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as the agric, cambic, and albic; salic and placic horizons require
0.5 kyr; spodic
>
and glossic horizons form in
1.0 kyr; argillic and ortstein in
2.0 kyr, calcic,
>
>
natric, and gypsic horizons in
6.0 kyr; kandic horizons in
10 kyr, and fragipans
>
>
in
>
18 kyr. However, horizons such as the sombric (
>
100 kyr), duripan
(
>
130 kyr), petrocalcic and petrogypsic (
>
200 kyr), and oxic (
>
2,000 kyr) require
considerably longer.
26.3 Geographic Distribution of Diagnostic Horizons
26.3.1 Epipedons
Although plaggen and anthropic epipedons have not been used in defining soil
series in the USA, we can project their distribution from an understanding of the
locations of archaeological sites. There are at least 88 such sites in the USA, 20 of
which represent prehistoric or early historical Native American cultures. The oldest
of these sites is the Clovis site in TX which ranges between 13,200 and 15,500 yr
BP. The Archaeological Institute of America shows these sites on Google Earths
The melanic epipedon is most evident in central and eastern WA and OR where
tephra has been deposited by eruptions of the the Cascade stratovolcanoes. The
mollic epipedon, ranked second in abundance behind the ochric epipedon, occurs
primarily in the North American grasslands and savannas. Soils containing mollic
epipedons are particularly common in OR, ND, SD, NE, KS, OK, TX, IA, and IL.
The umbric epipedon is predominant in soils derived from tephra but also from
cool, humid environments. Soils containing this diagnostic horizon occur primarily
in WA, OR, ID, CO, and NC, but they occur in many other US states. The ochric
epipedon prevails throughout the USA but is lacking in areas where Mollisols are
predominant.
The folistic epipedon has been identified in soils from seven states in the USA,
but this epipedon occurs primarily in soils on steep slopes mantled by conifers in
ME and in montane tropical forests of HI. The histic epipedon occurs in soils of
34 states but is most common in peatlands of MN, MI, WI, LA, and FL.
The agric epipedon has not been employed in identifying soil series of the USA.
However, it should be present in the major crop growing areas of the country,
especially the Midwest (SD, NE, MN, IA, WI, IL, IN).
26.3.2 Subsurface Horizons
The albic horizon is particularly common in the Spodosol regions of the USA (WA,
MI, ME, NH, VT, FL). The argillic horizon is the second most abundant diagnostic
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