soil erosion - agricultural...

Soil Erosion

“MAN HAS only a thin layer of soilbetween himself and

starvation.” This quote, attributed toBard of Cincinnati, clearly states theimportance of soil. The future ofmankind could depend on soil erosionand soil management practices.

Objective:

� Describe soil erosion and the soil erosion process.

Key Terms:

�

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.

E-unit: Soil Erosion

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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

Accelerated erosion usu-

ally 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 dif-

ferent 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.

Wind Erosion

Wind erosion is the loss of

soil due to the movement of wind

over the land. It occurs when per-

sistent or frequent high-velocity

winds blow across dry, resi-

due-free soil surfaces. It is com-

mon 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

safety hazards, and fills drainage

ditches. Wind moves soil by sus-

pension, 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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FIGURE 1. Although plowing is a good way to aerate soil, it can also

potentially cause accelerated erosion.

FIGURE 2. Wind erosion has caused these sand dunes.

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 aggre-

gates 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

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 ero-

sion from glaciers is of minor importance today except in areas where glaciers exist.


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FIGURE 3. Because this slope lacks any major vegetation, rill erosion has set

in.

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 parti-

cles. The second step is the moving of soil particles. The third step is the placement of soil par-

ticles. 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-


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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.

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 influ-

ences 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 vol-

ume. 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 run-

ning 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:

� Soil erosion is the process by which soil is moved by wind, water, or other naturalactions. Natural erosion occurs on land not disturbed by humans. Accelerated ero-sion 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. Windmoves soil by suspension, saltation, and surface creep. Water erosion is the loss ofsoil due to water movement. Three types of water erosion are sheet erosion, rillerosion, and gully erosion. Glacial erosion occurs when the front edge of a glacierpushes soil, rocks, and other materials. Land slippage occurs when soil that is satu-rated 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 textureand 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:

� Contact your local soil conservation district and ask a representative to speak toyour class about soil erosion. Be sure to have questions prepared in advance regard-ing 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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http://soils.usda.gov/technical/manual/contents/chapter3b.html

http://www.soilerosion.net/

http://www.nrcs.usda.gov/TECHNICAL/land/erosion.html

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