Geoscience Reference
In-Depth Information
S u m m a r y o f K e y C o n C e p t S
1.
Soil consists of the outermost layer of Earth and forms
through the complex interaction of additions, transfor-
mations, translocations, and losses. These processes
occur in various combinations that depend on the five
soil-forming factors: (a) climate, (b) organisms, (c) relief,
(d) parent material, and (e) time.
2.
Soils have a variety of distinctive characteristics, includ-
ing color, texture, structure, pH, and cation exchange
capacity, that can be measured and compared from one
soil to another.
3.
Soils are organized into horizons that form by distinctive
processes. The O horizon is the uppermost horizon and
consists of freshly added organic matter, such as leaves.
The A horizon contains decomposed organic matter
called humus. The E horizon forms through the eluvia-
tion of minerals, which recrystallize and collect through
the process of illuviation in the underlying B horizon. The
C horizon is unaltered parent material, which, in bedrock
landscapes, is known as regolith.
4.
Soils in the United States are classified on the basis
of their genetic properties in a scheme called
soil tax-
onomy
. The highest level of this classification system is
the soil order. Twelve soil orders are recognized: Oxi-
sols, Ultisols, Alfisols, Spodosols, Histosols, Vertisols,
Gelisols, Mollisols, Aridisols, Entisols, Inceptisols, and
Andisols.
C h e C K y o u r u n d e r S t a n d i n g
1.
How is regolith related to bedrock and soil?
2.
Describe the soil-forming factors and provide an exam-
ple of how they interact to produce soil.
3.
Compare and contrast the concepts of soil texture and
soil structure.
4.
Which texture will allow water to drain more rapidly:
sand or clay? Why?
5.
What are the various aspects of soil chemistry that are
used to characterize soil?
6.
Which parent material will have a higher cation exchange
capacity—one that is sandy, or one that is rich in clay?
Explain your answer.
7.
Explain how the various soil horizons form.
8.
Which pair of horizons is created through the process of
translocation? In what way does this evolution occur?
9.
It is conceivable that a soil could be classified as
an Entisol even though it is very “old.” In what
environment—tropical, subtropical, or arid—would this
kind of weak development most likely occur? Why?
10.
What is a soil profile, and why is it useful for determin-
ing the characteristics of a soil?
11.
How are Spodosols and Oxisols similar? How are they
different?
12.
Why is grass an important part of the calcification
pedogenic model?
13.
In which parent material would you most likely find an
Inceptisol: a sand dune that is 500 years old or one that
is 1000 years old? Why?
14.
Provide an example of how soils are important to
agriculture and how they are managed.
ANSWERS TO VISUAL CONCEPT CHECKS
V i s u a l C o n C e p t C h e C k 1 1 . 1
The answer is
b
. The photograph shows an example of soil additions. When leaves fall to the ground,
they slowly decompose and become incorporated into the soil.
V i s u a l C o n C e p t C h e C k 1 1 . 2
The answer is
d
; this soil is classified as a sandy clay.
V i s u a l C o n C e p t C h e C k 1 1 . 3
This soil is forming by the podzolization pedogenic process, which results in the eluviation of iron,
aluminum, and organic acids. The soil contains developing A, E, and Bs horizons. You can identify the
A horizon because it is dark, which occurs through the addition of humus. The E horizon is whitish and
forms through the eluviation of iron and aluminum. The Bs horizon is visible beneath the E horizon as
the light orange zone. This color reflects the illuviation of iron.
