Soil Orders

There are 12 soil orders, and all soils on the planet fall into one and only one of these orders. Orders are based mainly on the soils’ origin and development. You may be familiar with some of the soil orders and not even realize it; the 12 soil orders are:

• Entisols – young soils
• Histosols – characterized by deep accumulation of organic matter and wet conditions
• Gelisols – permafrost
• Inceptisols – characterized by mild weathering; young in age
• Andisols – mildly weathered volcanic soils
• Aridisols – soils of dry climates
• Vertisols – swelling clays of climates with dry seasons
• Alfisols – mildly acidic clays
• Mollisols – soft, dark soils of grassland regions
• Ultisols – strongly acid clays of subtropical forests
• Spodosols – characterized by aluminum accumulation in coniferous forests
• Oxisols – very old soils with extreme weathering and iron and aluminum oxides

Link to more information on each of the soil orders. A map showing the distribution of the 12 soil orders is shown below:

Map courtesy of U.S. Department of Agriculture, Natural Resources Conservation Service.

Much of Arizona is characterized by aridisols, soils of dry climates, and entisols, young soils. The Mogollon Rim region, exhibiting extensive pine forests, is dominated by alfisols, mildly acidic clays often associated with pine forests.

Soil suborders are grouped within each of the 12 soil orders. Suborders are mainly based on the moisture or temperature regime of the region. Each of the subsequent categories in the soil classification scheme falls within the class above it. Great groups are based mainly on the presence of specific characteristics within a soil. Subgroup, family, and series names similarly communicate information about the soil being described. The family level incorporates information on the soil’s temperature regime. An example of a complete soil classification is given below:

• Order Alfisol
• Suborder Xeralf
• Great Group Durixeralf
• Subgroup Abruptic Durixeralf
• Family Fine, mixed, active, thermic Abruptic Durixeralf
• Series San Joaquin