In 1979, the USDA standardized soil types, refining the categories until they established the current 12 classes. This standard, referred to as "Keys of Soil Taxonomy," is the most widely used soil classification method in the world. These categories give researchers a universal language when identifying and solving agricultural and ecological problems.
Arguably the most productive soils in the world, mollisols form from grassland plants growing and decomposing over long periods of time. The prairies of the Great Plains are mollisols, supporting the fields of North Dakota and stretching south into Oklahoma. At 20 percent of the U.S. land area, mollisols are the largest soil type in the United States and are valuable farmland.
Alfisols have high fertility and usually form under temperate hardwood forests. They consist of organic matter and clay, and have value for agricultural and timber production. Much of the land south of the Great Lakes region consists of alfisols.
The famous red clay of the Southeast is an ultisol. Compared to other soils, ultisols are not very fertile and are severely weathered, leached of potassium and other minerals. They do have iron oxides, which give the yellow or red cast to the clay. The vegetation-friendly climates allow heavy forest growth, but ultisols are not very productive unless heavily fertilized.
Underdeveloped inceptisols do not have the layers of organic matter and minerals that other soil types do. This soil type supports the largest percentage of the world's population, however, and is found in a variety of ecological areas. Most inceptisols exist in mountainous territory and find use as forested recreational areas and watershed.
Much of the dry Southwest consists of aridisols--soil with layers of calcium carbonate, clay, silica and gypsum. Unless irrigated, landowners leave aridisols as range land, wildlife habitat and outdoor recreation areas.
High in clay, vertisols crack and shrink during drought and swell with rain. This movement makes vertisols unsuitable for building, and the hard clay is resistant to both plant roots and farmer's plows. They are most often found in Texas.
Spodosols are high in iron-laced humus and are not very fertile. However, spodosols support large stands of timber and old-growth forest. They commonly occur underfoot in cool, humid regions, especially in conifer forests.
Volcanic activity in the northwestern United States created andisols--soils composed of volcanic glass, ash and other ejected minerals alongside organic matter. Andisols conserve water and support large forests, but make up less than 1 percent of the Earth's ice-free soil.
Peat bogs and wetlands make up the histosol category, where up to 30 percent of the soil is organic matter. The slow-draining soil preserves the organic debris, and thick layers build up over time. Histosols are not useful from an engineering standpoint, as they subside under weight, but they are a source of peat for fuel and garden use.
Gelisols exist at high elevations and polar latitudes, where permafrost occurs 2 meters or less from the surface. Organic matter in gelisols decomposes very slowly, making them second only to wetland soil for organic content. Despite the carbon-rich soil, the extreme climate prevents gelisols from human use, and gelisols support the lowest percentage of the Earth's population.
The poor soils under equatorial rainforests and jungles are often oxisols. Rain and rivers have leached nutrients from this soil type, and most of the valuable nutrients reside in the living and decomposing plants the soil supports. Highly fertilized oxisols are agriculturally productive, but their value lies in the rainforest above them.
The most widespread soil type, sandy entisols occupy roughly 18 percent of the world's land mass. They are the youngest soils and are very diverse, located from rocky, steep slopes to alluvial plains. River valleys and shoreline soil deposits are usually entisols, and they provide good cropland and general utility.