soil degradation: erosion, CoMPaCtion,
and ContaMination
Hard ground makes too great resistance, as air makes too little resistance, to the surfaces of roots.
—Jethro tull, 1733
EROSION
Water Erosion
Soil loss during agricultural production is mainly
Water erosion occurs on bare, sloping land when intense
caused by water, wind, and tillage. Additionally,
rainfall rates exceed a soil’s infiltration capacity and
landslides (gravitational erosion) may occur on very
runoff begins. The water concentrates into tiny stream-
steep slopes. While water erosion and landslides occur
lets, which detach the saturated soil and transport the
under extremely wet soil conditions, wind erosion is
particles downhill. Runoff water gains more energy as
a concern with very dry soil. Tillage erosion occurs on
it moves down the slope, scouring away more soil and
fields that are either steep or have undulating topog-
also carrying more agricultural chemicals and nutrients,
raphy and is not affected by soil moisture conditions,
which end up in streams, lakes, and estuaries (figure
because the soil movement downslope is caused by
6.1). Reduced soil health in many of our agricultural and
the action of farm implements.
urban watersheds has resulted in increased runoff dur-
Erosion is the result of the combination of an erosive
ing intense rainfall and increased problems with flood-
force (water, wind, or gravity), a susceptible soil, and
ing. Also, the lower infiltration capacity of degraded soils
several other management- or landscape-related factors.
reduces the amount of water that is available to plants,
A soil’s inherent susceptibility to erosion (its erodibil-
as well as the amount that percolates through the soil
ity) is primarily a function of its texture (generally, silts
into underground aquifers. This reduction in under-
more than sands and clays), its aggregation (the strength
ground water recharge results in streams drying up
and size of aggregates, which are related to the amount
during drought periods. Watersheds with degraded soils
of organic matter), and soil water conditions. Many
thus experience lower stream flow during dry seasons
management practices can reduce soil erosion, although
and increased flooding during times of high rainfall.
different types of erosion have different solutions.
Soil erosion is of greatest concern when the surface
Photo by Jerry DeWitt
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chAPter 6 soil degradation: erosion, CoMPaCtion, and ContaMination
Figure 6.1. Left: Water erosion on clean-tilled soil in Bulgaria. Topsoil has been lost in the background field. Right: A stream in Guarico, Venezuela, contaminated with dispersed sediment.
is unprotected and directly exposed to the destruc-
characteristics, leading to a reduced ability to sustain
tive energy of raindrops and wind (figure 6.1). While
crops and increased potential for harmful environmen-
degraded soils tend to promote erosion, the process of
tal impacts.
erosion in turn leads to a decrease in soil quality. Thus,
a vicious cycle is begun in which erosion degrades soils,
Wind Erosion
which then leads to further susceptibility to erosion, and
The picture of wind erosion from the Dust Bowl era
so on. Soil is degraded because the best soil material—
(figure 5.12, p. 55) provides a graphic illustration of
the surface layer enriched in organic matter—is removed
land degradation. Wind erosion can occur when soil
by erosion. Erosion also selectively removes the more
is dry and loose, the surface is bare and smooth, and
easily transported finer soil particles. Severely eroded
the landscape has few physical barriers to wind. The
soils, therefore, become low in organic matter and
wind tends to roll and sweep larger soil particles along
have less favorable physical, chemical, and biological
the soil surface, which will dislodge other soil particles
soIL And WAter conservAtIon In hIstorIcAL tIMes
Some ancient farming civilizations recognized soil erosion as a problem and developed effective methods for runoff and erosion control. Ancient terracing practices are apparent in various parts of the world, notably in the Andean region of South America and in Southeast Asia.
Other cultures effectively controlled erosion using mulching and intercropping that protected the soil surface. Some ancient desert civilizations, such as the Anasazi in the southwestern U.S.
(A.D. 600 to 1200), held back and distributed runoff water with check dams to grow crops in downhill depressions (see the picture of a now forested site). Their methods, however, were specific to very dry conditions. For most agricultural areas of the world today, erosion still causes extensive damage (including the spread of deserts) and remains the greatest threat to agricultural sustainability and water quality.
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chAPter 6 soil degradation: erosion, CoMPaCtion, and ContaMination
Figure 6.2. Wind erosion damaged young wheat plants through abrasion.
Figure 6.3. Sustained rains from Hurricane Mitch in 1998 caused super-
Photo by USDA Wind Erosion Research Unit.
saturated soils and landslides in Central America. Photo by Benjamin
Zaitchik.
and increase overall soil detachment. The smaller soil
the soil mass (all pores are filled with water), and it
particles (very fine sand and silt) are lighter and will
decreases the cohesion of the soil (see the compaction of
go into suspension. They can be transported over great
wet soil in figure 6.10, right, p. 64) and thereby its ability
distances, sometimes across continents and oceans.
to resist the force of gravity. Agricultural areas are more
Wind erosion affects soil quality through the loss of
susceptible than forests because they lack large, deep
topsoil rich in organic matter and can cause crop dam-
tree roots that can hold soil material together. Pastures
age from abrasion (figure 6.2). In addition, wind erosion
on steep lands, common in many mountainous areas,
affects air quality, which is a serious concern for nearby
typically have shallow-rooted grasses and may also
communities.
experience slumping. With certain soil types, landslides
The ability of wind to erode a soil depends on how
may becomes liquefied and turn into mudslides.
that soil has been managed, because strong aggregation
makes it less susceptible to dispersion and transpor-
Tillage Erosion
tation. In addition, many soil-building practices like
Tillage degrades land even beyond promoting water and
mulching and the use of cover crops protect the soil
wind erosion by breaking down aggregates and exposing
surface from both wind and water erosion.
soil to the elements. It can also cause erosion by directly
moving soil down the slope to lower areas of the field. In
Landslides
complex topographies—such as seen in figure 6.4—till-
Landslides occur on steep slopes when the soils have
age erosion ultimately removes surface soil from knolls
become supersaturated from prolonged rains. They are
and deposits it in depressions (swales) at the bottom
especially of concern in places where high population
of slopes. What causes tillage erosion? Gravity causes
pressure has resulted in farming of steep hillsides (figure
more soil to be moved by the plow or harrow downslope
6.3). The sustained rains saturate the soil (especially
than upslope. Soil is thrown farther downslope when
in landscape positions that receive water from upslope
tilling in the downslope direction than is thrown uphill
areas). This has two effects: It increases the weight of
when tilling in the upslope direction (figure 6.5a).
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erosion is that it is unrelated to extreme weather events
and occurs gradually with every tillage operation.
Soil loss from slopes due to tillage erosion enhances
the potential for further soil losses from water or wind
erosion. On the other hand, tillage erosion does not
generally result in off-site damage, because the soil is
merely moved from higher to lower positions within a
field. However, it is another reason to reduce tillage on
sloping fields.
SOIl TIlTH AND cOMPAcTION
Figure 6.4. Effects of tillage erosion on soils. Photo by USDA-NRCS.
A soil becomes more compact, or dense, when aggregates
Downslope tillage typically occurs at greater speed than
or individual particles of soil are forced closer together.
when traveling uphill, making the situation even worse.
Soil compaction has various causes and different visible
Tillage along the contour also results in downslope soil
effects. Compaction can occur either at or near the surface
movement. Soil lifted by a tillage tool comes to rest at
(surface compaction, which includes surface crusting as
a slightly lower position on the slope (figure 6.5b). A
well as plow layer compaction) or lower down in the soil
more serious situation occurs when using a moldboard
(subsoil compaction). See figure 6.6.
plow along the contour. Moldboard plowing is typically
performed by throwing the soil down the slope, as better
Surface Compaction
inversion is thus obtained than by trying to turn the fur-
Plow layer compaction—compaction of the surface
row up the slope (figure 6.5c). One unique feature of till-
layer—has probably occurred to some extent in all
age erosion compared to wind, water, and gravitational
intensively worked agricultural soils. It is the result of a
loss of soil aggregation that typically has three primary
causes—erosion, reduced organic matter levels, and
region of soil loss
region of soil
force exerted by the weight of field equipment. The first
accumulation
two result in reduced supplies of sticky binding materi-
als and a subsequent loss of aggregation.
a) up-and-downhill tillage
Surface crusting has the same causes as plow layer
compaction but specifically occurs when the soil surface
is unprotected by crop residue or a plant canopy and the
b) tillage (chisel, disc, etc.)
energy of raindrops disperses wet aggregates, pound-
along contour
ing them apart so that particles settle into a thin, dense
surface layer. The sealing of the soil reduces water infil-
tration, and the surface forms a hard crust when dried.
c) plowing along contour,
If the crusting occurs soon after planting, it may delay
throwing furrow downhill
or prevent seedling emergence. Even when the crust is
not severe enough to limit germination, it can reduce
Figure 6.5. Three causes of erosion resulting from tilling soils on slopes.
water infiltration. Soils with surface crusts are prone to
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chAPter 6 soil degradation: erosion, CoMPaCtion, and ContaMination
surface crust
germinating seed
porous
tightly packed crumbs
(loose-fitting)
crumbs and blocks
large blocks with
few cracks
subsoil compaction
a) good soil structure
b) compacted soil
Figure 6.6. Plants growing in (a) soil with good tilth and (b) soil with all three types of compaction.
high rates of runoff and erosion. You can reduce surface
seriously compacted if tilled or traveled on, because soil
crusting by leaving more residue on the surface and
aggregates are pushed together into a smeared, dense
maintaining strong soil aggregation.
mass. This compaction may be observed when you see
Compaction of soils by heavy equipment and tillage
shiny, cloddy furrows or deep tire ruts in a field (figure
tools is especially damaging when soils are wet. This
6.8). When the soil is friable (the water content is below
combination of factors is the primary cause for sub-
the plastic limit), it crumbles when tilled and aggregates
soil compaction and one of the causes for plow layer
resist compaction by field traffic. Thus, the potential for
compaction. To understand this, we need to know a little
compaction is strongly influenced by the timing of field
about soil consistence, or how soil reacts to external
forces. At very high water contents, a soil may behave
plastic limit
like a liquid (figure 6.7), because it has little internal
cohesion (figure 5.10, left, p. 54). On a slope it can sim-
sand
ply flow as a result of the force of gravity—as with mud-
loose
friable
slides during excessively wet periods. At slightly lower
water contents, soil has somewhat more cohesion (figure
clay
5.10, middle, p. 54), but it can still be easily molded and
hard
friable
plastic
is said to be plastic (figure 6.7). Upon further drying, the
liquid
soil will become friable—it will break apart rather than
mold under pressure (figure 6.7).
soil water content
The point between plastic and friable soil, the plastic
0
saturation
limit, has important agricultural implications. When
Figure 6.7. Soil consistency states for a sand and a clay soil (friable soil is a soil is wetter than the plastic limit, it may become
best for tillage).
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prevent the soil from dispersing. The result may be a soil
with a dense plow layer and a crust at the surface. Some
soils may hard-set like cement, even after the slightest
drying, thereby slowing plant growth. Although the soil
becomes softer when it re-wets, that moisture provides
only temporary relief to plants.
Subsoil Compaction
Subsoil compaction—dense soil below the normally
tilled surface layer—is usually referred to as a plow pan,
although it is commonly caused by more than just plow-
ing. Subsoil is easily compacted, because it is usually
Figure 6.8. Deep tire ruts in a hay field following harvest when soil was wetter, denser, higher in clay content, lower in organic
wet and plastic.
matter, and less aggregated than topsoil. Also, subsoil
operations as related to soil moisture conditions.
is not loosened by regular tillage and cannot easily be
A soil’s consistency is strongly affected by its texture
amended with additions of organic materials, so com-
(figure 6.7). For example, as coarse-textured sandy soils
paction in the subsoil is more difficult to manage.
drain, they rapidly change from being plastic to friable.
Subsoil compaction is the result of either direct load-
Fine-textured loams and clays need longer drying periods
ing or the transfer of compaction forces from the surface
to lose enough water to become friable. This extra drying
into deeper layers. Subsoil compaction occurs when
time may cause delays when scheduling field operations.
farmers run heavy vehicles with poor weight distribu-
Surface crusting and plow layer compaction are
tion. The load exerted on the surface is transferred into
especially common with intensively tilled soils. Tillage
the soil along a cone-shaped pattern (figure 6.10, p. 64).
operations often become part of a vicious cycle in which
With increasing depth, the compaction force is distrib-
a compacted soil tills up very cloddy (figure 6.9a), and
uted over a larger area, thereby reducing the pressure in
then requires extensive secondary tillage and pack-
deeper layers. When the loading force at the surface is
ing trips to create a satisfactory seedbed (figure 6.9b).
small, say through foot or hoof traffic or a light tractor,
Natural aggregates break down, and organic matter
the pressure exerted below the plow layer is minimal.
decomposes in the process—contributing to more com-
But when the load is high from heavy equipment, the
paction in the future. Although the final seedbed may be
pressures at depth are sufficient to cause considerable
ideal at the time of planting, rainfall shortly after plant-
soil compaction. When the soil is wet, the force causing
ing may cause surface sealing and further settling (figure
compaction near the surface is more easily transferred
6.9c), because few sturdy aggregates are present to
to the subsoil. Clearly, the most severe compaction
checK before tILLInG
To be sure that a soil is ready for equipment use, you can do the simple “ball test” by taking a handful of soil from the lower part of the plow layer and trying to make a ball out of it. If it molds easily and sticks together, the soil is too wet. If it crumbles readily, it is sufficiently dry for tillage or heavy traffic.
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chAPter 6 soil degradation: erosion, CoMPaCtion, and ContaMination
damage to subsoils occurs with the combination of
heavy vehicle traffic and wet soil conditions.
Direct loading is also caused by the pressure of a
tillage implement, especially a plow or disk, pressing
on the soil below. Plows cause compaction because the
weight of the plow plus the lifting of the furrow slices
results in strong downward forces. Disks have much of
their weight concentrated at the bottom of the disk and
thereby cause pans. Subsoil compaction may also occur
during moldboard plowing when a set of tractor wheels
is placed in the open furrow, thereby applying wheel
a) Stage 1: Cloddy soil after tillage makes for a poor seedbed.
pressure directly to the soil below the plow layer.
cONSEQUENcES OF cOMPAcTION
As compaction pushes particles closer together, the
soil becomes dense and pore space is lost. Notably,
the larger pores are eliminated. Loss of aggregation
from compaction is particularly harmful for fine- and
medium-textured soils that depend on those pores for
good infiltration and percolation of water, as well as air
exchange with the atmosphere. Although compaction
can also damage coarse-textured soils, the impact is less
severe. They depend less on aggregation, because the
pores between individual particles are sufficiently large
to allow good water and air movement.
b) Stage 2: Soil is packed and pulverized to make a fine seedbed.
Compacted soil becomes hard when it dries, as it
has many small pores that can hold water under high
suction and pull particles tightly together. This can
restrict root growth and the activity of soil organisms.
Compacted soils typically have greater resistance to
penetration at a given soil moisture level than a well-
structured soil (figure 6.11, p. 65), which has large pores
between aggregates that therefore easily pull apart.
The resistance to penetration for a moist, high-quality
soil is usually well below the critical level where root
growth ceases for most crops—300 pounds per square
inch (psi). As the soil dries, its strength increases, but
c) Stage 3: Raindrops disperse soil aggregates, forming a surface crust.
a high-quality soil may not exceed the critical level for
Figure 6.9. Three stages of tilth for a compacted soil that has become
most (or all) of the moisture range. A compacted soil, on
addicted to tillage.
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