55061_12_Ch12_p320_347 pp2.indd - Cengage Learning

Soils and Soil Development
Soil is an outstanding example of the interrelationships among
Earth’s subsystems.
■ Why is soil such a good example of the interaction and integration
of subsystems?
■ Why should soil be considered an open system?
Soil water is the means by which plants receive dissolved nutrients
that are essential for growth.
■ Why is gravitational water such an effective agent of solution?
■ How is capillary water important during periods of drought?
Soil fertility depends on many factors, and a soil that is fertile for
one vegetation type may not be for another.
■ What factors determine a soil’s fertility, and how is vegetation
involved?
■ How are acidity and alkalinity related to soil fertility?
On a global scale, climate exerts a major infl uence on the
formation and characteristics of soils.
■
■
CHAPTER PREVIEW
How do temperature, precipitation, and moisture regimes affect
the development of soils?
Why is the regional distribution of soils similar to regions of
climate and vegetation?
Soils are among the world’s most critical and widely abused yet
least understood natural resources.
■ How are soils abused or neglected as a resource?
■ What can be done to conserve soils?
Opposite: Scientists work to understand and control erosional losses
and other problems that threaten our precious natural resource—soil.
USDA/ARS/Photo by Jack Dykinga
▼
12
W hat are the four most important natural
constituents that permit life as we know it to exist
on Earth? Many people if asked that question would reply,
“air, water, and sunlight,” right away, but they might have
to think harder and longer about their fourth answer. Most
people give little attention to that fourth natural resource,
but it is essential for their life on Earth, and it lies right below
their feet. Soil is that critical resource. The soil mantle that
covers most land surfaces is indispensable, but fragile, and
threatened by erosion, pollution, or being covered over by
the human-built environment. Soil provides nutrients that
directly or indirectly support much of life on Earth.
Soil is a dynamic natural body capable of supporting
a vegetative cover. It contains chemical solutions, gases,
organic refuse, flora, and fauna. The physical, chemical, and
biological processes that take place among the components
of a soil are integral parts of its dynamic character. Soil
responds to climatic conditions (especially temperature and
moisture), to the land surface configuration, to vegetative
cover and composition, and to animal activity.
Soil has been called “the skin of the Earth.” The
condition and nature of a soil reflects both the ancient
environments under which it formed, and today’s
environmental conditions. A soil functions as an
environmental system, adapting, reflecting, and responding
321

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CHAPTER 12 SOILS AND SOIL DEVELOPMENT
to a great variety of natural and human-influenced processes. Soil
is an exceptional example of the integration, interdependence, and
overlap among Earth’s subsystems because the characteristics of a
soil reflect the atmospheric, hydrologic, lithologic, and biotic conditions
under which it developed ( ● Fig. 12.1). In fact, because
soils integrate these major subsystems so well, they are sometimes
considered a separate system called the pedosphere (from Greek:
pedon, ground).
Soil is also home to numerous living organisms, forming
the environments in which they live, both above and below the
ground surface. The life-forms that live in or on a soil play significant
roles in the development and characteristics of a soil,
and through human population growth and expanding civilizations,
potentially negative impacts on soils have increased
dramatically.
How a soil develops and its resulting characteristics depend
on a great number of factors. But when soils are viewed on a
world regional scale, a strong and significant influence is climate.
The relationships between soils, climate types, and their associated
environments were considered in Chapters 9 and 10.
Major Soil Components
What is soil actually made of ? What does a shovel contain when it
scoops up a load of soil? What soil characteristics support and influence
variations in Earth’s vegetational environments? Soils contain
four major components, and there are many processes that act
on these components. The four major components of soil are inorganic
materials, soil water, soil air, and organic matter ( ● Fig. 12.2).
Inorganic Materials
Soils contain varying amounts of insoluble materials—rock fragments
and minerals that will not readily dissolve in water. Soils
also contain soluble minerals, which supply dissolved chemicals
held in solution. Most minerals found in soils are combinations
● FIGURE 12.1
The intertwined links between soil and the major Earth subsystems.
Why is soil considered to be such an integrator of Earth systems?
Biosphere
Atmosphere
SOIL
Lithosphere
Hydrosphere
From Purves, et al., Life: The Science of Biology, Fourth Edition. Used with
permission of Sinauer Associates, Inc.
USDA/NRCS/Lynn Betts
of the common elements of Earth’s surface rocks: silicon, aluminum,
oxygen, and iron. Some of these constituents occur as solid
chemical compounds, and others are found in the air and water
that are also vital components of a soil. Soils sustain Earth’s land
● FIGURE 12.2
The four major components of soil. Soil contains a complex assemblage
of inorganic minerals and rocks, along with water, air, and organic matter.
The interaction among these components and the proportion of each
are important factors in the development of a soil.
How do each of these soil components contribute to making a soil
suitable to support plant life?
Microcolonies
of bacteria
Quartz
H 2O
Quartz
Air
Organic
matter
Clay particle
Air
Clay particle
Quartz
● FIGURE 12.3
Fertilizers increase the productivity of soils. This farmer is adding
nitrogen fertilizer into the soil on this Iowa farm.
Why can soil fertilizer be either useful or detrimental when it is
introduced into the soil system?

ecosystems by providing a great variety of necessary chemical elements
and compounds to life-forms. Carbon, hydrogen, nitrogen,
sodium, potassium, zinc, copper, iodine, and compounds of these
elements are important in soils. The chemical constituents of a
soil typically come from many sources—the breakdown (weathering)
of underlying rocks, deposits of loose sediments, and minerals
dissolved in water. Organic activities help to disintegrate rocks,
create new chemical compounds, and release gases into the soil.
Plants need many chemical substances for growth, and having
a knowledge of a soil’s mineral and chemical content is necessary
for determining its potential productivity. Soil fertilization
is the process of adding nutrients or other constituents in order to
meet the soil conditions that certain plants require ( ● Fig. 12.3).
Soil Water
The original source of soil water is precipitation. When precipitation
falls on the land, the water that is not evaporated away is either
absorbed into the ground or by vegetation, or it runs downslope.
Soil water is both an ingredient and a catalyst for chemical reactions
that sustain life and influence soil development. Water also
● FIGURE 12.4
The interrelationships between a soil and the environmental factors that influence soil development. Soil is an
example of an open system because it receives inputs of matter and energy, stores part of these inputs, and
outputs matter and energy. Note the inputs and outputs on this diagram.
What are some examples of energy and matter that flow into and out of the soil system?
Run-in gravitational water
zone of aeration
Capillary
fringe
water
Mineral particle
MAJOR SOIL COMPONENTS
provides nutrients in a form that can be extracted by vegetation. As
water moves through a soil it washes over and through various soil
components, dissolving some of these materials and carrying them
through the soil. Soil water is not pure, but is a solution that contains
soluble nutrients. Plants need air, water, and minerals to function,
live, and grow, and they depend on soil for much of these necessities.
A soil functions as an open system. Matter and energy flow
into and out of a soil, and they are also held in storage ( ● Fig. 12.4).
Understanding these flows—inputs and outputs, the components
and processes involved, and how they vary from soil to soil—is a
key to appreciating the complexities of this natural resource.
The water in a soil is found in several different circumstances
(see again Fig. 12.4). Soil water adheres to soil particles and soil
clumps by surface tension (the property that causes small water
droplets to form rounded beads instead of spreading out in a thin
film). This soil water, called capillary water, serves as a stored
water supply for plants. Capillary water can move in all directions
through soil because it migrates from areas with more water to
areas with less. Thus, during dry periods, when there is no gravitational
water flowing through the soil, capillary water can move
upward or horizontally to supply plant roots with moisture and
dissolved nutrients.
Water
Precipitation
Air Hygroscopic
water
Solid bedrock
Weathered bedrock
Groundwater flow
Joints
Organic additions
and soil faunal
activity
Water table
Runoff
(surface water)
Below water table
zone of saturation
323

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CHAPTER 12 SOILS AND SOIL DEVELOPMENT
Capillary water migrates upward and moves minerals from
the subsoil toward the surface. If this capillary water evaporates
away, the formerly dissolved minerals remain, generally as alkaline
or saline deposits in the topsoil. High concentrations of certain
mineral deposits, like these, can be detrimental to plants and animals
existing in the soil. Lime (calcium carbonate) deposited by
evaporating soil water can build up to produce a cementlike layer,
called caliche, which like a clay hardpan prevents the downward
percolation of water.
Soil water is also found as a very thin film, invisible to the
naked eye, that is bound to the surfaces of soil particles by strong
electrical forces. This is hygroscopic water, which does not
move through the soil, and it also does not supply plants with the
moisture that they need.
Water that percolates down through a soil, under the force of
gravity, is called gravitational water. Gravitational water moves
downward through voids between soil particles and toward the
water table—the level below which all available spaces are filled
with water. The quantity of gravitational water in a soil is related
to several conditions, including the amount of precipitation, the
time since it fell, evaporation rates, the space available for water
storage, and how easily the water can move through the soil.
Gravitational water performs several functions in a soil
( ● Fig. 12.5). As gravitational water percolates downward, it dissolves
soluble minerals and carries them into deeper levels of
the soil, perhaps to the zone where all open spaces are saturated.
Depleting nutrients in the soil by the through flow of
water is called leaching. In regions of heavy rainfall, leaching
is common and can be intense, robbing a topsoil of all but the
insoluble substances.
Gravitational water moving down through a soil also takes
with it the finer particles (clay and silt) from the upper soil layers.
This downward removal of soil components by water is called
eluviation. As gravitational water percolates downward, it deposits
the fine materials that were removed from the topsoil at
a lower level in the soil. Deposition by water in the subsoil is
called illuviation. Gravitational water also mixes soil particles
as it moves them downward. One result of eluviation is that the
texture of a topsoil tends to become coarser as the fine particles
are removed. Consequently, the topsoil’s ability to retain water is
reduced. Illuviation may eventually cause the subsoil to become
dense and compact, forming a clay hardpan.
Leaching and eluviation both strongly influence the characteristic
layered changes with depth, or stratification. Fine particles
and substances dissolved from the upper soil are deposited
in lower levels, which become dense and may be strongly colored
by accumulated iron compounds.
Soil Air
● FIGURE 12.5
Water plays several important roles in the processes that affect soil development. Water is important in
moving nutrients and particles vertically, both up and down, in a soil.
How does deposition by capillary water differ from deposition (illuviation) by gravitational water?
Plant litter
Faunal activity downward
mixing organic material
Leaching
of solubles
Redeposition
of some
solubles
Loss of some
solubles in
groundwater outflow
Precipitation
Much of a soil—in some cases, approaching 50%—consists of
spaces between soil particles and between clumps (aggregates of
soil particles). Voids that are not filled with water contain air or
certain gases. Compared to the composition of the lower atmosphere,
the air in a soil is likely to have less oxygen, more carbon
dioxide, and a fairly high relative
humidity because of the presence of
Organic additions (plant litter)
Organic additions (underground)
Eluviation of fine
particles (depletion)
Illuviation of fine
particles (additions)
Capillary rise and evaporation
deposits chemical load
Water table
capillary and hygroscopic water.
For most microorganisms and
plants that live in the ground, soil
air supplies oxygen and carbon
dioxide necessary for life processes.
The problem with a watersaturated
soil is not necessarily excess
water but, if all pore spaces are
filled with water, there is no air
supply. The lack of air is why many
plants find it difficult to survive in
water-saturated soils.
Organic Matter
Soil contains organic matter in addition
to minerals, gases, and water.
The decayed remains of plant and
animal materials, partially transformed
by bacterial action, are collectively
called humus. Humus is
an important catalyst in chemical
reactions that help plants to extract
soil nutrients. Humus also supplies
nutrients and minerals to the soil.
Soils that contain humus are quite

workable and have a good capacity to retain water. Humus also
provides an abundant food source for microscopic soil organisms.
Most soils are actually environments that teem with life, ranging
from microscopic bacteria and fungi, to earthworms, rodents,
and other burrowers. Animals contribute to the soil development
and enrichment by creating humus from plant litter. They also mix
organic material deeper into the soil, and move inorganic fragments
toward the surface. In addition, the functions of plants and
their root systems are integral parts of the soil-forming system.
Soils vary at local, regional, and global scales. Particularly strong
relationships exist between a soil and the vegetation and climate at
its location. For example, soils in middle-latitude grasslands normally
have a very high proportion of organic matter; those in deserts
are thin, and rich in minerals left behind by evaporating water,
like lime and salts; and tropical soils typically have a high content of
iron and aluminum oxides. Knowing a soil’s water, mineral, and organic
components and their proportions can help us determine its
productivity and what the best use for that particular soil might be.
Characteristics of Soil
Several soil properties that can be readily tested or examined are
used to describe and differentiate soil types. The most important
properties include color, texture, structure, acidity or alkalinity,
and capacity to hold and transmit water and air.
Color
Color is the most visible soil characteristic, but it might not be the
most important attribute. Most people are aware of how soils vary
in color from place to place. For example, the well-known red clay
soils of Georgia are not far from Alabama’s belt of black soils. Soils
vary in color from black to brown to red, yellow, gray, and nearwhite.
A soil’s color is generally related to its physical and chemical
characteristics. When describing soils in the field or samples in
the laboratory, soil scientists use a book of standardized colors to
clearly and precisely identify this coloration ( ● Fig. 12.6).
Decomposed organic matter is black or brown, so soils with
high humus contents tend to be dark. If the humus content
of soil decreases because of either low organic activity or loss
of organics through leaching, soil colors typically fade to light
brown or gray. Soils rich in humus are usually very fertile. For
this reason, dark brown or black soils are often referred to as
rich. However, this is not always true because some black or dark
brown soils have little or no humus, but are dark because of
other soil-forming factors.
Soils that are red or yellow typically indicate the presence of
iron. In moist climates, a light gray or white soil indicates that iron
has been leached out, leaving oxides of silicon and aluminum; in
dry climates, the same color typically indicates a high proportion
of calcium or salts.
Soil colors provide useful clues to the physical and chemical
characteristics of soils and make the job of recognizing different
soil types easier. But color alone does not answer all the important
questions about a soil’s qualities or fertility.
Courtesy of James P. Shoryer, Kansas State University Research and Extension
Texture
CHARACTERISTICS OF SOIL
● FIGURE 12.6
Determining soil color. A standardized classification system is used to
determine precise color by comparing the soil to the color samples
found in Munsell soil color books.
In general, how would you describe the color of the soils where
you live?
Soil texture refers to the particle sizes (or distribution of sizes)
that make up a soil ( ● Fig. 12.7). In clayey soils, the dominant
size is clay particles, defined as having diameters of less than
0.002 millimeter (soil scientists universally use the metric system).
In silty soils, the dominant silt particles are defined as being between
0.002 and 0.05 millimeter. Sandy soils have mostly sandsized
particles, with diameters between 0.05 and 2.0 millimeters.
Rocks larger than 2.0 millimeters are regarded as pebbles, gravel,
or rock fragments, and technically are not soil particles.
The proportion of particle sizes determines a soil’s texture.
For example, a soil composed of 50% silt-sized particles, 45% clay,
and 5% sand would be identified as a silty clay. A triangular graph
( ● Fig. 12.8) is used to discern different classes of soil texture
based on the plot of percentages for each soil grade (as sand,
silt, and clay are called) within each class. Point A within the silty
clay class represents the example just given. A second soil sample
(B) that is 20% silt, 30% clay, and 50% sand would be referred to
as a sandy clay loam. Loam soils, which occupy the central areas
of the triangular graph, are soils with a mix of the three grades
(sizes) of soil particles without any size being greatly dominant. It
is interesting to note that loam soils are generally best suited for
supporting vegetation growth.
Soil texture helps determine a soil’s capacity to retain moisture
and air that are necessary for plant growth. Soils with a higher
proportion of larger particles tend to be well aerated and allow
water to infiltrate (seep through) the soil quickly—sometimes
so quickly that plants are unable to use the water. Clay soils present
the opposite problem because they retard water movement,
becoming waterlogged and deficient in air. Aeration of the soil is
an important process in cultivation, and plowing a soil opens its
structure and increases its air content.
325

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CHAPTER 12 SOILS AND SOIL DEVELOPMENT
GEOGRAPHY’S PHYSICAL SCIENCE PERSPECTIVE
Basic Soil Analysis
After studying just one chapter on
soils, no one would expect any
introductory student to be able to
do a detailed soil analysis. However, there
are some basic observations that anyone
can perform to better understand a few
properties of a local soil. No equipment is
required to make analyses; only visual and
hands-on examinations are necessary.
Soil Color
Soil color can hold clues to the composition
and/or the formation processes of that soil.
Figure 12.6 shows the Munsell color book,
a standard guide for matching and recognizing
precise colors of soil types. The book includes
common soil colors, but each color
can also appear in a wide variety of tones.
Red: Reddish soil usually indicates that
oxidation has been an active process—
oxygen has chemically reacted with the
soil minerals. Red also indicates that iron
is in the soil. Just like rusting iron, many
iron-rich minerals turn red when oxidized.
The formula for this process is FeO + O 2 →
Fe 2 O 3 (ferrous oxide + oxygen becomes
hematite, a reddish iron oxide).
© Jeff Vanuga/USDA Natural Resources Conservation Service
Blue/Silver/Gray: These tones mean
that the soil has likely been reduced; in
other words, oxygen has been removed
from the soil.
Fe 2 O 3 → FeO + O 2 (the previous
formula in reverse)
White: Usually denotes that calcium
carbonate (CaCO 3 ) or salts (such as NaCl)
may be present in the soil.
Black: A very dark color may indicate a
high amount of organic material present
in the soil.
More sophisticated field or laboratory
analyses are required for absolute identification,
but these examples will allow good
working hypotheses for soil characteristics
represented by colors.
Soil Texture
The particle sizes in a soil determine its
texture. Soil texture is a property that you
can feel, and your fingers can help in the
analysis. Sand-sized particles can be easily
recognized because they feel gritty to the
touch. Wetting the soil and working it with
your hands can help in this process. If the
sample is not gritty but rather is smooth
Soil analysis in the field is a hands-on process.
to the touch, then the soil contains silt or
clay. If the sample feels sticky and you can
squeeze a small soil sample into a ribbon
(like with modeling clay), then clay-sized
particles are abundant. Actual percentages
of particle sizes in a soil sample are best
established in a laboratory.
Soil Structure
The shape of clumps that a soil makes
when it is broken apart is called structure
and can be examined by breaking up a
handful of soil. The peds (or small clumps
of soil) may take on some distinctive
shapes. Though the peds may form a
variety of shapes, some of the more common
are granular (denoting a presence
of sand) and platy (showing a presence
of clay). Other soil structures, such as
blocky, columnar, or prismatic, are shown
in Figure 12.9.
Although these simple procedures will
not yield a complete analysis of a soil sample,
they can certainly be the first steps in
the process. It is interesting to note that pedologists
(soil scientists) while in the field
perform many of these same procedures.

0 1 2
0
Sand
Silt
Clay
Invisible at
this scale
Structure
Comparative sizes of
soil/rock particles
0.05 – 2.0 mm
0.002 – 0.05 mm
1/16
Greatly
enlarged
mm
Inches
Actual size of sand grain 1 mm diameter
● FIGURE 12.7
Particle sizes in soil. Sand, silt, and clay are terms
that refer to the size of these particles for scientific
and engineering purposes. Here, the sizes of each
can be compared. Clay particles are tiny, sheetlike
particles that cannot be seen.
In most soils, particles clump into distinctive
masses known as soil peds, which give a soil
a distinctive structure. Soil structure influences
a soil’s porosity—the amount of space that
may contain fluids. Soil structure also influences
permeability—the rate at which fluids
such as water can pass through. Permeability
is usually greatest in sandy soils, and porosity
is usually greatest in clayey soils. Both of
these factors control soil drainage as well as
the available moisture in a soil. Soils with similar
textures may have different structures, and
vice versa.
Soil structure can be influenced by outside
factors such as moisture regime and the
nutrient cycles that plants use to interchange
chemicals with the soil, keeping certain ones
in the system while others are leached away.
We have all seen the structural change in
soils that occurs when soils are wet compared
to when they are dry. Human activities also
influence soil structure through cultivation,
irrigation, and fertilization. Fertilizers, as well
as lime or decayed organic debris, encourage
clumping of soil particles and the maintenance
of clumps. Excess sodium and magnesium have the opposite effect,
causing clay soils to become a sticky muck when wet and
like concrete when dry. The absence of smaller particles typically
hinders the development of a well-defined soil structure.
Scientists classify soil structures according to their form. These
range from columns, prisms, and angular blocks, to nutlike spheroids,
laminated plates, crumbs, and granules ( ● Fig. 12.9). Soils with massive
and fine structures tend to be less useful than aggregates of intermediate
size and stability, which permit good drainage and aeration.
Acidity and Alkalinity
CHARACTERISTICS OF SOIL
An important aspect of soil chemistry is a soil’s departure from
neutrality toward either acidity or alkalinity (baseness). Levels of
acidity or alkalinity are measured on the pH scale of 0 to 14. A
pH reading indicates the concentration of reactive hydrogen ions
present. The pH scale is logarithmic, meaning that each change in a
whole pH number represents a tenfold change. It is also an inverted
scale—a lower pH means a greater amount of hydrogen ions present
(higher acidity). Low pH values indicate an acid soil, and high
pH indicates alkaline conditions ( ● Fig. 12.10).
● FIGURE 12.8
The texture of a soil can be represented by a plotting a point on this diagram. Texture is
determined by sieving the soil to determine the percentage of particles falling into the size
ranges for clay, silt, and sand. Note that each of the three axes of the triangle is in a different
color and the line colors also correspond (clay-red, silt-blue, sand-green).
What would a soil that contains 40% sand, 40% silt, and 20% clay be classified as?
0
100
10
20
Sand
90
30
Loamy
sand
40
Percent clay
80
50
60
Sandy
clay
70
70
B
Sandy clay loam
80
60
90
100
0
Clay
50
10
Clay loam
Loam
Percent sand
20
40
30
40
Silty
clay
30
50
Silty clay
loam
Sandy loam Silty loam
A
Percent silt
60
20
70
Silt
80
10
90
100
0
327

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CHAPTER 12 SOILS AND SOIL DEVELOPMENT
Soil acidity or alkalinity helps determine the available nutrients
that affect plant growth. Plants absorb nutrients that are dissolved
in liquid. However, soil water that lacks some degree of acidity has
little ability to dissolve these nutrients. As a result, even though nutrients
are in the soil, plants may not have access to them.
Most complex plants will grow only in soils with levels
between pH 4 and pH 10, although the optimum pH for vegetation
growth varies with the plant species. Around the world,
vegetation has evolved in and adapted to a variety of climates
and soil environments, both of which can affect soil pH. Certain
species tolerate alkaline soils, and others thrive under more
acid conditions.
Leaching caused by high rainfall gradually replaces soil
elements such as sodium (Na), potassium (K), magnesium (Mg),
and calcium (Ca) with hydrogen. Falling rain picks up atmospheric
carbon dioxide and becomes slightly acidic: H 2 O + CO 2 = H 2 CO 3
(carbonic acid), so desert soils tend to be alkaline and soils in humid
regions tend to be acidic ( ● Fig. 12.11). The humus content in the
soils of humid areas also contributes to higher soil acidity.
To correct soil alkalinity, common in the arid regions, and
to make the soil more productive, the soil can be flushed with
irrigation water. Strongly acidic soils are also detrimental to plant
growth. In acidic soils, soil moisture dissolves nutrients, but they
may be leached away before plant roots can absorb them. Soil acidity
can generally be corrected by adding lime to the soil. In addition
to affecting plant growth, soil acidity or alkalinity also affects
microorganisms in the soil. Microorganisms are highly sensitive to a
soil’s pH, and each species has an optimum environmental setting.
● FIGURE 12.9
This guide to classifying the structure of a soil on the basis of soil peds can be used to help
determine other characteristics of a soil.
How does soil structure affect a soil’s usefulness or suitability for agriculture?
Blocky
Platelike
Platy Lenticular
Prismatic
Blocklike
Prismlike
Columnar
Spheroidal
Nuciform Granular Crumb
Development of Soil Horizons
Soil development begins when plants and animals colonize rocks
or deposits of rock fragments, the parent material on which soil
will form. Once organic processes begin among mineral particles
or rock fragments, differences begin to develop from the surface
down through the parent material.
Initially, vertical differences result from the surface accumulation
of organic litter and the removal of fine particles and dissolved
minerals from upper layers by percolating water that
deposits these materials at a lower level. The vertical cross section
of a soil from the surface down to the parent material is
called a soil profile ( ● Fig. 12.12). Examining soil profiles and
the vertical differences they contain is important to recognizing
different soil types and how those soils developed. As climate,
vegetation, animal life, and characteristics of the land surface
affect soil formation over time, this vertical differentiation becomes
more and more apparent.
Soil Horizons
Within their soil profiles, well-developed soils typically exhibit
several distinct layers, called soil horizons, that are distinguished
by their physical and chemical properties. Soils are classified largely
on the differences in their horizons and in the processes responsible
for those differences ( ● Fig. 12.13). Soil horizons are designated
by a set of letters that refer to their composition, dominant
process, and/or position in the soil profile.
At the surface, but only in locations where
there is a sufficient cover of decomposed vegetation
litter, there will be an O horizon. This
is a layer of organic debris and humus; the
“O” designation refers to this horizon’s high
organic content. Immediately below is the
A horizon, commonly referred to as “topsoil.” In
general, the A horizon is dark because it contains
decomposed organic matter. Beneath the
A horizon, certain soils have a lighter-colored
E horizon, named for the action of strong eluvial
processes. Below this is a zone of accumulation,
the B horizon, where much of the
materials removed from the A and E horizons
are deposited. Except in soils with a high organic
content that has been mixed vertically,
the B horizon generally has little humus. The
C horizon is the weathered parent material from
which the soil has developed—either fragments
of the bedrock, or deposits of rock materials
that were transported to the site by water, wind,
glacial, or other surface process.
The lowest layer, sometimes called the
R horizon, is unchanged parent material, either
bedrock or transported deposits of rock fragments.
Certain horizons in some soils may not
be as well developed as others, and some horizons
may be missing altogether. Because soils

USDA/NRCS/Tim McCabe
(a)
(b)
Increasingly acidic
Neutral
solution
Increasingly basic or alkaline
pH Solution
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Battery acid
Normal stomach acidity (1.0 to 3.0)
Lemon juice (2.3), acid fog (2 to 3.5)
Vinegar, wine, soft drinks, beer
Orange juice
Tomatoes, grapes, acid deposition (4 to 5)
Black coffee, most shaving lotions
Bread
Normal rainwater (5.6)
Milk (6.6)
Saliva (6.2 to 7.4)
Pure water
Blood (7.3 to 7.5), swimming pool water
Eggs
Seawater (7.8 to 8.3)
Shampoo
Baking soda
Phosphate detergents
Chlorine bleach, antacids
Milk of magnesia (9.9 to 10.1)
Soap solutions
Household ammonia (10.5 to 11.9)
Nonphosphate detergents
Washing soda (Na2CO3) Hair remover
Oven cleaner
● FIGURE 12.10
(a) The pH scale of acidity, neutrality, and alkalinity. The degree of acidity
or of alkalinity, called pH, can be easily understood when numbers on
the scale are linked to common substances. Low pH means acidic, and
high pH means alkaline; a reading of 7 is neutral. (b) Alkalinity in a soil
can be tested in the field with drops of a dilute acid. If the soil fizzes in
the acid solution, alkalinity is typically high.
© Hari Eswaran, USDA/NRCS
Alkaline
soils
DEVELOPMENT OF SOIL HORIZONS
30 inches of
rain per year
Acidic
soils
● FIGURE 12.11
The distribution of alkaline and acidic soils in the United States is generally
related to climate. Soils in the East tend to be acidic and those in
the West, alkaline. The dividing line corresponds fairly well with the
30-inch annual precipitation isohyet.
Other than climate, what environmental factors might cause this
east–west variation, and why are some places west of the 30-inch
line acidic?
● FIGURE 12.12
A soil profile is examined by digging a pit with vertical walls to clearly show
variations in color, structure, composition, and other characteristics that
occur with depth. This soil is in a grassland region of northern Minnesota.
329

330
Zone of
eluviation
Zone of
illuviation
CHAPTER 12 SOILS AND SOIL DEVELOPMENT
O i or O c
O a or O e
A
E
B
BC or CB
C
R
Loose leaves and organic debris
Partly decomposed organic debris
Topsoil; dark in color;
rich in organic matter
Zone of intense leaching or eluviation
Zone of accumulation
Lime or Gypsum in mollisols
deeper colored zone of
maximum accumulation
Transition to C
Partly weathered parent material
Regolith or rock layer
● FIGURE 12.13
Soils are categorized by the degree of development and the physical
characteristics of their horizons. Regolith is a generic term for broken
bedrock fragments at or very near the surface.
Which soil profiles shown in Figures 12.28–12.31 display horizons
that are easy to recognize?
and the processes that form them vary widely and can be transitional
between horizons, the horizon boundaries may be either
sharp or gradual. Variations in color and texture within a horizon
are also not unusual.
Factors Affecting Soil Formation
Because of the great variety among the components of soils and
the processes that affected them, no two soils are identical in all
of their characteristics. One important factor is rock weathering,
which refers to the many natural processes that break down rocks
into smaller fragments (weathering will be discussed in detail in
Chapter 15). Chemical reactions can cause rocks and minerals to
decompose and physical processes also cause the breakup of rocks.
Just as statues, monuments, and buildings become “weatherbeaten”
over time, rocks exposed to the elements eventually break
up and decompose.
Hans Jenny, a distinguished soil scientist, observed that soil
development was a function of climate, organic matter, relief, parent
material, and time—factors that are easy to remember by their
initials arranged in the following order: Cl, O, R, P, T. Among
these factors, parent material is distinctive because it is the raw
material. The other factors influence the type of soil that forms
from the parent material.
Parent Material
All soil contains weathered rock fragments. If these weathered
rock particles have accumulated in place—through the physical
and chemical breakdown of bedrock directly beneath the soil—
we refer to the fragments as residual parent material.
If the rock fragments that form a soil have been carried to
the site and deposited by streams, waves, winds, gravity, or glaciers,
this mass of deposits is called transported parent material.
The development and action of organic matter through the life
cycles of organisms and the climatic conditions are primarily
responsible for changing the fragmented rocks or other parent
material into a soil.
Parent material influences the characteristics of a soil in
varying degrees. Some parent materials, such as a sandstone
that contains extremely hard and resistant sand-sized fragments,
are far less subject to weathering than others. Soils that
develop from weathering-resistant rocks tend to have a high
level of similarity to their parent materials. If the bedrock is
easily weathered, the soils that develop tend to be more similar
to soils in other regions that have a similar climate than
to those of comparable parent materials, which formed in a
different climate.
On a global basis, climate and the associated plant communities
produce greater variations in soil characteristics than do parent
materials. Soil differences that are related to variations in parent
material are most visible on a local level.
The longer a soil develops, the influence of parent material
on its characteristics diminishes. Given the same soil-forming
conditions, recently developed soils will show more similarity to
its parent material, compared to a soil that has developed over a
long time.
Many of the chemicals and nutrients in a soil reflect the
composition of its parent material ( ● Fig. 12.14). For example,
calcium-deficient parent materials will produce soils that
are low in calcium, and its natural fauna and plant cover will
be types that require little calcium. Likewise, a parent material
with a high aluminum content will produce a soil that is rich
in aluminum. In fact, the ore of aluminum is bauxite, found in
tropical soils where it has been concentrated by intense leaching
away of the other bases.
The particle sizes that result from the breakdown of parent
material are a prime determinant of a soil’s texture and structure.
A rock material such as sandstone, which contains little clay and
weathers into relatively coarse fragments, will produce a soil of
coarse texture. Parent materials are also an important influence on
the availability of air and water to a soil’s living population.
Organic Activity
Plants and animals affect soil development in many ways. The
life processes of plants growing in a soil are as important as its
microorganisms—the microscopic plants and animals that live
in a soil.
Generally, a dense vegetative cover protects a soil from being
eroded away by running water or wind. Forests form a protective
canopy and produce surface litter, which keeps rain from

R. Gabler
● FIGURE 12.14
Despite strong leaching under a wet tropical climate, Hawaiian soils
remain high in nutrients because their parent material is of recent
volcanic origin.
What other parent materials provide the basis for continuously
fertile soils in wet tropical climates?
beating directly on the soil and increases the proportion of rainwater
entering the soil rather than running off its surface. Variations
in vegetation species and density of cover can also affect
the evapotranspiration rates. A sparse vegetative cover will allow
greater evaporation of soil moisture and dense vegetation tends
to maintain soil moisture.
The characteristics of a plant community affect the nutrient
cycles that are involved in soil development ( ● Fig. 12.15). As
plants die and decompose, or leaves fall to the ground, nutrients
are returned to the soil. Soils, however, can become impoverished
if soluble nutrients that are not used by plants are lost through
leaching. The roots of plants help to break up the soil structure,
making it more porous, and roots also absorb water and nutrients
from the soil.
Leaves, bark, branches, flowers, and root networks contribute
to the organic composition of soil, through litter and
through the remains of dead plants. The organic content of soil
depends on its associated plant life. For example, a grass-covered
prairie supplies much more organic matter than the thin vegetative
cover of desert regions. There is some question, however,
as to whether forests or grasslands (with their thick root networks
and annual life cycle) furnish the soil with greater organic
content. Many of the world’s grassland regions, like the North
American prairies, provide some of the world’s most fertile soils
for cultivation in part because of the high amount of organic
matter that a grass cover generates.
In terms of their contribution to soil formation, bacteria are
perhaps the most important microorganisms that live in soils. Bacteria
break down organic matter, humus, and the debris of living
things into organic and inorganic components, allowing the formation
of new organic compounds that promote plant growth. It
has been suggested that the number of bacteria, fungi, and other
microscopic plants and animals living in a soil may be 1 billion
per gram (a fifth of a teaspoon) of soil. The activities and remains
of these microorganisms, minute though they are individually, add
considerably to the organic content of a soil.
Earthworms, nematodes, ants, termites, wood lice, centipedes,
burrowing rodents, snails, and slugs also stir up the soil,
mixing mineral components from lower levels with organic
components from the upper portion. Earthworms contribute
greatly to soil development because they take soil in, pass it
through their digestive tracts, and excrete it in casts. The process
not only helps mix the soil but also changes the texture,
structure, and chemical qualities of the soil. In the late 1800s,
Charles Darwin estimated that earthworm casts produced in a
year would equal as much as 10–15 tons per acre. As for the
number of earthworms, a study suggested that the total weight
of earthworms beneath a pasture in New Zealand equaled the
weight of the sheep grazing above them.
Climate
FACTORS AFFECTING SOIL FORMATION
Chapters 9 and 10 demonstrated that, on a world regional scale,
climate is a major factor in soil formation. Of course, if the climate
is the same in a region where the soils vary, other factors
must be responsible for the local variation. Soil differences that
are apparent at a local level tend to reflect the influence of factors
such as parent materials, land surface configurations, vegetation
types, and time.
Temperature directly affects soil microorganism activity,
which in turn affects the decomposition rates of organic matter.
In hot equatorial regions, intense activities by soil microorganisms
preclude thick accumulations of organic debris or humus.
● Figure 12.16 shows that the amounts of organic matter and
humus in a soil increase toward the middle latitudes and away
from polar regions and the tropics. In the mesothermal and microthermal
climates (C and D), microorganism activity is slow
enough to allow decaying organic matter and humus to accumulate
in rich layers. Moving poleward into colder regions, retarded
microorganism activity and limited plant growth tends to result
in thin accumulations of organic matter.
Chemical activity increases and decreases directly with temperature,
given equal availability of moisture. As a result, parent
materials of soils in hot, humid equatorial regions are altered to a
far greater degree by chemical means than are parent materials in
colder zones.
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332
CHAPTER 12 SOILS AND SOIL DEVELOPMENT
Temperature affects soil indirectly through its influence on vegetation
associations that are adapted to certain climatic regimes. Soils
generally reflect the character of plant cover because of nutrient
cycles that tend to keep both vegetation and soil in chemical equi-
● FIGURE 12.15
The nutrient cycle in a forest. Trees take up nutrients from the soil through soil water
absorbed into their root systems. Nutrients are supplied by the breakdown of rocks and
minerals, as well as by leaf and other organic litter.
Organic matter
produced or destroyed
8
6
4
Rain of dead
forest detritus
Output into groundwater
and stream flow
Rate of production
(macroflora)
Forest canopy
Tree
trunks
Forest litter
● FIGURE 12.16
The relationship of temperature to production and destruction of organic matter in the soil.
What range of mean annual temperatures is most favorable for the accumulation of humus?
Rate of destruction
by aerobic bacteria
librium. The combined effects of vegetative cover and the climatic
regime tend to produce soil profiles and characteristics that tend to
share certain characteristics among different regions that have similar
climates and vegetation associations ( ● Fig. 12.17).
Moisture conditions affect the development
and character of soils more directly than any other
Humus
absent
2
Destruction by
anaerobic bacteria
0
20 40 60
Mean annual temperature (°F)
80 100
Polar Middle Latitudes
Tropics
Humus accumulates
Root and decay zone
Input from
rock weathering
climatic factor. Without precipitation, and the soil
water it provides, terrestrial plant life would be impossible.
Ample precipitation supports plant growth
that can greatly increase the organic content and
thereby the fertility of a soil. However, extremely
high rainfall will cause leaching of nutrients, and a
relatively infertile soil.
Gravitational and capillary water have pronounced
effects on soil development, structure,
texture, and color. Precipitation is the original
source of soil water (disregarding the minor contribution
of dew), and the amount of precipitation
received affects leaching, eluviation, and illuviation
and thereby rates of soil formation and horizon
development. The evaporation rate is a very
important factor as well. Salt and gypsum deposits
from the upward migration of capillary water are
more extensive in hot, dry regions—such as the
southwestern United States where evaporation
rates are high—than in colder, dry regions (see
again Fig. 12.17).
Land Surface Configuration
The slope of the land, its relief, and its aspect (the
direction it faces) all influence soil development.
Steep slopes are generally better drained than gentler
ones, and they are also subject to rapid runoff
of surface water. As a consequence, there is less infiltration
of water on steeper slopes, which
inhibits soil development, sometimes to the
extent that there will be no soil. In addition,
rapid runoff on steep slopes can erode surfaces
as fast or faster than soil can develop on
them. On gentler slopes, where there is less
runoff and higher infiltration, more water is
available for soil development and to support
vegetation growth, so erosion is not as intense.
In fact, erosion rates in areas of gently
rolling hills may be just enough to offset the
development of soils. Well-developed soils
typically form on land that is flat or has a
gentle slope.
Slope aspect has a direct effect on microclimates
in areas outside of the equatorial
tropics. North-facing slopes in the middle and
high latitudes of the Northern Hemisphere
have microclimates that are cooler and wetter
than those on south-facing exposures, which
receive the sun’s rays at a steeper angle and are
therefore warmer and drier. Local variations

Tropical Rain Forest Soil
(humid, tropical climate)
Desert Soil
(hot, dry climate)
Acidic
lightcolored
humus
Iron and
aluminum
compounds
mixed with
clay
Mosaic
of closely
packed
pebbles,
boulders
Weak humus–
mineral mixture
Dry, brown to
reddish-brown
with variable
accumulations
of clay, calcium
carbonate, and
soluble salts
Deciduous Forest Soil
(humid, mild climate)
Grassland Soil
(semiarid climate)
Forest litter
leaf mold
Humus–mineral
mixture
Light, grayishbrown,
silt loam
Dark brown
firm clay
Alkaline,
dark,
and rich
in humus
Clay,
calcium
compounds
● FIGURE 12.17
Idealized diagrams of five different soil profiles illustrate the effects of climate and vegetation on the development
of soils and their horizons.
Which two environments produce the most humus and which two produce the least?
in soil depth, texture, and profile development result directly from
microclimate differences.
Topography, through its effects on vegetation, indirectly influences
soil development. Steep slopes prevent the formation of
a soil that would support abundant vegetation, and a modest plant
cover yields less organic debris for the soil.
Time
FACTORS AFFECTING SOIL FORMATION
Coniferous Forest Soil
(humid, cold climate)
Acid litter
and humus
Light-colored
and acidic
Humus and
iron and
aluminum
compounds
Soils have a tendency to develop toward a state of equilibrium
with their environment. A soil is sometimes called “mature” when
it has reached such a condition of equilibrium. Young soils are still
in the process of developing toward being in equilibrium with
333

334
From Derek Elsom, Earth, 1992. Copyright © 1992 by Marshall Editions Developments Limited. New York. Macmillan. Used by permission.
CHAPTER 12 SOILS AND SOIL DEVELOPMENT
their environmental conditions. Mature soils have well-developed
horizons that indicate the conditions under which they formed.
Young or “immature” soils typically have poorly developed horizons
or perhaps none at all ( ● Fig. 12.18).
Another effect of time is that, as soils develop, their influence
of their parent material decreases and they increasingly
reflect their climate and vegetative environments. On a global
scale, climate typically has the greatest influence on soils,
provided sufficient time has passed for the soils to become
well developed.
The importance of time in soil formation is especially clear
in soils developed on transported parent materials. Depositional
surfaces are in many cases quite recent in geologic terms and have
not been exposed to weathering long enough for a mature soil
to develop. Deposition occurs in a variety of settings: on river
floodplains where the accumulating sediment is known as alluvium;
downwind from dry areas where dust settles out of the atmosphere
to form blankets of wind-deposited silts, called loess;
and in volcanic regions showered by ash and covered by lava. Ten
thousand years ago, glaciers withdrew from vast areas, leaving
behind jumbled deposits of rocks, sand, silt, and clay.
Because of the great number and variability of materials
and processes involved in the formation of soils, there is no fixed
amount of time that it takes for a soil to become mature. The
Natural Resources Conservation Service, however, estimates that
it takes about 500 years to develop 1 inch of soil in the agricultural
regions of the United States. Generally, though, it takes
thousands of years for a soil to reach maturity.
Soil-Forming Regimes
● FIGURE 12.18
The time that a soil has been developing is important to its composition and physical character. Given enough
time and the proper environmental conditions, soils will become more maturely developed with a deeper
profile and stronger horizon development.
What major changes occur as the soil illustrated here becomes better developed over time?
O horizon
Leaf litter
A horizon
Topsoil
B horizon
Subsoil
C horizon
Parent
material
Oak tree
Fern
Root system
Wood
sorrel
Lords and
ladies
Mature soil
Earthworm
Millipede
Honey
fungus
Red earth
mite
Dog violet
Mole
The characteristics that make major soil types distinctive and
different from one another result from their soil-forming regimes,
which vary mainly because of differences in climate and
vegetation. At the broadest scale of generalization, climate differences
produce three primary soil-forming regimes: laterization,
podzolization, and calcification.
Grasses and
small shrubs
Pseudoscorpion
Mite
Nematode
Organic debris
builds up
Moss and
lichen
Young soil
Actinomycetes
Springtail
Fungus
Bacteria
Regolith
Rock
fragments
Bedrock
Immature soil

R. Gabler
Laterization
Laterization is a soil-forming regime that occurs in humid
tropical and subtropical climates as a result of high temperatures
and abundant precipitation. These climatic environments encourage
rapid breakdown of rocks and decomposition of nearly all
minerals. A soil of this type is known as laterite, and these soils
are generally reddish in color from iron oxides; the term laterite
means “brick-like.” In tropical areas laterite is quarried for building
material ( ● Fig. 12.19).
Despite the dense vegetation that is typical of these climate
regions, little humus is incorporated into the soil because the
plant litter decomposes so rapidly. Laterites do not have an O
horizon, and the A horizon loses fine soil particles as well as
most minerals and bases except for iron and aluminum compounds,
which are insoluble primarily because of the absence
of organic acids ( ● Fig. 12.20). As a result, the topsoil is reddish,
coarse textured, and tends to be porous. In contrast to the A horizon,
the B horizon in a lateritic soil has a heavy concentration
of illuviated materials.
In the tropical forests, soluble nutrients released by weathering
are quickly absorbed by vegetation, which eventually returns
them to the soil where they are reabsorbed by plants. This rapid
cycling of nutrients prevents the total leaching away of bases, leaving
the soil only moderately acidic. Removal of vegetation permits
total leaching of bases, resulting in the formation of crusts of
iron and aluminum compounds (laterites), as well as accelerated
erosion of the A horizon.
Laterization is a year-round process because of the small
seasonal variations in temperature or soil moisture in the humid
tropics. This continuous activity and strong weathering of parent
material cause some tropical soils to develop to depths of as much
as 8 meters (25 ft) or more.
● FIGURE 12.19
Laterite cut for building stone and stacked along a village road in the state of
Orissa, India.
Why is building with brick or stone rather than wood so important in heavily
populated, less developed nations such as India?
A
B
C
Podzolization
Podzolization occurs mainly in the high middle latitudes where
the climate is moist with short, cool summers and long, severe winters.
The coniferous forests of these climate regions are an integral
part of the podzolization process.
Where temperatures are low much of the year, microorganism
activity is reduced enough that humus does accumulate; however,
because of the small number of animals living in the soil,
there is little mixing of humus below the surface. Leaching and
eluviation by acidic solutions remove the soluble bases and aluminum
and iron compounds from the A horizon ( ● Fig. 12.21).
The remaining silica gives a distinctive ash-gray color to the
E horizon (podzol is derived from a Russian word meaning
“ashy”). The needles that coniferous trees drop are
chemically acidic and contribute to the soil acidity. It is
difficult to determine whether the soil is acidic because
of the vegetative cover or whether the vegetative cover is
adapted to the acidic soil.
Podzolization can take place outside the typical cold,
moist climate regions if the parent material is highly
acidic—for example, on the sandy areas common along
the East Coast of the United States. The pine forests that
grow in such acidic conditions return acids to the soil,
promoting the process of podzolization.
Calcification
Little or no organic
debris, little silica,
much residual iron
and aluminum,
coarse texture
SOIL-FORMING REGIMES
Some illuvial bases,
much accumulated
laterite
Much of the soluble
material lost to
drainage
● FIGURE 12.20
Soil profile horizons in laterite, one of three major soil-forming regimes. Laterization
is a soil development process that occurs in tropical and equatorial
zones that experience warm temperatures year round and wet climates.
A third distinctive soil-forming regime is called
calcification. In contrast to both laterization and podzolization,
which require humid climates, calcification
occurs in regions where evapotranspiration significantly
exceeds precipitation. Calcification is important in the
climate regions where moisture penetration is shallow.
The subsoil is typically too dry to support tree growth,
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336
CHAPTER 12 SOILS AND SOIL DEVELOPMENT
O i
O e
A
E
B
C
Well-developed
organic horizons
Thin, dark
Badly leached, light
in color, largely Si
Darker than E;
often colorful;
accumulations of
humus; Fe, Al, N,
Ca, Mg, Na, K
Some Ca, Mg, Na,
and K leached
down from B is
lost to lateral
movement of water
below water table
● FIGURE 12.21
Soil profile horizons in a podzol soil, another of the three major soilforming
regimes. Podzolization typically occurs under cool, wet climates
in regions of coniferous trees or in boggy environments, and is a very
acidic soil type.
and shallow-rooted grass or shrubs are the primary forms of
vegetation. Calcification is enhanced as grasses use calcium,
drawing it up from lower soil layers and returning it to the soil
when the annual grasses die. Grasses and their dense root networks
provide large amounts of organic matter, which is mixed
deep into the soil by burrowing animals. Middle-latitude grassland
soils are rich in bases and in humus and are the world’s
most productive agricultural soils. The deserts of the American
West generally have no humus, and the rise of capillary water
can leave not only calcium carbonate but also sodium chloride
(salt) at the surface.
In many areas of low precipitation, the air is often loaded
with alkali dusts such as calcium carbonate (CaCO 3 ). When
calm conditions prevail or when it rains, the dust settles and
accumulates in the soil. The rainfall produces an amount of
soil water that is just sufficient to translocate these materials
to the B horizon ( ● Fig. 12.22). Over hundreds to thousands
of years, the CaCO 3 -enriched dust concentrates in the B horizon,
forming hard layers of caliche. Much thicker accumulations
called calcretes ( ● Fig. 12.23) form by the upward (capillary)
movement of dissolved calcium in groundwater when the water
table is near the surface.
Regimes of Local Importance
Two additional localized soil-forming regimes merit attention.
Both characterize areas with poor drainage although they occur
under very different climate conditions. The first, salinization,
or the concentration of salts in the soil, is often detrimental to
plant growth. Salinization occurs in stream valleys, interior basins,
and other low-lying areas, particularly in arid regions with high
groundwater tables. The high groundwater levels can be the result
of water from adjacent mountain ranges, stream flow originating
in humid regions, or a wet–dry seasonal precipitation regime
( ● Fig. 12.24). Salinization can also be a consequence of intensive
irrigation under arid conditions. Rapid evaporation leaves
© State of Victoria, Department of Primary Industries, 1997
● FIGURE 12.22
Soil profile horizons in a calcified soil, formed by the third major soilforming
regime. Calcification is a soil development process that is most
prominent in cool to hot subhumid or semiarid climate regions, particularly
in grassland regions, but also in deserts.
O
A
B
C
Dark color,
granular structure,
high content of
residual bases
Lighter color,
very high content
of accumulated
bases, caliche
nodules
Relatively
unaltered,
rich in base
supply, virtually
no loss to
drainage water
● FIGURE 12.23
This shallow-rooted grass is growing in a thin layer of topsoil
over a much thicker layer of calcium carbonate called calcrete.
What precipitation characteristics are associated with the
calcification soil-forming process?

Awidely used analogy for
understanding the amount of soil
that exists on Earth uses an apple
to demonstrate that soil, which is adequate
for the world’s agricultural needs, is a rather
limited natural resource. If the apple is cut
into quarters, three of those pieces (75%)
represent the water bodies on Earth and
should be put away. The piece that remains
represents the Earth’s land area (25%)
where soil could exist. Half of the remaining
piece should be cut off and put away, representing
rocky desert, polar, or high mountain
regions, where soil does not exist or will
not grow much because of harsh climatic
conditions. Now there is an eighth (12.5%)
of the apple remaining. This eighth of the
apple should then be cut into four pieces,
and three of them put away as they represent
areas with local conditions that preclude
agriculture (too rocky, steep, wet, and
so on) and the areas that we have already
covered over with cities, towns, and roads.
The remaining apple slice (about 3% of
Earth) represents areas with good soils for
agriculture to feed the world’s population.
Soil Degradation and Soil Loss
Today, even areas with good soils are
under continuing threat. It may take
200–1000 years or more to develop
2.5 centimeters (1 in.) of soil, but through
erosion by water or wind, thousands of
years of soil development may be lost in
a season or in days. Land degradation by
humans is a major cause of soil loss, but
impacts from changing climates, particularly
desertification, also play a role. Overgrazing,
deforestation, overuse of the land, and
poor agricultural practices are the major
human-related causes of soil loss. The
United Nations estimates that, just through
erosion, the world loses between 5 and
7 million hectares (12.36–17.30 million
acres) of farmland every year. This is an
area equal to the size of West Virginia.
SOIL-FORMING REGIMES
GEOGRAPHY’S ENVIRONMENTAL SCIENCE PERSPECTIVE
Soil Resources Are Limited and Threatened: How
Much Good Soil Is There on Earth?
NRCS, USDA Philippe Rekacewicz, UNEP/GRID-Arendal. Data from UNEP,
International Soil Reference and Information Centre (ISRIC), World Atlas of
Desertification, 1997. http://maps.grida.no/go/graphic/degraded-soils
A world map done by the United Nations shows global patterns—
the geography—of soil degradation by various degrees.
What major geographic factors explain the areas with the
highest levels of degradation, and what explains areas with
the lowest impact on soils?
USDA/NRCS/Lynn Betts
In addition to the degradation of soil
that is ongoing, farmlands with excellent
soils are being taken out of production
by urbanization/suburbanization. Many
farmlands are attractive to land developers,
because they are already cleared, the soils
are good for lawns and landscaping, and
cities and suburbs are expanding into surrounding
agricultural lands.
Our planet is losing its arable soil, while
the population is growing, a trend that cannot
continue forever. One estimate is that
soils are being lost at a rate 17 times faster
than the rate at which they form. Many
government and private agencies around
the world are working to educate people
about the critically important problem of
soil degradation, and to support solutions
to minimize human impacts on soils. The
problems associated with soil degradation
are globally significant, but the solutions
are proper conservation and management
at local levels.
These prime farmlands in Iowa are being converted into
suburbs as populations grow and towns expand.
Are there agricultural lands surrounding the place where
you live that are currently being converted into housing,
commercial, or industrial areas?
337

338
USDA/NRCS/Tim McCabe
CHAPTER 12 SOILS AND SOIL DEVELOPMENT
● FIGURE 12.24
Salinization is indicated by these white deposits on this field in Colorado.
Surface salinity has resulted from the upward capillary movement of
water and evaporation at the surface causing deposits of salt. The soil
cracks also indicate shrinkage caused by evaporative drying of the soil.
What negative soil effects can result when humans practice irrigated
agriculture in regions that experience great evaporation rates?
behind a high concentration of soluble salts and may destroy a
soil’s agricultural productivity. An extreme example of salinization
exists in certain areas of the Middle East, where thousands of
years of irrigated agriculture in the desert have made the soils too
saline to cultivate today.
Another localized soil regime, gleization, occurs in poorly
drained areas under cold, wet environmental conditions. Gley
soils, as they are called, are typically associated with peat bogs
where the soil has an accumulation of humus overlying a bluegray
layer of thick, gummy, water-saturated clay. In poorly drained
regions that were formerly glaciated, such as northern Russia, Ireland,
Scotland, and Scandinavia, peat has long been harvested and
used as a source of energy.
Soil Classification
Soils, like climates, can be classified by their characteristics and
mapped by their spatial distributions. In the United States the Soil
Survey Division of the Natural Resources Conservation Service
(NRCS), a branch of the Department of Agriculture, is responsible
for soil classification (termed soil taxonomy). As with any
classification system, the methods and categories are continually
being updated and refined.
Soil classifications are published in soil surveys, books that
outline and describe the kinds of soils in a region and include
maps that show the distribution of soil types, usually at the county
level. These documents, available for most parts of the United
States, are useful references for factors such as soil fertility, irrigation,
and drainage.
The NRCS Soil Classification System
The NRCS soil classification system is based on the development
and composition of soil horizons. The largest division in the classification
of soils is the soil order, of which 12 are recognized
by the NRCS. To provide greater detail, soil orders can also be
subdivided into suborders, great groups, subgroups, families, and
series. More than 10,000 soil series have been recognized in the
United States.
The NRCS system of soil types uses names derived from
root words of classical languages such as Latin, Arabic, and Greek
to refer to the different soil categories. The names, like the system,
are precise and consistent and were chosen to describe the characteristics
that distinguish one soil from another. Some soil orders
reflect regional climate conditions; however, other soil orders reflect
the recency or type of parent material, so the distribution of
these soils does not conform to climate regions.
When examining a soil for classification under the NRCS
system, particular attention is paid to characteristic horizons and
textures. Some of these horizons are below the surface (subsurface
horizons); others, called epipedons, are surface layers that
usually exhibit a dark shading associated with organic material
(humus). Examples of some of the more common horizons, illustrating
how names were chosen to represent actual soil properties,
are found in Table 12.1.
NRCS Soil Orders
The 12 soil orders are based on a variety of characteristics and
processes that can be recognized by examining a soil and its profile.
The soil descriptions that follow are based on the sequence
shown in ● Figure 12.25, which illustrates the links between
climate and soils. ● Figure 12.26 is a map showing the distribution
of dominant soil orders in the United States. Global soils
based on the NRCS classification of soil orders are shown in
● Figure 12.27. Frequent comparison of Figure 12.27 with the
map of world climates in Figure 8.6 will illustrate the relationships
between the global distributions of soil and climate.
Entisols are soils that have undergone little or no soil development
and lack horizons, because they have only recently begun
to form ( ● Fig. 12.28a). They are often associated with the continuing
erosion of sloping land in mountainous regions or with
the frequent deposition of alluvium by flooding, or in areas of
windblown sand.
Inceptisols are young soils with weak horizon development
(Fig. 12.28b). The processes of A horizon depletion
(eluviation) and B horizon deposition (illuviation) are just beginning,
usually because of a very cold climate, repeated floodrelated
deposition, or a high rate of soil erosion. In the United

From Nature and Properties of Soils, 12/e by Brady and Weil, © 1998. Reprinted by permission of Pearson Education, Inc.,
Upper Saddle River, NJ.
USDA/NRCS
Little or no
development
Entisols
(recent)
Various
conditions
Organic
plant material,
wet environment
Mild
weathering
Inceptisols
(beginning)
Various
conditions
Swampy,
boggy areas
Histosols
(organics,peat)
Permanently
frozen
subsoil
Mild
weathering
on volcanic
ejecta
Tundra regions
Short
grasses,
desert
shrubs
Warm to hot,
high base
status, high
activity clays
Andisols
(volcanic
ejecta)
Gelisols
(permafrost)
Dry to
desert
Aridisols
(dry)
Semi arid/
moist
grasslands
Moist, mildly
acid
Vertisols
(swelling
clays)
Mollic
epipedon
Mollisols
(soft)
Cool, wet,
sandy,
acid
Silicate
clays, Fe,
Al oxides
Alfisols
(mild
forest soil)
Broadleaf
forests
Degree of weathering and soil development
Coniferous
forests
Spodosol
(spodic
horizon)
Ultiisols
(ultimate)
Wet tropical and
subtropical forests
Slight Intermediate
Strong
● FIGURE 12.25
The NRCS soil orders. The soil orders of the NRCS can be linked to the parent materials, climate, and vegetation
of the region in which they formed. The linkages form a treelike pattern, as seen here.
How is degree of weathering related to climatic characteristics?
● FIGURE 12.26
The distribution of soils in the United States according to the National Resource Conservation Service’s
classification.
What kind of soil dominates the place where you live, according to this map?
SOIL CLASSIFICATION
Wet, tropical
forests, extreme
weathering
Oxisols
clays
Fe Al oxides
339

340
CHAPTER 12 SOILS AND SOIL DEVELOPMENT
TABLE 12.1
Common Soil Horizons (NRCS Soil Classification System)*
Oxic horizon (from oxygen)
Subsurface horizon, in low-elevation tropical and subtropical climates, that contains oxides of iron and aluminum.
Argillic horizon (from Latin: argilla, clay)
Layer formed beneath the A horizon by illuviation that contains a high content of accumulated clays.
Ochric epipedon (from Greek: ochros, pale)
A surface horizon that is light in color and either very low in organic matter, or very thin.
Albic horizon (from Latin: albus, white)
An A2 horizon, sandy and light-colored due to the removal of clay and iron oxides, that is above a spodic horizon.
Spodic horizon (from Greek: spodos, wood ash)
Beneath an A2 horizon, this layer is dark-colored from illuviated humus, and oxides of aluminum and/or iron.
Mollic epipedon (from Latin: mollis, soft)
A dark-colored surface layer with a high content of basic substances (calcium, magnesium, potassium).
Calcic horizon (from calcium)
A subsurface horizon that is rich in accumulated calcium carbonate or magnesium carbonate.
Salic horizon (from salt)
A soil layer, common in desert basins, that is at least 6 inches thick and contains at least 2% salt.
Gypsic horizon (from gypsum)
A subsurface soil horizon that is rich in accumulated calcium sulfate (gypsum).
* This table includes only some of the more common horizons.
States, Inceptisols are most common in Alaska, the lower
Mississippi River floodplain, and the western Appalachians.
Globally, Inceptisols are especially important along the lower
portions of the great river systems of South Asia, such as the
Ganges-Brahmaputra, the Irrawaddy, and the Mekong. In these
areas, sediments associated with periodic flooding constantly
enrich Inceptisols, and form the basis for agriculture that supports
millions of people.
Histosols develop in poorly drained areas, such as swamps,
meadows, or bogs, as a product of gleization (Fig. 12.28c). They
are largely composed of decomposing plant material. The waterlogged
soil conditions deprive bacteria of the oxygen necessary to
decompose the organic matter. Although Histosols may be found
in low areas with poor drainage at all latitudes, they are most common
in tundra areas or in recently glaciated, high-latitude locations
such as Scandinavia, Canada, Ireland, and Scotland. Histosols
in the subpolar latitudes are commonly acidic and only suitable
for special bog crops such as cranberries. Histosols are the primary
source of peat, which is a fuel source in some regions and also is
used in landscaping.
Andisols are soils that develop on volcanic parent materials,
usually volcanic ash, the dust-sized particles emitted by volcanoes
( ● Fig. 12.29a). Because many of these soils are replenished
by eruptions, they are often fertile. Intensive agriculture atop
Andisols supports dense populations in the Philippines, Indonesia,
and the West Indies. In the United States, Andisols are
most common on the slopes of and downwind from volcanoes
in the Pacific Northwest and to a lesser extent in Hawaii
and Alaska.
Gelisols are soils that experience frequent freezing and thawing
of the ground, above permafrost, permanently frozen subsoil
(Fig. 12.29b). When soil freezes, the ice that forms takes up 9%
more space than the liquid water that it replaces. To accommodate
the increased space taken up by ice, the soil and the particles
in it are pushed upward and outward, away from forming
ice cores. When the surface soil thaws, gravity pulls the waterlogged
ground back downward. Repeated cycles of freezing
and thawing mix and churn the upper soil in a process called
cryoturbation—mixing (turbation) related to freezing (cryo). Only
the upper part of the soil undergoes freeze–thaw cycles. Permafrost
does not permit soil water to percolate downward, so Gelisol
soils are typically water saturated when they are not frozen at
the surface.
Gelisols occur in tundra and subarctic climate regions where
soil development tends to be slow because chemical processes operate
slowly in cold environments. This soil type is found in north

and central Alaska and Canada, in Siberia, and in high-altitude
tundra areas.
Aridisols are soils of desert regions that develop primarily
under conditions where precipitation is much less than half of
potential evaporation (Fig. 12.29c). Consequently, most Aridisols
reflect the calcification process. Where groundwater tables are
high, evidence of salinization may also be present.
Although Aridisols tend to have weak horizon development
because of limited water movement in the soil, there is often a
subsurface accumulation of calcium carbonate (calcic horizon),
salt (salic horizon), or calcium sulfate (gypsic horizon). Soil humus
is minimal because vegetation is sparse in deserts; therefore,
Aridisols are often light in color. Aridisols are usually alkaline,
but because few nutrients have been leached, they can support
productive agriculture if irrigated to reduce the pH and salinity.
Geographically, Aridisols are the most common soils on Earth
because deserts cover such a large portion of the land surface.
Vertisols are typically found in regions of strong seasonality
of precipitation such as the tropical wet and dry climates
( ● Fig. 12.30a). In the United States, they are most common
where the parent materials produce clay-rich soils. The combination
of clayey soils in a wet and dry climate leads to the
drying of the soil and consequent shrinkage that forms deep
cracks during the dry season, followed by expansion of the soil
during the wet season. The constant shrink–swell process disrupts
horizon formation to the point that soil scientists often
describe Vertisols as “self-plowing” soils. Vertical soil movement
may damage highways, sidewalks, foundations, and basements
that are built on shrink–swell soils. Vertisols are dark-colored, are
high in bases, and contain considerable organic material derived
from the grasslands or savanna vegetation with which they are
normally associated. Although they harden when dry and become
sticky and difficult to cultivate when swollen with moisture,
Vertisols can be agriculturally productive.
Mollisols are most closely associated with grassland regions
and are among the best soils for sustained agriculture
(Fig. 12.30b). Because they are located in semiarid climates,
Mollisols are not heavily leached, and they have a generous
supply of bases, especially calcium. The characteristic horizon
of a Mollisol is a thick, dark-colored surface layer rich
in organic matter from the decay of abundant root material.
Grasslands and associated Mollisols served as the grazing lands
for countless herds of antelope, bison, and horses. Before the
invention of the steel plow, the thick root material of grasses
made this soil nearly uncultivable in the United States and
thus led to the widespread public image of the Great Plains as
a “Great American Desert.”
In regions of adequate precipitation, such as the tall-grass
prairies of the American Midwest, the combination of soils and
climate is unexcelled for agriculture. In areas of lesser precipitation,
periodic drought is a constant threat, and the temptation of
fertile soils was the downfall of many farmers prior to the advent
of center pivot irrigation.
Alfisols occur in a wide variety of climate settings. They are
characterized by a subsurface clay horizon (argillic B horizon), a
SOIL CLASSIFICATION
medium to high base supply, and a light-colored ochric epipedon
(Fig. 12.30c). The five suborders of Alfisols reflect climate types
and exemplify the hierarchical nature of the classification system:
Aqualfs are seasonally wet and can be found in mesothermal areas
such as Louisiana, Mississippi, and Florida; Boralfs are found
in moist, microthermal climates such as Montana, Wyoming, and
Minnesota; Udalfs are common in both microthermal and mesothermal
climates that are moist enough to support agriculture
without irrigation, such as Wisconsin, Ohio, and Tennessee; Ustalfs
are found in mesothermal climates that are intermittently dry,
such as Texas and New Mexico; and Xeralfs are found in California’s
Mediterranean climate, which is characterized by wet winters
and long, dry summers.
Because of their abundant bases, Alfisols can be very productive
agriculturally if local deficiencies are corrected: irrigation for
the dry suborders, properly drained fields for the wet suborders.
Spodosols are most closely associated with the podzolization
soil-forming process. They are readily identified by their strong
horizon development ( ● Fig. 12.31a). There is often a white or
light-gray E horizon (albic horizon) covered with a thin, black
layer of partially decomposed humus and underlain by a colorful
B horizon enriched in relocated iron and aluminum compounds
(spodic horizon).
Spodosols are generally low in bases and form in porous substrates
such as glacial drift or beach sands. In New England and
Michigan, Spodosols are also acidic. In these regions, as well as
in similar regions in northern Russia, Scandinavia, and Poland,
only a few types of agricultural plants, such as cucumbers and
potatoes, can tolerate the microthermal climates and sandy, acidic
soils. Consequently, the cuisine of these regions directly reflects
the Spodosols that dominate the areas.
Ultisols, like Spodosols, are also low in bases because they develop
in moist or wet regions. Ultisols are characterized by a subsurface
clay horizon (argillic horizon) and are often yellow or red
because of residual iron and aluminum oxides in the A horizon
(Fig. 12.31b). In North America, the Ultisols are most closely associated
with the southeastern United States. When first cleared of
forests, these soils can be agriculturally productive for several decades.
But a combination of high rainfall with the associated runoff
and erosion from the fields decreases the natural fertility of the soils.
Ultisols remain productive only with the continuous application
of fertilizers. Today, forests cover many former cotton and tobacco
fields of the southeastern United States because of a reduction in
soil fertility and extensive soil erosion (see again Fig. 10.9).
Oxisols have developed over long periods of time in tropical
regions with high temperatures and heavy annual rainfall. They are
almost entirely leached of soluble bases and are characterized by
a thick development of iron and aluminum oxides (Fig. 12.31c).
The soil consists mainly of minerals that resist weathering (for example,
quartz, clays, hydrated oxides). Oxisols are most closely associated
with the humid tropics, but they also extend into savanna
and tropical thorn forest regions. In the United States, Oxisols
are present only in Hawaii. Oxisols are dominated by laterization
and retain their natural fertility only as long as the soils and forest
cover maintain their delicate equilibrium. The bases in the tropical
341

342
CHAPTER 12 SOILS AND SOIL DEVELOPMENT
USDA/NRCS
● FIGURE 12.27
The global distribution of soils by NRCS soil orders.
How do these patterns resemble the spatial distribution of world climates?

SOIL CLASSIFICATION
343

344
© Hari Eswaran, USDA/NRCS
© Hari Eswaran, USDA/NRCS
CHAPTER 12 SOILS AND SOIL DEVELOPMENT
(a) (b)
● FIGURE 12.28
Soil profile examples: (a) Entisols, (b) Inceptisols, and (c) Histosols.
(a) (b)
● FIGURE 12.29
Soil profile examples: (a) Andisols, (b) Gelisols, and (c) Aridisols.
rainforests are stored mainly in the vegetation. When a tree dies,
epiphytes and insects must recycle the bases rapidly before the
heavy rainfall leaches them from the system.
The burning of vegetation associated with slash-and-burn
agriculture in rainforests releases the nutrients necessary for crop
© Hari Eswaran, USDA/NRCS
© Hari Eswaran, USDA/NRCS
© Hari Eswaran, USDA/NRCS
© Hari Eswaran, USDA/NRCS
(c)
(c)
growth but quickly results in their loss from the ecosystem. Many
tropical Oxisols that once supported lush forests are now heavily
dissected by erosion and only support a combination of weeds,
shrubs, and grasses.

© Hari Eswaran, USDA/NRCS
© Hari Eswaran, USDA/NRCS
(a) (b)
● FIGURE 12.30
Soil profile examples: (a) Vertisols, (b) Mollisols, and (c) Alfisols.
(a) (b)
● FIGURE 12.31
Soil profile examples: (a) Spodosols, (b) Ultisols, and (c) Oxisols.
Soil as a Critical
Natural Resource
Regardless of their composition, origin, or state of development,
Earth’s soils remain one of our most important and vulnerable
resources. The word fertility, so often associated with soils, has a
meaning that takes into consideration the usefulness of a soil to
© Hari Eswaran, USDA/NRCS
© Hari Eswaran, USDA/NRCS
SOIL AS A CRITICAL NATURAL RESOURCE
© Hari Eswaran, USDA/NRCS
© Hari Eswaran, USDA/NRCS
(c)
(c)
humans. Soils are fertile in respect to their effectiveness in producing
specific vegetation types or associations. Some soils may
be fertile for corn and others for potatoes. Other soils retain their
fertility only as long as they remain in delicate equilibrium with
their vegetative cover.
It is clearly the responsibility of all of us who enjoy the
agricultural products of farms, ranches, and orchards, as well
as appreciate the natural beauty of Earth’s diverse biomes,
345

346
USDA/NRCS/Lynn Betts
CHAPTER 12 SOILS AND SOIL DEVELOPMENT
● FIGURE 12.32
Gully erosion on farmlands is a significant problem that can often
be avoided or overcome by proper agricultural practices. Gullying,
if unchecked, can alter the landscape to the point that the original
productivity of the land cannot be regained.
What could have been done to prevent the kind of soil loss shown
in this example?
to help protect our valuable soils. Soil erosion, soil depletion,
and the mismanagement of land are problems that we should
have great concern about in the world today ( ● Fig. 12.32).
We should also be aware that these problems have reasonable
Chapter 12 Activities
Define & Recall
soil
soil fertilization
capillary water
hygroscopic water
gravitational water
leaching
eluviation
illuviation
hardpan
stratification
humus
soil texture
clayey
clay
silty
silt
sandy
sand
soil grade
USDA/NRCS/Lynn Betts
loam
infiltrate
soil ped
porosity
permeability
pH scale
parent material
soil profile
soil horizon
Cl, O, R, P, T
residual parent material
transported parent material
soil-forming regime
laterization
laterite
podzolization
calcification
salinization
● FIGURE 12.33
This farm’s contour farming techniques, and the use of buffer zones
between fields and along the water course, are excellent examples of
soil conservation methods.
What other soil conservation practices are often used to preserve
the soil resource?
solutions ( ● Fig. 12.33). Conserving soils and maintaining soil
fertility are critical challenges that are essential to maintaining
natural environments, as well as supporting life on Earth
today and for the future.
gleization
soil taxonomy
soil survey
soil order
subsurface horizon
epipedon
Entisol
Inceptisol
Histosol
Andisol
Gelisol
Aridisol
Vertisol
Mollisol
Alfisol
Spodosol
Ultisol
Oxisol

Discuss & Review
1. Why is soil an outstanding example of the integration and
interaction among Earth’s subsystems?
2. Describe the different circumstances in which water is found
in soil.
3. Under what conditions does leaching take place? What is
the effect of leaching on the soil and, consequently, on the
vegetation that it supports?
4. How can capillary water contribute to the formation of
caliche? What is the effect of caliche on drainage?
5. How is humus formed? What relation does humus have to
soil fertility?
6. How is texture used to classify soils? Describe the ways
scientists have classified soil structure.
7. What pH range indicates soil suitable for most complex
plants?
Consider & Respond
1. Refer to Figure 12.13 and associated pages in the text.
a. What horizons make up the zone of eluviation?
b. What are two processes that occur in the zone of eluviation?
c. The various B horizons are in what zone?
d. Weathered parent material is the major constituent of
what horizon?
e. Partly decomposed organic debris makes up which horizon?
2. Refer to Table 12.1.
a. What materials accumulate in an argillic horizon?
Apply & Learn
1. Refer to Figure 12.8. Using the texture triangle, determine
the textures of the following soil samples.
Sand Silt Clay
a. 35% 45% 20%
b. 75% 15% 10%
c. 10% 60% 30%
d. 5% 45% 50%
What are the percentages of sand, silt, and clay of the
following soil textures? (Note: Answers may vary, but they
should total 100%.)
e. Sandy clay
f. Silty loam
2. Obtain a small sample of soil (a handful or so) and try to discover
its texture by using the following method: Wet the soil
a bit and work it in your hand.
I. First, if you can form a ribbon of soil by kneading it
with your fingers:
CHAPTER 12 ACTIVITIES
8. What are the general characteristics of each horizon in a soil
profile? How are soil profiles important to scientists?
9. What factors are involved in the formation of soils? Which is
most important on a global scale?
10. How does transported parent material differ from residual
parent material? List those factors that help determine how
much effect the parent material will have on the soil.
11. What are the most important effects of parent material on
soil?
12. How does the presence of earthworms and other burrowing
animals affect soil?
13. Describe the various ways in which temperature and
precipitation are related to soil formation.
14. Describe the three major soil-forming regimes.
b. Which would generally be better suited for agriculture—a
soil with an ochric epipedon or a mollic epipedon? Why?
c. What name would be given to a 7-inch-thick horizon
that contained at least 2% salt?
3. Where would you rank soils in terms of importance among
a nation’s environmental resources?
4. Give your opinion of the overall value of soils in the United
States and the extent to which these soils are preserved and
protected.
If you can form a ribbon that is long relatively strong and
flexible: the soil is a clay.
If you can form a ribbon that is weak and breaks easily, and
the soil can be rolled into a coherent ball: the soil is a clay
loam.
If the clay loam looks powdery when dry: the soil is a silty
clay loam.
If the clay loam has a gritty feel with visible sand: it is a sandy
clay loam.
II. Second, if you cannot form a ribbon because the soil
breaks up:
If damp soil breaks up easily, but is gritty, yet still sticky
enough to make a ball: the soil is a loam.
If it feels gritty and sand can be seen, and the ball breaks up
easily in your hands: the soil is a sandy loam.
III. Third, if the soil is very loose with visible grains of
about the same size:
If you can see grains in the soil, and the mass in your hand
breaks up easily: the soil is a sand.
347