Twelve Soil Orders
Soil Taxonomy is a soil classification system developed by the United States Department of Agriculture’s soil survey staff.
Learn about AlfisolsTwelve Soil Orders
Soil Taxonomy is a soil classification system developed by the United States Department of Agriculture's soil survey staff. This system is based on measurable and observable soil properties and was designed to facilitate detailed soil survey. Although it is not the only system for classifying soils, Soil Taxonomy is widely used worldwide and many of its features have been adopted into other systems.
At the highest level of classification, Soil Taxonomy places soils into one of 12 categories known as “orders.” Each of these orders represents a grouping of soils with distinct characteristics and ecological significance. To illustrate this, we have put together a collection of images and information to highlight the distribution, properties and land use for each of the 12 soil orders.
Alfisols
Alfisols are moderately leached soils that have relatively high native fertility.
Andisols
Andisols (from Japanese ando, "black soil") are soils that have formed in volcanic ash or other volcanic ejecta.
Aridisols
Aridisols (from Latin aridus, "dry") are CaCO3-containing soils of arid regions that exhibit subsurface horizon development.
Entisols
Entisols are soils of recent origin.
Gelisols
Gelisols (from Latin gelare, "to freeze") are soils of very cold climates that contain permafrost within two meters of the surface.
Histosols
Histosols (from Greek histos, "tissue") are soils that are composed mainly of organic materials.
Inceptisols
Inceptisols (from Latin inceptum, "beginning") are soils that exhibit minimal horizon development.
Mollisols
Mollisols (from Latin mollis, "soft") are the soils of grassland ecosystems.
Oxisols
Oxisols (from French oxide, "oxide") are very highly weathered soils that are found primarily in the intertropical regions of the world.
Spodosols
Spodosols (from Greek spodos, "wood ash") are acid soils characterized by a subsurface accumulation of humus that is complexed with Al and Fe.
Ultisols
Ultisols (from Latin ultimus, "last") are strongly leached, acid forest soils with relatively low native fertility.
Vertisols
Vertisols (from Latin verto, "turn") are clay-rich soils that shrink and swell with changes in moisture content.
Image sources and resources
Sources of all images used on this website are noted. Additional information about the soils is derived from the following sources unless otherwise noted.
- Soil Survey Staff. 1999. Soil Taxonomy, 2nd ed. USDA-NRCS, Washington, DC.
- Soil Survey Staff. 2014. Keys to Soil Taxonomy. 12th ed. USDA-NRCS, Washington, DC.
- USDA. 2006. Land Resource Regions and Major Land Resource Areas of the United States, the Caribbean, and the Pacific Basin. U.S. Department of Agriculture Handbook 296. USDA-NRCS, Washington, DC.
- USDA-NRCS. 2018. Official soil series descriptions. Accessed at: https://soilseries.sc.egov.usda.gov
The second edition of Soil taxonomy, a basic system of soil classification for making and interpreting soil surveys (pdf) is available for download or order a printed copy through the Natural Resources Conservation Service, United States Department of Agriculture.