PARENT MATERIALS, SOIL FORMING FACTORS, AND SOIL PROFILE by zdn12589

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									PARENT MATERIALS, SOIL
FORMING FACTORS, AND
     SOIL PROFILE
    DESCRIPTIONS
PARENT MATERIALS, SOIL FORMING FACTORS, AND SOIL PROFILE
DESCRIPTIONS


Let’s start with an ideal, simplified weathering sequence - keep in mind
this is just one of an infinite number of ways a soil can form. In this
example we’ll consider a soil forming in place over bedrock. You start
off with some sort of rock. As that rock is exposed to weathering
forces - particularly water and oxygen - it begins to breakdown.
Eventually, over the bedrock, there will be a layer of material that is
similar to the bedrock, but it has been physically broken down into
smaller pieces, and may be chemically altered. This layer is called the
“C” horizon . The “C” horizon is the soil’s parent material. Eventually
vegetation will begin to take root in this layer. Over time, the
vegetation acts to further disintegrate and decompose the mineral
material, in addition to adding organic matter to the soil as the plants
die and are worked into the soil. Eventually a distinct layer forms at the
surface of the soil. This layer is called the “A” horizon . The “A” horizon
consists primarily of mineral materials with an accumulation of
decomposed organic matter. As the bedrock, the “C” horizon , and the
“A” horizon continue to weather, another layer develops between the
“A” and the “C” horizons. This layer, the “B” horizon is characterized
as having strong accumulations of weathering products. In typical
southeastern soils, these weathering products are clays and oxides. In
drier climate, the accumulations are carbonate minerals or gypsum.
Two very important soil-forming processes are responsible for the
development of “B” horizons - “eluviaton” and “illuviation” . Both
processes result from the movement of water down the soil profile.
“Eluviation” occurs when materials are removed, or leached, from the
upper part of the soil. “Illuviation” is the deposition of that material
further down in the soil. Illuviation is largely responsible for the
accumulation of materials that characterize “B” horizons . After very
long periods of time, a layer may develop below the “A” horizon that is
characterized by significant eluviation - this is called the “E” horizon .
E horizons tend to be extremely light colored, and coarser in texture
than the underlying material. Under certain conditions, particularly in
 wooded areas, organic debris will accumulate on top of the mineral soil.
This layer is called the “O” horizon .

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Many times it will be necessary to attach a small letter (to the capital
letter) to complete the horizon designation. There are 24 different
subordinate symbols that may be used. This list identifies some of the
more commonly used symbols. The underlined symbols are ones that
we will commonly be using in lab. (except for x). In some cases you
may use a combination of letters; rarely are more than three used
together.




b - buried soil horizon               p - plowing or other
                                          disturbance
k - an accumulation of
    carbonates                        n - an accumulation of
                                          soluble salts
y - an accumualtion of
    calcium sulfate                   q - cementation by
                                          siliceous material
f - frozen soil
                                      t - illuvial clay
h - illuvial humus
                                      x - fragipan character
s - illuvial Fe and Al
    oxides                            g - gleyed

m - strong cementation                w - weakly developed




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TYPICAL WELL-DEVELOPED SOIL PROFILE FOUND
THROUGHOUT THE
This profile illustrates a typical soil found
throughout the southeast - a soil that is
old, well-developed, occurs on gently slop-
                                                Ap
ing landscapes, developed under woods,
and is now being used for agriculture. In       E
the area around Blacksburg this would be
a typical valley soil. First note that you
have the master horizons A, E, B, C, and R.
In general, "O" horizons are found only in
wooded areas. Next, note that some of the
master symbols have been amended with a
subordinate designation. The surface hori-      Bt
zon is designated Ap because the soil sur-
face is disturbed - in this case due to
plowing . Whenever the surface of a soil
has been disturbed, the surface layer is
assigned the Ap designation. In an undis-
turbed soil you’ll find “A” horizon materi-
als at the top of a soil, but in a disturbed
soil you could have “E”, “B”, or “C” type
material at the surface. Regardless, the        C
surface horizon is assigned the “Ap”
designation. For example, in an agricul-
tural field you may find that the original A
horizon is completely gone, and what you
see at the surface is “B” material. Again,
                                                R
when writing a description even though
the surface soil may be obviously “B”
material, it is designated Ap. The other
subordinate symbol used here is the “t”
which indicates the accumulation of
translocated clay . This usually means that
clay has eluviated from the upper part of
the profile, and was subsequently illuviat-
ed where the “B” horizon is now located.


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