Geoscience Reference
In-Depth Information
Chapter 3
Anthropic and Plaggen Epipedons: Products
of Human Disturbance
3.1
Introduction
A summary of the general requirements for diagnostic epipedons is given in
Table
3.1
. Epipedons are not to be confused with master soil horizons, such as
the A horizon, as they may also include E and B horizons. It should also be noted
that three additional terms are used for variations in epipedons, including thaptic,
cumulic, and pachic. Thaptic is used strictly for Andisols where burial of mollic-
like epipedons has occurred below a depth of 25 cm. The soil organic carbon (SOC)
content of the thaptic horizon must be
3 %. The cumulic horizon is thick (
40 cm
>
to
60 cm), may qualify as a mollic or umbric epipedon, and has an irregular
decrease in SOC below 25 cm usually from tephra (volcanic ejecta) deposition.
This horizon is used only in Gelisols, Inceptisols, and Mollisols. The pachic horizon
may be umbric or mollic, is thick (
>
40 to
60 cm), contains abundant SOC
(
6 %), and is used only in the Andisols, Inceptisols, and Mollisols.
The anthropic and plaggen epipedons are distinguished from the other epipedons
by having been significantly disturbed through human activities. Both epipedons
are mineral in origin, have dark colors, have abundant SOC, and are moderately
thick. A main difference between the two epipedons is that the anthropic epipedon
has abundant citrate-extractable phosphate and a low
n
-value (the
n
-value charac-
terizes the relation between the percentage of water in a soil under field conditions
and its percentages of inorganic clay and humus. The
n
-value is helpful in
predicting whether a soil can support other loads and in predicting what degree of
subsidence would occur after drainage) and the plaggen contains either a raised
surface, artifacts, or spade marks. In addition, the plaggen is considerably thicker
(
25 cm, depending on texture).
The anthropic epipedon is recognized in ST at the suborder level (Anthrepts),
great-group level (Anthracambids), and subgroup level (Anthropic Torrifluvents,
50 cm) than the anthropic epipedon (
10 to
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