Geoscience Reference
In-Depth Information
see the distinct layers that occur in the profile? These layers are
referred to as soil horizons because they typically blend into
one another; in other words, it is very rare to see abrupt bound-
aries between different parts of the soil.
The specific character of soil horizons varies dramatically
among different parts of the world, but, in general, any well-
developed soil contains, from top to bottom, O, A, E, B, C, and
R horizons (Figure 11.20). These horizons form by different
processes and have the following characteristics.
O horizon
(leaf and plant litter)
A horizon
(enriched with humus)
1.
O horizon
—The
O horizon
is the uppermost horizon of
the soil, existing at ground level and consisting mainly
of undecomposed organic matter. A good example of an
O horizon is the leaf litter that accumulates on the forest
floor during the fall season. Thus, an O horizon forms
through the addition of organic matter.
2.
A horizon
—The
A horizon
is the uppermost mineral
horizon of the soil. This horizon is distinctive because it
contains abundant humus, which consists of decomposed
organic matter added to the soil. Given their relatively
high humus content, A horizons are typically a darkish
gray color. In general, A horizons have granular structure
due to the high concentration of humus. In Figure 11.13,
for example, the dark horizon at the top of the soil profiles
is an A horizon.
3.
E horizon
—The
E horizon
lies immediately below the A
horizon (see Figure 11.20) and is the zone of eluviation;
thus, this horizon forms largely through the process of
translocation. In other words, this horizon evolves due to
the downward movement of minerals leaching from it. As
a result of this process, the E horizon is usually relatively
light in color compared to the horizons above and below it.
4.
B horizon
—The
B horizon
lies below the E horizon and
is the zone of illuviation and thus forms largely through
the process of translocation. In other words, this part of
the soil evolves because minerals recrystallize at this
level. Recall that the zone of illuviation is deeper in more
humid climates than in relatively dry ones, which is to say
that the B horizon is deeper and thicker where precipita-
tion is greater (see Figure 11.10). Given the accumula-
tion of eluviated minerals in the B horizon, this horizon
usually has stronger structure (such as “blocky”) than the
overlying horizons. The base of this horizon corresponds
Solum
E horizon
(zone of eluviation)
B horizon
(zone of illuviation)
C horizon
(unaltered parent
material)
Regolith
R horizon
(bedrock)
Bedrock
Figure 11.20 Soil horizons in a fully developed soil on a
bedrock-dominated landscape.
With time, soils evolve to
produce a predictable sequence of horizons. Keep in mind that
the soil in this illustration formed within regolith.
to the base of the soil. The full thickness of the soil, from
the top of the A horizon to the bottom of the B horizon, is
referred to as the
solum
.
5.
C horizon
—The
C horizon
consists of unaltered parent
material that has yet to be affected by soil-forming pro-
cesses. In other words, this horizon is regolith and is not
part of the solum.
6.
R horizon
—Where soils form in landscapes dominated
by bedrock, such as those you find in the Appalachian
Mountains or parts of the Great Plains, an R horizon oc-
curs. Very simply, the
R horizon
is the bedrock that is the
source of the regolith/parent material, which is the sedi-
ment in which the soil forms.
O horizon
The uppermost soil horizon that consists of unde-
composed plant litter.
Solum
The A, E, and B horizons of a soil, which form through
pedogenic processes.
The soil horizon that is enriched with humus.
A horizon
C horizon
Unaltered soil parent material.
E horizon
The soil horizon that is progressively depleted of
minerals through dissolution and translocation.
R horizon
Unweathered bedrock that underlies soil.
B horizon
The soil horizon that forms below the E horizon
because translocated minerals recrystallize.
