Introduction - Soil Geography of the USA: A Diagnostic-Horizon Approach - page 5

Table 1.3 (continued)
Subsurface
horizon Subgroup b Order Suborder Great group

Petrogypsic Aridisols Aridisols, Gelisols

Placic Andisols,

Spodosols

Andisols, Inceptisols,
Spodosols

Salic Alfisols (Salidic), Gelisols,

Inceptisols (Salidic),
Mollisols (Salidic)
Sombric Oxisols, Ultisols Ultisols
Spodic Spodosols Andisols, Entisols, Gelisols,
Inceptisols, Ultisols
a Dominant subsurface horizon; from descriptions of suborders in ST (Soil Survey Staff 1999 )
and KST (Soil Survey Staff 2010 )
b Subgroup name shown in parentheses only for extragrades

Aridisols Aridisols,

Vertisols

Table 1.5 shows use of diagnostic subsurface horizons among the 12 order in ST.
Subsurface horizons used most commonly for the various orders at different levels
include the argillic (14), duripan (13), calcic (12), kandic (11), natric (9), and others
with 8 or less. The orders containing the greatest number of diagnostic subsurface
horizons at different levels include the Aridisols (22), Alfisols, Inceptisols and
Mollisols (16 each), Ultisols (11), Andisols and Spodosols (10 each), and others
with 8 or less. Diagnostic subsurface horizons currently are used 6, 9, 37, and
73 times at the order, suborder, great-group, and subgroup levels, respectively.

1.1 Diagnostic Horizons and the Soil-Forming Factors

In discussing the role of soil-forming processes in modern soil taxonomic systems,
Bockheim and Gennadiyev ( 2000 ) showed how the diagnostic horizons originated
from the progression of soil-forming factors to soil-forming processes. Soil climate
is the most important factor in ST. It is used at the highest level to define two of the
12 soil orders, including the Aridisols and the Gelisols, and at the suborder level in
eight of the remaining orders (Table 1.6 ). Parent material is used to fully define two
orders, the Histosols (organic materials) and the Andisols (andic materials), and
partially to define two suborders in the Entisols (Fluvents, Psamments). Only one
group of organisms, the Annelidae (worms), is used at the great-group and subgroup
levels in several orders. Time is used indirectly in pale- great groups within the
Alfisols, Aridisols, Mollisols, Oxisols, and Ultisols. Relief is not used in defining
taxa in ST except at the phase level for percent slope.
Three of the eight epipedons are defined on the basis of parent material (folistic,
histic, melanic), two from human activities (anthropic and plaggen), and two from
the interaction of climate and vegetation (mollic and umbric). It is difficult to assign
the diagnostic subsurface horizons to one or more soil-forming factors.