Conclusions - Soil Geography of the USA: A Diagnostic-Horizon Approach - page 303

Table 26.1 (continued)
No. of soil
series

%of
total Dominant orders

Placic 7 0.0 Andisols, Inceptisols Spodosols

>

Petrogypsic 2 0.0 Aridisols

Agric 0 0.0 -

Sombric 0 0.0 -

Total 21,316 100.0

considered a subsurface horizon, it often exists at the surface, especially in
Spodosols and Alfisols. The duripan, spodic, glossic, and fragipan horizons each
accounted for less than 4 % of the soil series. The natric, gypsic, and salic accounted
for less than 1 % each of the soil series. The oxic horizon was present in only 64 soil
series representing the Oxisols in the USA and its territories. The petrocalcic,
placic, and petrogypsic occur to a very limited extent in the USA. The agric and
sombric horizons have not been used in the USA.

26.2 Soil-Forming Factors and Diagnostic Horizons

Climate plays an important role in differentiating among the diagnostic horizon but
only in a general. The following horizons form where precipitation (P) exceeds
evapotranspiration (ET): histic, argillic, spodic, fragipan, oxic, ortstein, placic,
sombric (Table 26.2 ). The following horizons form where P < ET: calcic, duripan,

salic, gypsic, petrocalcic, petrogypsic.
Vegetation is key to the occurrence of diagnostic horizons, especially the
epipedons, including grassland (mollic, duripan), peatland (histic), forest (folistic,
kandic), coniferous forest (spodic, ortstein, placic), and tropical forest and savanna
(oxic horizon) (Table 26.2 ). Relief is important in that histic, albic, salic, and placic
horizons often occur in depression or other areas of restricted drainage; folistic,
gypsic, oxic, and petrogypsic appear to require well-drained conditions; and
ortstein and plinthite develops where there is a seasonally high water table. Differ-
ences in parent material influence diagnostic horizons in that organic-rich materials
yield histic and sombric horizons; base-rich materials yield mollic, calcic, and
petrocalcic horizons; base-depleted materials result in umbric and ochric horizons;
high-activity clays yield argillic horizons; low-activity clays produce kandic hori-
zons; silica-rich materials feature melanic and duripan horizons; iron-rich materials
form spodic, placic, and ortstein; and sodium-rich materials yield natric horizons.
The time required for diagnostic horizons to form reflects the overall soil
evolutionary process. Anthropogenic (anthropic, plaggen) and ochric epipedons
that may form is less than 0.1 kyr; major epipedons, such as mollic, umbric, folistic,
and histic, require 0.5 kyr; and the melanic epipedon may require 3.0 kyr to

> >

form (Table 26.2 ). Some subsurface horizons form rather quickly ( 0.1 kyr), such

<