Soil Orders

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. Six of these that are commonly encountered are described below.

Alfisols

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.

Aridisols

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.

Inceptisols

Usually wet during the growing season, these young soils are greatly variable. They often produce well when amended.

Mollisols

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.

Spodosols

With a high sand content, these soils are usually moist and moderately to strongly acidic. Add lime and fertilizers.

Ultisols

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.