133
Controls of Soil Formation
C O N C E P T C H E C K 5 . 5
Why did the headstones in Figure 5.11
weather so differently?
How does climate influence weathering?
Soil
Soil covers most land surfaces. Along with
air and water, it is one of our most indispensable resources. Also, like air and water,
soil is taken for granted by many of us. The
following quote helps put this vital layer in
perspective.
Science, in recent years, has focused more and more on
the Earth as a planet, one that for all we know is unique—
where a thin blanket of air, a thinner film of water, and
the thinnest veneer of soil combine to support a web of life
of wondrous diversity in continuous change.
*
Soil has accurately been called “the
bridge between life and the inanimate
world.” All life—the entire biosphere—
owes its existence to a dozen or so elements
that must ultimately come from Earth’ s
crust. Once weathering and other processes
create soil, plants carry out the intermediary role of assimilating the necessary
elements and making them available to
animals, including humans.
An Interface
in the Earth System
When Earth is viewed as a system, soil is
referred to as an interface—a common
boundary where different parts of a system
2
1
interact. This is an appropriate designation
because soil forms where the geosphere,
the atmosphere, the hydrosphere, and the
biosphere meet. Soil is a material that
develops in response to complex environmental interactions among different parts of
the Earth system. Over time, soil gradually
evolves to a state of equilibrium or balance
with the environment. Soil is dynamic and
sensitive to almost every aspect of its surroundings. Thus, when environmental
changes occur, in climate, vegetative cover,
or animal (including human) activity, the
soil responds. Any such change produces a
gradual alteration of soil characteristics
until a new balance is reached. Although
thinly distributed over the land surface,
soil functions as a fundamental interface,
providing an excellent example of the
integration among many parts of the Earth
system.
What Is Soil?
With few exceptions, Earth’ s land surface
is covered by regolith, the layer of rock
and mineral fragments produced by
weathering. Some would call this material
soil, but soil is more than an accumulation
of weathered debris. Soil is a combination
of mineral and organic matter, water, and
air—that portion of the regolith that supports the growth of plants. Although the
proportions of the major components in
soil vary, the same four components always
are present to some extent (FIGURE 5.14).
About one-half of the total volume of a
good quality surface soil is a mixture of
disintegrated and decomposed rock
(mineral matter) and humus, the decayed
remains of animal and plant life (organic
matter). The remaining half consists of pore
spaces among the solid particles where air
and water circulate.
Although the mineral portion of the
soil is usually much greater than the
organic portion, humus is an essential
component. In addition to being an important source of plant nutrients, humus
enhances the soil’ s ability to retain water.
Because plants require air and water to live
and grow, the portion of the soil consisting
of pore spaces that allow for the circulation
of these fluids is as vital as the solid soil
constituents.
Soil water is far from “pure” water;
instead, it is a complex solution containing
many soluble nutrients. Soil water not only
provides the necessary moisture for the
chemical reactions that sustain life, it also
supplies plants with nutrients in a form
they can use. The pore spaces that are not
filled with water contain air. This air is the
source of necessary oxygen and carbon
dioxide for most microorganisms and
plants that live in the soil.
C O N C E P T C H E C K 5 . 6
Why is soil considered an interface in the
Earth system?
How is regolith different from soil?
Controls
of Soil Formation
Soil is the product of the complex interplay
of several factors. The most important of
these are parent material, time, climate,
plants and animals, and topography.
Although all of these factors are interdependent, their roles will be examined
separately.
Parent Material
The source of the weathered mineral matter
from which soils develop is called the
parent material and is a major factor influencing a newly forming soil. Gradually it
undergoes physical and chemical changes
as the processes of soil formation progress.
Parent material might be the underlying
bedrock, or it can be a layer of unconsolidated deposits, as in a stream valley. When
the parent material is bedrock, the soils are
termed residual soils. By contrast, those
developed on unconsolidated sediment are
called transported soils (FIGURE 5.15). Note
that transported soils form in place on
parent materials that have been carried
from elsewhere and deposited by gravity,
water, wind, or ice.
The nature of the parent material influences soils in two ways. First, the type of
parent material affects the rate of weathering and thus the rate of soil formation.
(Consider the weathering rates of granite
2
1
* Jack Eddy. “A Fragile Seam of Dark Blue Light,” in
Proceedings of the Global Change Research Forum,
U.S. Geological Survey Circular 1086, (1993),
p. 15.
D I D Y O U K N O W ?
Sometimes soils become buried and
preserved. Later, if these ancient soils,
called paleosols, are uncovered, they
can provide useful clues to climates and
the nature of landscapes thousands or
millions of years ago.
Controls of Soil Formation
C O N C E P T C H E C K 5 . 5
Why did the headstones in Figure 5.11
weather so differently?
How does climate influence weathering?
Soil
Soil covers most land surfaces. Along with
air and water, it is one of our most indispensable resources. Also, like air and water,
soil is taken for granted by many of us. The
following quote helps put this vital layer in
perspective.
Science, in recent years, has focused more and more on
the Earth as a planet, one that for all we know is unique—
where a thin blanket of air, a thinner film of water, and
the thinnest veneer of soil combine to support a web of life
of wondrous diversity in continuous change.
*
Soil has accurately been called “the
bridge between life and the inanimate
world.” All life—the entire biosphere—
owes its existence to a dozen or so elements
that must ultimately come from Earth’ s
crust. Once weathering and other processes
create soil, plants carry out the intermediary role of assimilating the necessary
elements and making them available to
animals, including humans.
An Interface
in the Earth System
When Earth is viewed as a system, soil is
referred to as an interface—a common
boundary where different parts of a system
2
1
interact. This is an appropriate designation
because soil forms where the geosphere,
the atmosphere, the hydrosphere, and the
biosphere meet. Soil is a material that
develops in response to complex environmental interactions among different parts of
the Earth system. Over time, soil gradually
evolves to a state of equilibrium or balance
with the environment. Soil is dynamic and
sensitive to almost every aspect of its surroundings. Thus, when environmental
changes occur, in climate, vegetative cover,
or animal (including human) activity, the
soil responds. Any such change produces a
gradual alteration of soil characteristics
until a new balance is reached. Although
thinly distributed over the land surface,
soil functions as a fundamental interface,
providing an excellent example of the
integration among many parts of the Earth
system.
What Is Soil?
With few exceptions, Earth’ s land surface
is covered by regolith, the layer of rock
and mineral fragments produced by
weathering. Some would call this material
soil, but soil is more than an accumulation
of weathered debris. Soil is a combination
of mineral and organic matter, water, and
air—that portion of the regolith that supports the growth of plants. Although the
proportions of the major components in
soil vary, the same four components always
are present to some extent (FIGURE 5.14).
About one-half of the total volume of a
good quality surface soil is a mixture of
disintegrated and decomposed rock
(mineral matter) and humus, the decayed
remains of animal and plant life (organic
matter). The remaining half consists of pore
spaces among the solid particles where air
and water circulate.
Although the mineral portion of the
soil is usually much greater than the
organic portion, humus is an essential
component. In addition to being an important source of plant nutrients, humus
enhances the soil’ s ability to retain water.
Because plants require air and water to live
and grow, the portion of the soil consisting
of pore spaces that allow for the circulation
of these fluids is as vital as the solid soil
constituents.
Soil water is far from “pure” water;
instead, it is a complex solution containing
many soluble nutrients. Soil water not only
provides the necessary moisture for the
chemical reactions that sustain life, it also
supplies plants with nutrients in a form
they can use. The pore spaces that are not
filled with water contain air. This air is the
source of necessary oxygen and carbon
dioxide for most microorganisms and
plants that live in the soil.
C O N C E P T C H E C K 5 . 6
Why is soil considered an interface in the
Earth system?
How is regolith different from soil?
Controls
of Soil Formation
Soil is the product of the complex interplay
of several factors. The most important of
these are parent material, time, climate,
plants and animals, and topography.
Although all of these factors are interdependent, their roles will be examined
separately.
Parent Material
The source of the weathered mineral matter
from which soils develop is called the
parent material and is a major factor influencing a newly forming soil. Gradually it
undergoes physical and chemical changes
as the processes of soil formation progress.
Parent material might be the underlying
bedrock, or it can be a layer of unconsolidated deposits, as in a stream valley. When
the parent material is bedrock, the soils are
termed residual soils. By contrast, those
developed on unconsolidated sediment are
called transported soils (FIGURE 5.15). Note
that transported soils form in place on
parent materials that have been carried
from elsewhere and deposited by gravity,
water, wind, or ice.
The nature of the parent material influences soils in two ways. First, the type of
parent material affects the rate of weathering and thus the rate of soil formation.
(Consider the weathering rates of granite
2
1
* Jack Eddy. “A Fragile Seam of Dark Blue Light,” in
Proceedings of the Global Change Research Forum,
U.S. Geological Survey Circular 1086, (1993),
p. 15.
D I D Y O U K N O W ?
Sometimes soils become buried and
preserved. Later, if these ancient soils,
called paleosols, are uncovered, they
can provide useful clues to climates and
the nature of landscapes thousands or
millions of years ago.