Geoscience Reference
In-Depth Information
Again, you might be saying to yourself, “Why does it matter
how soil particles group together?” The fact is that it is extremely
important to how water percolates through the soil and how eas-
ily plant roots can penetrate it. Which soil structure presented in
Figure 11.16 would be the most difficult for plants to penetrate?
The answer is the platy structure because the roots cannot drive
straight down through the soil. As far as water's infiltration is con-
cerned, it flows best in soil with granular and blocky structure.
Granular structure
Blocky structure
Soil Chemistry
Given that soils contain a variety of mineral, organic, water, and
atmospheric components, each soil has a distinct chemical com-
position that is important regarding its formation and fertility. The
chemical composition of any given soil is determined by a complex
set of chemical reactions that occur within the soil solution. As noted
before, when water comes into contact with minerals under the right
conditions, it causes many minerals to dissolve. Similarly, when water
mixes with carbon dioxide and various forms of organic matter, it
creates carbonic and organic acids, respectively, that promote further
changes within the soil. A primary product of these kinds of transfor-
mations is chemical ions that are free to be absorbed by plants.
Platy structure
Columnar structure
Figure 11.16 Basic soil structures.
Particles within the soil
clump together in various ways, resulting in a distinct classifica-
tion system for soil structure.
•
Granular
structure occurs when the peds break up into
very small, rounded clumps. You can recognize granular
structure when the soil is slightly cohesive but still easily
spills between your fingers.
•
Blocky
structure occurs when the clumps break up into
distinct blocks that can be easily held.
•
Platy
structure occurs when the peds are stacked in dis-
tinct plate-like forms.
•
Columnar
structure occurs when the peds are organized
vertically.
Soil pH
One of the ways to characterize the chemical composition of
soil is by measuring its
pH
, which is the relative concentration
of hydrogen ions (H
+
) present in solution. The pH scale ranges
pH
The measure of acidity or alkalinity of a solution, ranging
from 0 to 14, based on the activity of hydrogen ions (H
+
).
VISUAL CONCEPT CHECK
11.2
100
What is the texture of a soil that is 50% sand,
40% clay, and 10% silt?
10
90
a)
Loam
80
20
b)
Silt
30
70
c)
Sandy clay loam
d)
Sandy clay
Clay
60
40
Percent
clay
Percent
silt
50
50
Silty
clay
Sandy
clay
40
60
Silty clay
loam
Clay loam
30
70
Sandy clay
loam
20
80
Loam
Silt loam
10
Sandy loam
90
Loamy
sand
Sand
Silt
100
Percent sand
