The first soil categories were developed in the 1930s by Curtis Marbut, according to Physical Geography. More modern categories were developed by the U.S. Soil and Water Conservation Service in the 1950s. This system, which wasn't complete until 1960, was an improvement but still lacked the credibility and standards that were necessary. Currently, the Natural Resource Conservation Service under the direction of the Department of Agriculture has soil standards and maintains this department.
The current soil division contains 12 groups of soils, according to the Natural Resources Conservation Service. These groups are further broken down into soil types such as clay, loam and sand. Other regions of the world have varying soil types and a differing classification system.
Alfisols & Mollisols
Alfisol soil is generally found stretching from Minnesota to eastern Texas. This soil develops under forest vegetation where the material has undergone severe weathering. There is an illuviation of clay in the deeper horizon with a light colored surface.
Mollisol soil in most areas has been converted to agricultural fields for crop growth, but thrives in grassland environments, such as the Midwest. Spreading from the Midwestern states to the coast of California, Mollisol soil can be located under grass or vegetation.
Ultisols & Spodosols
Ultisol soil is located primarily in the Southeastern United States. This soil thrives in warm temperatures, and abundant moisture enhances the weathering process and improved leaching of the soil. This soil can be found formed under forest vegetation.
Spondosol soil, like Ultisol soil, can be found under vegetation, but more specifically under coniferous vegetation. It is modified by podzolization and is rich in sand. There is little clay and a small amount of humus in the soil.
Andisols & Ardisols
Andisol soil develops from a volcanic parent material. The unique process of weathering causes an accumulation of allophane in the developing soil. Andisol soil is most generally found in the Northwestern United States and southern Alaska.
Ardisol soil, like Andisol, develops in dry environments. The soil horizon is poor and shallow, leaving the soil light colored. This is primarily because of the limited humus additions from vegetation.
Entisol & Inceptisol
Entisol soil is an immature soil with a lack of developed vertical horizons. There are no definite layers to the soil. This is associated with recently developed sediment from wind, water and ice erosion. In time, weathering will turn the soil. It is found spread across the United States in various parts of the country.
Inceptisol soil is a young soil developed from Entisol soil. It is found primarily in the Alaskan Arctic tundra region. Glacial deposits have only begun the weathering process of this young soil.
Histol & Oxisol
Histol soil is found in the northern Midwest and southern Florida. This organic soil forms in areas with poor draining, making it a wet moist soil. There is a thick organic material base because of decomposition of materials around the soil.
Oxisol soil is found in the tropical regions of Hawaii and Puerto Rico. This wet soil develops in areas with high precipitation and high temperatures. The Oxisol soil group encompasses quartz, clay and iron.
Gelisol & Vertisol
Gelisol soil is generally found across Alaska. It thrives in high altitude areas and develops from permafrost on the surface. Mainly consisting of organic or mineral materials, Gelisol soil can encompass both.
Vertisol soil is a heavy clay soil. This thick soil results from a lack of or excessive moisture. It is common in areas with shale parent material and heavy precipitation. This soil is rampant in the Texas area where cotton growth was popular because this crop grows well in a hard soil. l