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Soil Erosion
“M
AN HAS only a thin layer of soil
between himself and
starvation.” This quote, attributed to
Bard of Cincinnati, clearly states the
importance of soil. The future of
mankind could depend on soil erosion
and soil management practices.
Objective:
þ
Describe soil erosion and the soil erosion process.
Key Terms:
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accelerated erosion
deposition
detachment
glacial erosion
gully erosion
land slippage
natural erosion
rill erosion
runoff
saltation
sediment
sheet erosion
soil erosion
splash erosion
surface creep
suspension
transport
water erosion
wind erosion
The Process of Soil Erosion
Soil erosion is the process by which soil is moved by wind, water, or other natural
actions. Vegetation and other coverings help prevent soil erosion. When the soil is exposed, it is
vulnerable to erosion. Land that is eroded loses fertility and productivity. Plus, the soil that is
lost may pollute water or air. Two basic classes of erosion are natural erosion and accelerated
erosion.
Natural erosion, sometimes referred to as geologic erosion, occurs on land not disturbed
by humans. Examples are changes in the earth, such as rounding off mountains and filling in
valleys. Soil deposits from erosion can form highly fertile areas, such as the Mississippi Delta.
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Accelerated erosion usually results from human activity
on the land. Construction sites
and plowing are two activities that
lead to accelerated erosion. The
outcome of these and similar
activities is a rapid rate of erosion
and a loss of soil fertility.
CAUSES OF EROSION
Erosion is caused by many different factors, including wind,
water, glacial movement, and land
slippage. When land is cleared of
protective covering, it is much
more susceptible to erosion
caused by wind and water.
FIGURE 1. Although plowing is a good way to aerate soil, it can also
potentially cause accelerated erosion.
Wind Erosion
Wind erosion is the loss of
soil due to the movement of wind
over the land. It occurs when persistent or frequent high-velocity
winds blow across dry, residue-free soil surfaces. It is common in dry climates where the
soil is loose. Wind erosion also
takes place on newly plowed
fields, construction sites cleared
by large equipment, and land
where vegetation has been grazed
too short. Wind erosion causes air
pollution, produces highway
FIGURE 2. Wind erosion has caused these sand dunes.
safety hazards, and fills drainage
ditches. Wind moves soil by suspension, saltation, and surface creep.
Suspension is the phenomenon in which very small soil particles become airborne. These
soil particles are carried in the same general direction as the wind. Because the soil particles are
small, they remain in suspension. Dust storms are examples of suspension.
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Saltation occurs when the wind lifts medium-sized soil particles into the air. They are too
heavy to remain in suspension, so they bounce and hop along the ground. In the process, they
loosen other soil particles.
The soil particles that are too heavy to be moved by saltation are moved along the surface by
the impact of soil particles being displaced by saltation. This movement of heavy soil particles
is called surface creep.
Water Erosion
Water erosion is the loss of soil due to water movement. It is the major cause of soil loss
in North America. Water erosion occurs when excess rainfall creates runoff that carries soil
away. Runoff occurs when rain falls faster than it can be absorbed into the soil. Runoff water
carries soil particles into streams and rivers. Runoff also leads to water pollution and sediment.
Sediment is the deposition of soil in the bottom of streams, riverbeds, ditches, and so on.
Four kinds of water erosion can occur. These are splash erosion, sheet erosion, rill erosion, and
gully erosion.
Splash erosion is the direct movement of soil by splashing. A soil grain can be thrown as
far as 5 feet by a raindrop splash. These splashed particles fill the voids between other aggregates and seal the soil surface.
Sheet erosion results when thin layers, or sheets, of soil are worn away. Sheet erosion
can arise on nearly level land or on sloping land. Evidence of sheet erosion is muddy water
moving off a field. Sheet erosion may go unnoticed, since no channels form. However, it may
be just as problematic as erosion
that is more apparent.
Rill erosion usually occurs
on sloping land where small
channels are formed by running
water. The signs of rill erosion
can be masked by normal tillage
practices.
Gully erosion occurs when
rills continue to wash away and
become more severe. It is more
likely to occur on steeper slopes.
Gullies cannot be smoothed by
normal tillage practices.
Glacial Erosion
FIGURE 3. Because this slope lacks any major vegetation, rill erosion has set
in.
Glacial erosion occurs when the front edge of a glacier pushes soil, rocks, and other
materials. Glacial erosion during the ice ages played a huge role in shaping the land. Soil erosion from glaciers is of minor importance today except in areas where glaciers exist.
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Land Slippage
Land slippage occurs on sloping land that is wet. Soil that is saturated with water slips
down the hillside or mountain slope. Land slippage is also known as mudslides or landslides.
Banks along highways, streams, and waterfronts are often subject to slides.
SOIL EROSION PROCESS
The process of soil erosion follows a simple, systematic progression each and every time.
The erosion process involves three distinct steps. The first step is the loosening of soil particles. The second step is the moving of soil particles. The third step is the placement of soil particles. Terms that describe these three steps are detachment, transport, and deposition.
Detachment involves the impact of raindrops that shatters surface aggregates and loosens
soil particles. Some of the particles float into soil voids, sealing the soil surface so that water
cannot readily infiltrate the soil. The scouring action of running water also detaches some soil
particles.
Transport is the second step in which the detached soil grains move in flowing water and
are carried down slopes.
Deposition is the final step and occurs as the soil is deposited when the water slows down.
SUSCEPTIBILITY TO EROSION
There are four factors that determine how susceptible soil is to soil erosion. These are soil
texture and structure, slope, surface roughness, and soil cover.
Soil Texture and Structure
Soil texture has two effects on soil erosion. Texture affects the rate at which water can enter
the soil. Less water running on the surface of the soil means less soil can be transported. Tex-
UNDER INVESTIGATION…
LAB CONNECTION: Erosion Control
A simple experiment can be set up to study the influence of slope and surface materials on
soil erosion. Materials needed are: trays (such as cafeteria trays or lasagna pans), soil, annual
ryegrass seed, enough dried leaves to cover a pan of soil, watering can, bucket, and water.
Fill three trays with soil. Sow ryegrass seed in one tray, water, and place under bright light.
Cover the soil of a second tray with dried leaves so that the soil is no longer visible. Leave the
soil of the third tray bare. After the ryegrass in the first tray has grown a couple inches in height,
you are ready to conduct the erosion tests. Place each pan at about a 5 percent slope. Place
the bucket under a spot where water would be expected to run off. Using the watering can,
sprinkle the surface of each tray and collect the runoff. Inspect the runoff for the presence of
soil. Inspect the soil surfaces for evidence of erosion. Try increasing the slope.
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ture influences the ease of detachment of soil particles. Soil particles of different sizes vary in
how easily they can be detached. Silt particles are the most easily detached. Soil structure influences infiltration of water. Good soil structure allows water to enter the soil, thus reducing the
amount of water runoff.
Slope
There are two components that determine slope. These are length and grade. Water runoff
velocity will increase as slope, or grade, increases. This causes an increase in the erosive energy
of the runoff water. On a long slope, a greater surface area collects water, increasing flow volume. Running water can also pick up speed as it flows down a long slope.
Surface Roughness
A rough soil surface slows the downhill flow of water. Surface roughness depends a great
deal on the tillage practice used on the land. In conventional tillage, the seedbed is left smooth
with very few ridges. Chisel plowing leaves the seedbed rough. Tillage across slopes or on the
contour acts to slow water flow.
Soil Cover
Bare soil is fully exposed to the erosive forces of raindrop impact and the scouring of running water. Soil cover reduces the energy available to cause erosion to the soil. A mulch, cover
crop, or crop residue absorbs the energy of the falling raindrop and reduces the amount of soil
detachment.
Summary:
2
Soil erosion is the process by which soil is moved by wind, water, or other natural
actions. Natural erosion occurs on land not disturbed by humans. Accelerated erosion usually results from human activity on the land.
There are four basic causes of erosion—wind, water, glaciers, and land slippage.
Wind erosion is the loss of soil due to the movement of wind over the land. Wind
moves soil by suspension, saltation, and surface creep. Water erosion is the loss of
soil due to water movement. Three types of water erosion are sheet erosion, rill
erosion, and gully erosion. Glacial erosion occurs when the front edge of a glacier
pushes soil, rocks, and other materials. Land slippage occurs when soil that is saturated with water slips down the hillside or mountain slope.
Soil erosion follows a simple, systematic progression that involves detachment,
transport, and deposition.
Four factors that determine how susceptible a soil is to soil erosion are soil texture
and structure, slope, surface roughness, and soil cover.
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Checking Your Knowledge:
´
1. What is soil erosion?
2. What is the three-step process of erosion?
3. What are the types of wind erosion?
4. What are the types of water erosion?
5. What are four factors that determine how susceptible a soil is to soil erosion?
Expanding Your Knowledge:
L
Contact your local soil conservation district and ask a representative to speak to
your class about soil erosion. Be sure to have questions prepared in advance regarding the causes of erosion and the methods to reduce erosion.
Web Links:
:
Erosion
http://soils.usda.gov/technical/manual/contents/chapter3b.html
Soil Erosion Site
http://www.soilerosion.net/
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