U.S. Soil Taxonomy System

Soil Taxonomy was published by the United States Department of Agriculture's Soil Survey Staff in 1975. While this system for classifying soils has undergone numerous changes since that time, it remains one of the most widely used soil classification systems in the world. At the highest level, Soil Taxonomy places soils in one of twelve categories known as orders. Somewhat analogous to more familiar biological taxonomic systems, the orders are subdivided into more refined categories and so on. Following, each hierarchy is described in some detail.

Order: The broadest category in the system. Distinctions between orders are based largely on horizon morphology, with (unfortunately in some cases) soil genesis as an underlying factor. In general, each order is presumed to contain soils whose common properties suggest similar genesis. There are 12 orders in the taxonomy, but the key hasn't been updated to include the 12th yet. Mississippi has a very diverse physiography, and 8 of the 12 soil orders occur in our state.

Suborder: The suborders are subdivisions of the order based on factors such as wetness, climate (temperature and moisture), mode of deposition, texture, or diagnostic horizons. The number of suborders varies from four to seven within orders.

Great Group: Diagnostic horizons are often used to differentiate great groups within a suborder. For example, the presence or absence of an argillic horizon might distinguish one great group from another. (Argillic horizons are layers with observable clay accumulation in the soil profile. There are many soils in Mississippi containing argillics).

Subgroup: The subgroups are subdivisions of the great groups. The typical, or central, concept of the great group makes one subgroup (Typic). Often, other subgroups are intergrades between the current great group and the central concepts of other great groups (i.e., mollic subgroups of Alfisols).

Family: The family category allows the grouping of members of a subgroup by such things as common texture, mineralogy, pH, soil temperature, coarse fragment content, or soil depth. This level of the system is often the most useful one for interpretations because it is the most descriptive.

Series: Soil series represents a collection of soils essentially uniform in most differentiating characteristics and the arrangement of horizons. This is level most often identified by farmers and the NRCS. Names are usually based on towns near where the soil was first identifed. There are about 700 named soils in Mississippi. Common series in the Mississippi Delta are Alligator, Sharkey, Dundee, Dobbs, and Forestdale. Prentiss, Ruston, Grenada, Memphis, Oktibbeha, and Smithdale are found in the hill portions of the state.