How soils are classified. Pedologists classify soils according to the characteristics of a polypedon. The Soil Survey Staff of the United States Department of Agriculture uses a system that consists of 10 orders (groups) of soils. They are (1) alfisols, (2) aridisols, (3) entisols, (4) histosols, (5) inceptisols, (6) mollisols, (7) oxisols, (8) spodosols, (9) ultisols, and (10) vertisols.
- Alfisols develop under forests and grasslands in humid climates. Some agricultural soils are alfisols.
- Aridisols occur in dry regions and contain small amounts of organic matter. Desert soils are aridisols.
- Entisols show little development. They resemble the parent material and occur in many climates.
- Histosols are organic soils. They form in water-saturated environments, including swamps and bogs.
- Inceptisols are only slightly developed. They are more common in subhumid and humid climates, but also occur in most other kinds of climates.
- Mollisols develop in prairie regions. They have thick, organically rich topsoils.
- Oxisols are the most chemically weathered soils. They have a reddish color and occur in tropical regions.
- Spodosols contain iron, aluminum, and organic matter in their B horizons. They form in humid climates.
- Ultisols occur in warm, humid climates. They are moist, well-developed, acid soils.
- Vertisols form in subhumid and arid warm climates. They develop wide, deep cracks during dry seasons.
