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A Closer Look: Soil Horizons
 What
are soil horizons?
As you drive along highways, you are likely to pass places where the
roadway cuts through soil. Have you noticed layers in these road cuts?
During soil formation, inorganic materials (e.g., minerals) and organic
materials (e.g., decaying plants and animals) are transformed and,
with the flow of water through the soil, settle downward which creates
layers called soil horizons. An established method for describing soil
horizons labels them, form the surface down, O, A, E, B, C, and R.
There can, however, be subdivisions of the major horizons in some soil
profiles. Also, not all of the soil horizons are represented in any
given location; the presence or absence of these layers helps to classify
soil types. Soils have different and unique profiles depending on a
number of factors, including: climate, organisms, parent material,
topography, and time.
What are the characteristics of different
soil horizons?
O Horizon: The O, or organic, horizon is found in
soils formed under forest vegetation. It is composed mostly of vegetation
that has fallen to the ground and the remains of animals such as insects,
causing it to be dark in color.
A Horizon: Below the O horizon lies the A horizon,
which is commonly called topsoil. It is the first soil horizon made
mostly of minerals from the weathering of the underlying parent material,
but it can also contain decomposed organic material, which gives it
a dark color.
E Horizon: In areas that are or once were forested,
there can sometimes be an E horizon beneath the A horizon. E stands
for “eluviation,” which is the movement of dissolved or
suspended material out of a horizon. Water entering the soil moves
downward through the O and A horizons, and dissolves various soil materials
(iron and aluminum oxides, clay particles, and organic matter) before
carrying them to the E horizon, and then on to deeper levels. This
process of leaching creates the E horizon’s white or grayish
color. It is lighter in color than the layers above or below it.
B Horizon: The B horizon is commonly called subsoil.
The B horizon is the illuviated zone, where the soil material that
has been leached out of the upper horizons accumulates. In highly weathered
soils, the B horizon is commonly rich in clay, iron, or aluminum, and
is often colored yellow or red by the iron oxides that are transported
down from above. B horizons can be very thick and can be broken down
into multiple layers within the horizon.
C Horizon: Directly below the B horizon is the C
horizon. It consists of the “raw” or “parent” material
from which the soil was created. This includes partially weathered
bedrock, materials that were deposited by the wind or water, volcanic
material, organic matter, glacial deposits, and other materials.
R Horizon: If unaltered bedrock is within
a few feet of the surface, it is called the R horizon.
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Can you identify possible horizons
in this soil profile?
Roll your mouse over the image above to see horizon lines.
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