Types of Soils
Dana Desonie, Ph.D.
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Printed: January 3, 2015
AUTHOR
Dana Desonie, Ph.D.
www.ck12.org
C HAPTER
Chapter 1. Types of Soils
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Types of Soils
• Describe the characteristics of types of soil and where each is found.
What makes soil good?
Some types of soils are good for growing crops and some are not. When good soils are found in good climates where
water is available, a variety of crops will grow. If one of these things is missing, the possibilities are much more
limited.
Types of Soils
Although soil scientists recognize thousands of types of soil –each with its own specific characteristics and name
- let’s consider just three soil types. This will help you to understand some of the basic ideas about how climate
produces a certain type of soil, but there are many exceptions to what we will learn right now ( Figure 1.1).
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FIGURE 1.1
Just some of the thousands of soil types.
Pedalfer
Deciduous trees, the trees that lose their leaves each winter, need at least 65 cm of rain per year. These forests
produce soils called pedalfers, which are common in many areas of the temperate, eastern part of the United States
( Figure 1.2). The word pedalfer comes from some of the elements that are commonly found in the soil. The "Al"
in ped alfer is the chemical symbol of the element aluminum, and the "Fe" in pedal fer is the chemical symbol for
iron. Pedalfers are usually a very fertile, dark brown or black soil. Not surprisingly, they are rich in aluminum clays
and iron oxides. Because a great deal of rainfall is common in this climate, most of the soluble minerals dissolve
and are carried away, leaving the less soluble clays and iron oxides behind.
FIGURE 1.2
A pedalfer is the dark, fertile type of soil
that will form in a forested region.
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Chapter 1. Types of Soils
Pedocal
Pedocal soils form in drier, temperate areas where grasslands and brush are the usual types of vegetation ( Figure
1.3). The climates that form pedocals have less than 65 cm rainfall per year. Compared to pedalfers there is less
chemical weathering and less water to dissolve away soluble minerals, so more soluble minerals are present and
fewer clay minerals are produced. It is a drier region with less vegetation, so the soils have lower amounts of organic
material and are less fertile.
A pedocal is named for the calcite enriched layer that forms. Water begins to move down through the soil layers, but
before it gets very far, it begins to evaporate. Soluble minerals, like calcium carbonate, concentrate in a layer that
marks the lowest place that water was able to reach. This layer is called caliche.
FIGURE 1.3
A lizard on soil typical of an arid region in
Mexico.
Laterite
In tropical rainforests where it rains literally every day, laterite soils form ( Figure 1.4). In these hot, wet, tropical
regions, intense chemical weathering strips the soils of their nutrients. There is practically no humus. All soluble
minerals are removed from the soil and all plant nutrients are carried away. All that is left behind are the least soluble
materials, like aluminum and iron oxides. These soils are often red in color from the iron oxides. Laterite soils bake
as hard as a brick if they are exposed to the Sun.
FIGURE 1.4
A laterite is the type of thick, nutrient-poor
soil that forms in the rainforest.
Many climate types have not been mentioned here. Each produces a distinctive soil type that forms in the particular
circumstances found there. Where there is less weathering, soils are thinner but soluble minerals may be present.
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Where there is intense weathering, soils may be thick but nutrient-poor. Soil development takes a very long time, it
may take hundreds or even thousands of years for a good fertile topsoil to form. Soil scientists estimate that in the
very best soil-forming conditions, soil forms at a rate of about 1mm/year. In poor conditions, soil formation may
take thousands of years!
Summary
• Pedalfer is the soil common in deciduous forests and is rich in aluminum and iron. Pedalfers are dark brown
and fertile.
• Pedocal is the soil common in grasslands where the climate is drier and is rich in calcium.
• Laterite forms in tropical rain forests. Chemical weathering strips the soils of their nutrients, so when the
forest is removed the soil is not very fertile.
Explore More
Use these resources to answer the questions that follow.
https://www.youtube.com/watch?v=MQF5wlpvIfo Start at 3:52
1. What defines the two different soil types in the United States?
2. What are the characteristics of pedalfer? What is this due to?
3. What happens in the western half of the U.S. that inhibits soil development?
Review
1. What is pedocal and under what conditions does it form?
2. What is pedalfer and under what conditions does it form?
3. What is laterite and under what conditions does it form?
References
1. Oregon Department of Transportation. Samples of soil types . CC BY 2.0
2. User:Laitche/Wikimedia Commons. A pedalfer is the dark, fertile type of soil that will form in a forested re
gion . Public Domain
3. Image copyright Tim Pleasant, 2014. This arid region in Mexico has pedocal soil . Used under license from
Shutterstock.com
4. Image copyright samkin, 2013. A laterite is the type of thick, nutrient-poor soil that forms in the rainforest .
Used under license from Shutterstock.com
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