Soil scientists classify soils into groups much as biologists group plants and animals. Individual soils are grouped into series, series into families, and so on until the largest grouping is reached—that of orders. The United States Department of Agriculture recognizes 12 soil orders.
12 Soil Orders
Soils of very cold climates that contain permafrost within 2 meters of the surface. These soils are limited geographically to the high-latitude polar regions and localized areas at high mountain elevations. Because of the extreme environment in which they are found, Gelisols support only ~0.4% of the world’s population – the lowest percentage of any of the soil orders.
Soils that are composed mainly of organic materials. They contain at least 20-30% organic matter by weight and are more than 40 cm thick. Bulk densities are quite low, often less than 0.3 g cm3.
Soils that have formed in volcanic ash or other volcanic ejecta. They differ from those of other orders in that they typically are dominated by glass and short-range-order colloidal weathering products such as allophane, imogolite, and ferrihydrite.
Very highly weathered soils that are found primarily in the intertropical regions of the world. These soils contain few weatherable minerals and are often rich in Fe and Al oxide minerals. Oxisols occupy ~7.5% of the global ice-free land area. In the US, they only occupy ~0.02% of the land area and are restricted to Hawaii.
Clay-rich soils that shrink and swell with changes in moisture content. During dry periods, the soil volume shrinks, and deep wide cracks form. The soil volume then expands as it wets up. This shrink/swell action creates serious engineering problems and generally prevents formation of distinct, well-developed horizons in these soils.
These gray to brown soils over clay subsoil are among the most fertile and productive soils in the country. They may require the addition of lime. Fertilizers and irrigation during dry periods will increase yields.
Soils are dry for long periods with only short periods of wetness, which reduces leaching and may allow accumulation of soluble salts. Arid conditions reduce plant growth and therefore also organic content. When irrigated and fertilized, soils may be very productive.
Usually wet during the growing season, these young soils are greatly variable. They often produce well when amended.
Soils of recent origin. The central concept is soils developed in unconsolidated parent material with usually no genetic horizons except an A horizon. All soils that do not fit into one of the other 11 orders are Entisols. Thus, they are characterized by great diversity, both in environmental setting and land use.
Many Entisols are found in steep, rocky settings. However, Entisols of large river valleys and associated shore deposits provide cropland and habitat for millions of people worldwide.
These dark, fertile soils of grasslands and some hardwood forests are relatively high in humus and nitrogen. They are highly productive but may need lime to correct acidity.
With a high sand content, these soils are usually moist and moderately to strongly acidic. Add lime and fertilizers.
Soils of humid warm regions, ultisols are often acidic and heavily weathered. When managed well, they can be very productive. Fertilizing and liming are needed.