Classifications: International WRB and U.S. Soil Taxonomy - Major Soil Groups of the World - page 133

Again, the definitions provided are very much simplified. Let us give,
as example, the almost complete definition of the argillic horizon:

An 'argillic' horizon is normally a subsurface horizon with a
significantly higher percentage of phyllosilicate clay than the overlying
soil material. It shows evidence of clay illuviation. The argillic horizon
forms below the soil surface, but it may be exposed at the surface
later by erosion (USDA 2006). Morphological, micromorphological and
analytical criteria for identification are summarized below.
Conditions of thickness : the horizon whose classification is considered
to be 'argillic' must have a minimum thickness of x cm (there are
several values of x according to the texture and content of silt and
clay, and the structure of the horizon to be classified).
Conditions relating to signs of illuviation : it is necessary to observe
in the presumably 'argillic' horizon oriented clay or clay films on the
peds or in the pores or to determine that it is proportionately richer
in fine clay where the soil is affected by disturbances that destroy
the signs of illuviation.
Conditions of clay content : this involves verifying that the horizon
presumed to be 'argillic' is richer in clay by y % than the eluvial
horizon above it; there are several values of y according to the clay
content of the said eluvial horizon.

In addition to the diagnostic horizons, the American taxonomy
distinguishes diagnostic properties that do not necessarily pertain to
identified horizons. For example, an abrupt textural change may
involve two superposed horizons at the same time. These diagnostic
characteristics are grouped into three sets: those that apply to mineral
soils (Table 4.11), those that apply to organic soils (Table 4.12) and those
that can involve all soils (Table 4.13).

Diagnostic characteristics

The Orders are presented in the form of a determinative key (Table 4.14).
The user checks if the soil to be classified has the properties of the first
Order listed, then the second, and so on. Once again, the definitions
given here are simplified. They are sufficient for understanding the
methodology but are too brief to actually classify a soil. The names of
the Orders provide the suffixes that are used to compose the names of
the various taxa (see the following section).

Presentation of the Orders