Geoscience Reference
In-Depth Information
Soil Colloids and pH
The chemical composition of any given soil is determined by a
complex set of reactions taking place within the soil solution. The
ultimate outcome is often dependent on the nature of soil col-
loids, which are microscopic particles that occur in organic and
inorganic form. These particles have an important role as far as
cation exchange and soil pH are concerned. To better under-
stand these relationships, it is useful to view them in animated
form. Go to the
Geo Media Library
and select
Soil Colloids
and pH
. Once you complete the exercise, be sure to answer the
questions at the end to test your understanding of this concept.
depletions. The nice thing about these processes is that they
cause the soil to become progressively organized through time
in a way that can be easily seen by looking at a
soil profile
. Just
as the profile of your face refers to a side view that ranges from
the top of your head to your chin, a soil profile displays all the
soil characteristics in a vertical “image.” A soil profile can be
obtained by cleaning a natural exposure such as a creek bank,
or, more commonly, by excavating a soil pit with a shovel or
backhoe. Although it may not seem like it, this kind of work
is really a great deal of fun. Figure 11.19 shows examples of
exposing a soil profile.
As soils form, they organize internally into distinct
soil
horizons
. If the term “horizon” seems confusing, simply think
of the term as being equivalent to “layer” or “band” for the time
being. Look again at the soil profile in Figure 11.19a. Do you
KEY CONCEPTS TO REMEMBER ABOUT
MEASURABLE SOIL CHARACTERISTICS
AND SOIL CHEMISTRY
1.
All soils have physical and chemical properties that can
be measured, including color, texture, structure, soil
chemistry, and soil pH.
2.
Physical and chemical properties differ among and
within soils.
3.
Soil pH refers to the acidity or basicity of the soil.
4.
Colloids are collections of microscopic soil particles
that hold and exchange cations.
Soil Profiles (Reading the Soil)
A vertical exposure in which all soil components
Soil profile
can be seen.
With the various soil components, processes, and chemistry in
mind, let's now investigate the progressive development of soil
that results from additions, transformations, translocations, and
The distinct layers within a soil that result from
Soil horizons
pedogenesis.
(a)
(b)
Figure 11.19 Soil profiles.
(a) After a soil pit is excavated you can then see the soil profile and its specific characteristics, such as the
dark zone near the surface of this particular soil and the reddish zone beneath it. (b) Sometimes soils are exposed naturally, as along this
stream cutbank. In these situations, they can easily be studied.
