Geoscience Reference
In-Depth Information
Chapter 7
Ochric Epipedon
7.1
Introduction
The ochric epipedon refers to the “other” category, i.e., to epipedons that do not
have the thickness, color, SOC concentrations, and evidence of human disturbance
of the other seven epipedons.
The ochric epipedon is the dominant epipedon in the following orders: Aridisols,
Alfisols, Entisols, Inceptisols, Oxisols, Ultisols, and in some Spodosols; it likely is
present in more than half of the ~21,000 soil series identified to date in the USA and
its territories. The ochric epipedon is dominant in the Aridisols because of the aridic
or torric soil-moisture regime and generally low quantities of SOC in these soils.
The accumulation of salts in Aridisols also contributes to the colors of high value
and chroma. The Alfisols, Spodosols, and Ultisols often feature a thin A horizon or
an E horizon because of leaching of SOC that results in an ochric epipedon. Oxisols
may contain a thin organic-rich surface mineral horizon, but because of rapid
biogeochemical cycling, the horizon often is of insufficient thickness to be classi-
fied as an umbric epipedon. Entisols are the most poorly developed soils and lack
sufficient “pedogenic inertia” (Bryan and Teakle
1949
) to develop a surface mineral
horizon other than an ochric. Finally, cultivation has led to dramatic decreases in
SOC and may contribute to the prevalence of ochric epipedons in some regions. The
WRB no longer recognizes an ochric epipedon on the argument that in the absence
of other epipedons, it is unnecessary.
7.2 Properties of Soils with an Ochric Epipedon
A photograph of a Typic Haplocryepts (Fairbanks silt loam) bearing an ochric
epipedon is given in Fig.
7.1
. The mean thickness of the ochric epipedon in
13 pedons (Table
7.1
) is 11 cm, the mean SOC is 2.2 %, and the mean base
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