Name
CHAPTER 14
Class
Date
Weathering and Erosion
3 Soil
SECTION
KEY IDEAS
As you read this section, keep these questions in mind:
•
•
•
•
How does soil form?
How does parent rock affect soil composition?
What are the layers of mature residual soils?
What types of soil form in arctic and tropical
climates?
How Does Soil Form?
Weathering forms a layer of rock fragments that covers much of Earth’s surface. This layer is called regolith.
The solid, unweathered rock that lies beneath the regolith
is called bedrock. The upper rock fragments in regolith
weather to form a layer of fine particles. These particles
are an important component of soil. Soil is a mixture of
minerals, water, gases, and the remains of organisms.
Much of the material in soil comes from weathered
rock. The original rock that produced the weathered
material is called the parent rock.
The characteristics of soil depend on its parent rock.
Some soil forms and stays directly over its parent rock.
This soil is residual soil. Sometimes, wind and water
carry soil particles away from their parent rock. Such
soil is called transported soil. Transported soil may have
different characteristics than the rock underneath it.
Soil composition refers to the materials that make up
soil. The color of soil is related to the soil’s composition.
For example, black soils are generally rich in organic
material. Soil moisture can also affect color. Moist soils
are generally darker than dry soils.
Rock particles in soil can be grouped by size, as shown
below.
Particle Type
Diameter of Particle
Clay
less than 0.002 mm
Silt
from 0.002 mm to 0.05 mm
Sand
from 0.05 mm to 2 mm
READING TOOLBOX
Ask Questions Before you
read this section, create a
three-column chart. Fill in
the first two columns of the
chart with what you already
know about soil and what
you want to know about soil.
After you read the section,
fill out the last column in the
chart with what you learned
about soil. Discuss your chart
with a partner.
READING CHECK
1. Compare How do
residual soil and transported
soil differ?
LOOKING CLOSER
2. Identify Which type of
soil particle is largest?
The amount of clay, silt, and sand in a soil determines
the soil’s texture.
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SECTION 3
Class
Date
Soil continued
What Is a Soil Profile?
READING CHECK
3. Describe Describe the
characteristics of the A
horizon.
Residual soils typically develop distinct layers over
time. Scientists use a soil profile to study the layers of
soil. A soil profile is a cross section of, or slice through,
soil and its bedrock. The different layers of soil are called
horizons.
Residual soils generally have three main horizons. The
A horizon, or topsoil, is a mixture of organic materials
and small rock particles. Most organisms that live in
soil live in the A horizon. When the organisms die, their
remains decay. Decay produces humus, a dark, organic
material.
The B horizon, or subsoil, has minerals from the
topsoil, clay, and sometimes humus. In dry climates,
minerals collect in the B horizon as water evaporates.
The C horizon has partially weathered bedrock. The
first stages of mechanical and chemical change happen in
this layer. The figure below shows a soil profile.
Soil Horizons of Residual Soils
A
Horizon
B
Horizon
LOOKING CLOSER
As it sinks into the ground,
water carries minerals to
lower layers of soil.
C
Horizon
4. Identify What lies under
the C horizon?
Bedrock
solid rock layer
How Does Climate Affect Soil?
Climate determines the weathering processes that
happen in an area. These weathering processes affect
soil composition.
READING CHECK
5. Explain How does dense
plant growth affect the soil in
tropical climates?
TROPICAL SOILS
In humid tropical climates, where it is hot and rainy,
chemical weathering happens quickly. Thick soils called
laterites form. Heavy rains remove minerals and nutrients
from the topsoil, making the topsoil layer thin. However,
tropical climates also have dense plant life. These plants
add organic material to the soil. As a result, a thin layer
of humus covers the B horizon.
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Soil continued
TEMPERATE SOILS
In temperate climates, temperatures range between
low and high, and rainfall is moderate. Both chemical and
physical weathering are important in temperate regions.
Temperate soils have the thickest A horizon.
Two main soil types form in temperate climates:
pedalfer and pedocal. Pedalfer soils form in areas with
more than 65 cm of rain per year. Pedalfer soils contain
iron, clay, and quartz. Pedocal soils form in areas with
less than 65 cm of rain per year. Pedocal soils contain
large amounts of calcium carbonate.
DESERT AND ARCTIC SOILS
Desert and arctic regions do not get much rain. In
these climates, chemical weathering happens slowly. The
soil forms mainly by mechanical weathering. For this
reason, the soil is thin and made up mostly of regolith.
Desert and arctic regions have relatively little plant or
animal life, so their soils have little humus.
Tropical
Temperate
READING CHECK
6. Identify The Gulf Coast
states generally receive more
than 65 cm of rain per year.
Which kind of soils would
you expect to find in this
region?
Desert/Artic
A Horizon
B Horizon
Tropical climates produce
thick, infertile soils. Temperate
climates produce thick, fertile
soils. Desert and arctic climates
produce thin soils.
C Horizon
LOOKING CLOSER
7. Compare How is tropical
soil different from temperate
soil?
Bedrock
How Does Topography Affect Soil?
Topography, or the shape of the land, also affects soil
formation. As rainwater runs down a slope, it washes
away topsoil. As a result, the soil on the slope is thin. The
soil at the top and bottom of the slope is thicker.
Topsoil on a slope is often too thin to support dense
plant life. For this reason, the soil is poor and lacks
humus. The land at the bottom of slopes often has thick,
wet soil. The soil supports organisms and is rich in
humus.
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Section 3 Review
SECTION VOCABULARY
horizon a horizontal layer of soil that can be distinguished from the layers above and below it
humus dark, organic material formed in soil from
the decayed remains of plants and animals
soil a loose mixture of rock fragments and organic material that can support the growth of
vegetation
soil profile a vertical section of soil that shows
the layers, or horizons
1. Identify List the three main layers in mature residual soils.
2. Explain How does weathering help form soil?
3. Explain How does the composition of the parent rock affect soil composition?
4. Compare Complete the table to compare soils in different climates.
Characteristics of Climate
Tropical
Most Important Type of
Weathering
Characteristics of Soil
chemical
thin topsoil;
a layer of humus;
high rates of chemical
weathering
Temperate cool to warm;
moderate amounts of rain
Desert
and arctic
5. Apply Concepts Which climate would likely be best for growing crops with deep
roots? Explain your answer.
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