Classifications: International WRB and U.S. Soil Taxonomy - Major Soil Groups of the World - page 130

Taxonomy . Several editions followed. One has to be careful, for they often
coexist on the Internet. Here we rely on the tenth edition (USDA 2006).
For finding the version, search the USDA and NRCS ( Natural Resource
Conservation Service ) sites.
The Americans invented the idea of 'diagnostic' horizons. These
are typical horizons, very precisely described. The presence of any of
them is enough, in principle, for classifying a soil at the highest level
in the taxonomy. Although it might be backtracking a little to present
the details before the main point, we will have to first study these
diagnostic horizons since we cannot enter the system of classification
without passing through that stage.

4.3.2 Structure of the U.S. Taxonomy

The taxonomy comprises four principal levels (Table 4.8). The numerals
within brackets refer to the number of taxa found in each. But the
American soil classification includes two lower subdivisions:
—firstly, Families characterized by physical and chemical properties
identical at the planning level;
—secondly, some 17,000 Series characterized by specific soil profiles,
named by the place where they were first observed or studied, for
example, 'Nashville series'.

The hierarchical levels

Table 4.8 Structure of the American taxonomy.

Orders (12)

defined by diagnostic horizons representing a

soil-forming process (e.g. argillic horizon)

Suborders (60)

Correspond to the moisture regime of the soil or to

environmental conditions: climate, geology, vegetation

Great Groups (more than 300)

Defined by an important property that could be presence

of another less-differentiated diagnostic horizon

Subgroups (more than 1200)

Supplementary information, for example typic soil for

all properties, or soil having some characters of another taxon