A soil
profile with the horizons labeled.
The ability to correctly name soil horizons requires that the person be part
"detective" and part "artist". The detective part requires
that the person use all the soil morphological clues (color, texture,
structure, etc.) to correctly determine the influence of the soil forming
factors and the pedogenic process that have acted on the parent material. Then
the person puts all the clues together to make their interpretation of what
they are seeing.
Not all soil scientists will write the exact same soil profile description, because their previous experiences will have been different, just like all artists will not paint the same picture when looking at the same landscape scene. However, you will see very similar descriptions, that could be duplicated by another scientist.
As you start gaining experience with naming the horizons for soil profiles, you will become more adept at correctly using all of the clues to write a correct "Soil Profile Description."
Surface Horizons
Surface soil horizons are mainly influenced by the addition of organic matter. The organic matter that is added to a soil will first be undecomposed. Gradually organisms in the soil will begin to decompose this litter until it is no longer recognizable as leaves. Eventually this organic matter is decomposed so much it becomes colloidal in size and coats the mineral particles of the soil.
O-horizons are all organic matter and are mainly found in forest soils where the yearly addition of leaves builds up a layer of undecomposed leaves. In fact, the O horizon contains very little mineral soil.
The i subscript, or Oi horizons, would be mainly undecomposed plant parts. The Oe horizon would be of moderate decomposition, and the Oa horizon would be almost completely decomposed.
Besides being in the leaf litter of forested soils, O horizons can also be found in the peat bogs of swamps and wetlands. Minnesota has extensive peat soils in the northern part of the state.
The A horizon is a mineral horizon that is dark in
color due to the incorporation of decomposed organic matter, which coats the
mineral soil particles with colloidal size organic matter. In general, the
darker the mineral soil, the greater the percent organic matter. Many soils in
southern Minnesota that developed under a tall grass prairie will have 4 to 5%
organic matter in the A horizon for a depth of one foot or more. When an A
horizon is plowed, it is called an Ap horizon.
The AB horizon here is a transition horizon that has characteristics like the A and B, but more like the A; thus, it is called an AB.
Subsurface Horizons
Subsurface horizons develop due to translocation, transformations and losses. The are zones of accumulations or changes in the parent material where color or structure develop.
The E horizon develops when materials
are eluviated out of a zone in the soil. The effect of this is to remove the
organic matter and iron out of this zone, which causes it to be lighter in
color than the horizons above or below it. E horizons are found mainly in
forested soils (Nebish). The E horizon (8 inches thick) is seen below a 3 inch
A horizon. This horizon is called azone of eluviation. Not all soils
have E horizons.
Below the E horizon or A horizons, there are zones of illuviation or
accumulations. These are called B horizons. In the Nebish soil, the B horizon
is below the E and has accumulated clay and developed very strong structural
peds. This kind of B horizon is called a Bt, and is a zone of illuviation.
In some acid sandy soils where there is not very much clay, the significant
translocation of iron, humus and aluminum takes place very rapidly. Often the
vegetation is an evergreen forest that contributes a very acid O layer. In the
Orthod soil, the E horizon has become very developed, and the zone of
illuviation is labeled a Bhs horizon (h for humus and s for iron and aluminum).
The Bhs horizon is often dark red in color and easily recognized in the field.
Another example of a B horizon is where the soil has a minimal
amount of illuviation. However, the soil undergoes
significant transformations and develops different color and structure from the
C horizon or parent material. This is called a Bw horizon, as seen in the Udoll
soil.
In this Udoll, below the 8 inch A horizon, the Bw horizon has developed. This zone does not have enough clay accumulation to be a Bt, but is different from the parent material. The Bk horizon is a zone of accumulation, but instead of clay, calcium carbonate (CaCO3) is accumulating (k). Calcium carbonate can be identified in a soil profile because it is very light in color and reacts with a 10% solution of HCl to give off CO 2 as a gas or the soil "bubbles". The parent material for this soil is Des Moines Till, which is high in CaCO3.
The solum describes the A and B horizons (or the zone of pedogenic activity) and does not include the C horizon.
C-horizons are outside the zone of "pedogenesis"
(soil development). C horizons are little altered by the soil forming
processes. C horizons are referred to as the parent material of the
soil.
This soil near Red Wing, MN. has a C horizon of loess. The loess has changed very little and looks very similar to the way it looked when it was deposited by the wind over 20,000 years ago
Soil scientists use their scientific knowledge of the soil and their grasp
of soil forming factors to "describe" the soil in the field. There is
considerable "art" in describing soils or the interpretation of all
the facts to arrive at a depiction of the soil. Artists over the years have
used their visual skills to "depict" the soil. Go to
SOIL ART to see more about the soil and the
artist.
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