur only in parts of Alaska; they are characterized by having permafrost within 100 cm of the surface.
Histosols are organic soils lacking permafrost within 100 cm of the surface; they are characteristically formed on wet sites, e.g. bogs, some fens and some muskeg areas. Some Histosols have been drained, especially to permit cultivation.
In the US, Mollisols occur mostly on the Great Plains, and in some areas of the west. There is a considerable variety of Mollisols, including soils very closely resembling the Chernozem ("black earth") of eastern Europe (parts of Russia, the Ukraine and neighboring regions), and the Chernozemic soils of the Canadian prairies.
Oxisols occur only in tropical environments, which have very limited extent in the US.
Spodosols often occur under coniferous forest in cool, moist climates, such as southeastern Alaska, the Great Lakes region, the northeastern states, and higher elevations of the northwestern states. Spodosols are also found in warm, moist environments such as Florida and in fact are the most prominent soil order of the state. Spodosols have a B horizon containing a relatively high concentration of illuviated aluminum with accompanying illuviated organic matter, and in many cases, illuviated iron. Such horizons form under certain acidic leaching conditions influenced by acid decomposition products of litter accumulations under certain tree and/or shrub species. The Spodosols correspond to the Podzols of Russia, Central Europe and Northern Europe and to the Podzolic soils found in much of Canada's boreal forest.)
Ultisols are rather extensive in warm, humid regions of the US. They tend to represent rather advanced soil development, and thus are found on relatively old land surfaces.
Vertisols are not extensive in the US, being confined to areas where there is a great abundance of swelling clays, e.g. montmorillonite, that cause churning of soils as a consequence of wetting and drying cycles.