Geoscience Reference
In-Depth Information
Chapter 6
Umbric Epipedon
6.1
Introduction
Along with the ochric and mollic epipedons, the umbric epipedon is one of the most
common diagnostic surface horizons employed in
Soil Taxonomy
. In ST the umbric
epipedon is defined as follows: (1) a thickness of
10-
25 cm (commonly
18 cm), depending on texture; (2) a moist color value and chroma of
3; (3) a
base saturation (by NH
4
OAc) of
50 %; (4) a SOC concentration of
0.6 %; (5) a
<
phosphate concentration of
1,500 mg per kg (by citric-acid extraction); (6) an
<
n
value of
0.7; and (7) a lack of artifacts, spade marks, and raised surfaces that are
characteristic of the plaggen epipedon. In the
Seventh Approximation
(Soil Survey
Staff
1960
), soils with an umbric epipedon were recognized in the Umbrepts
suborder. The WRB recognizes soils with umbric epipedons at the highest level
in the Umbrisols. Soils with umbric epipedons have been characterized in CA
(Popenoe et al.
1992
), Pennsylvania (PA; Waltman and Ciolkosz
1995
), Wisconsin
(Quideau and Bockheim
1996
), and North Carolina (NC; Richter et al.
1989
;
Vepraskas and Caldwell
2008
).
<
6.2 Properties of Soils with Umbric Epipedons
Figure
6.1
shows a soil containing an umbric epipedon from a Humicryepts in the
subalpine zone of west-central Rocky Mountains, Colorado (CO). Laboratory data
are provided for six soil series with umbric epipedons in Table
6.1
. The umbric
epipedons are highlighted in boldface. The mean thickness of the umbric epipedon
for the six pedons is 32 cm, the mean SOC concentration is 4.7 %, and the mean
base saturation is 41 %.
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