Reducing Soil Erosion
So long! It's been good to know you.
This dusty old dust is a gettin' my home.
And I've got to be drifting along.
Woody Guthrie, 1940
The
dust storms that hit the center of the U.S. during the 1930s were responsible
for one of the great migrations in our history. As Woody Guthrie pointed
out in his songs, soil erosion was so bad that people saw little alternative
to abandoning their farms. They moved to other parts of the country in
search of work. Although changed climatic conditions and agricultural
practices improved the situation for a time, there was another period
of accelerated wind and water erosion during the 1970s and 1980s.
Erosion by wind
and water has occurred since the beginning of time. Although we should
expect some soil loss to occur on almost all soils, agriculture often
increases erosion. Erosion is the major hazard or limitation to the use
of about one-half of all cropland in the United States! On much of this
land, erosion is occurring fast enough to reduce future productivity.
As we discussed earlier, erosion is also an organic matter issue, because
it removes the soil layer highest in organic matter, the topsoil. The
soil removed from fields also has huge negative effects off the farm,
as sediment accumulates in streams, rivers, and reservoirs or blowing
dust reaches towns and cities.
A small amount
of erosion is acceptable, as long as new topsoil can be created as rapidly
as soil is lost. The maximum amount of soil that can be lost to erosion
each year, while still maintaining reasonable productivity is called the
soil loss tolerance or T value. For a deep soil with a rooting depth of
greater than 5 feet, the T value is 5 tons per acre each year. Although
this sounds like a huge amount of soil loss, keep in mind that the weight
of an acre of soil to 6 inches depth is about 2 million pounds, or 1,000
tons. So 5 tons is equivalent to about .03 inches ([5/1,000] x 6 inches
= 0.03 inch), enough to fill 200 bushel baskets with soil. If soil loss
continues at this rate, at the end of 33 years about 1 inch will be lost.
On deep soils with good management of organic matter, the rate of topsoil
creation can balance this loss. The soil loss tolerance amount is gradually
reduced for soils with less rooting depth. When the rooting depth is shallower
than 10 inches, the T value is about 1 ton per acre each year. This is
the same as 0.006 inch per year and is equivalent to 1 inch of loss in
167 years.
When your soil
loss is greater than the tolerance value, productivity suffers in the
long run. Yearly losses of 10 or 15 tons or more per acre occur in many
fields. Management practices are available to help reduce runoff and soil
losses. For example, researchers in Washington state found that erosion
on winter wheat fields was about 4 tons each year when a sod crop was
included in the rotation, compared to about 15 tons when sod was not included.
An Ohio experiment where runoff from conventionally tilled and no-till
continuous-corn fields was monitored showed that over a four-year period,
runoff averaged about 7 inches of water each year for conventional tillage
and less than one- tenth (0.1) inch for the no-till planting system.
Solving
Erosion Problems
Effective erosion
control is possible without compromising crop productivity. However, controlling
erosion is not always easy. It may require considerable investment (as
with terracing) or new management strategies (as with no-till systems).
The numerous approaches to controlling erosion can be generally grouped
into structural solutions and agronomic management practices. Structures
for reducing erosion generally involve engineering practices, where an
initial investment is made to build terraces, diversion ditches, drop
structures, etc. Agronomic practices to reduce erosion focus on changes
in soil and crop management. Appropriate conservation methods may vary
among fields and farms. Recently, there has been a clear trend away from
structural measures in favor of agronomic management practices. The primary
reasons for this change are:
- Management measures help
control erosion, while also improving soil quality and crop productivity.
- Significant advances
have been made in farm machinery and methodologies for alternative
soil and crop management.
- Structures generally
focus on containing runoff and sediment once erosion has been initiated,
whereas management measures try to prevent erosion from occurring
in the first place.
- Structures are often
expensive to build and maintain.
- Most structures do not
reduce tillage erosion.
Erosion: A Short-Term Memory Problem?
It's difficult to fully appreciate erosion's
damage potential, because the most severe erosion occurs during
rare weather events and climate anomalies. Wind erosion during
the dust-bowl days of the 1930s was especially damaging, resulting
from several extremely dry years in a row. And about one-third
of water erosion damage that occurs in a particular field
during a 30-year period commonly results from a single extreme
rainfall event! We must do our best to adequately protect
our soils from the damage that weather extremes can cause.
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For long-term sustainability of crop production, use of agronomic management
practices is usually preferred, although structural measures can effectively
complement them.
Erosion reduction
works by either decreasing the shear forces of water and wind or by keeping
soil in a condition that can't erode easily. Many conservation practices
actually provide both. The soil organic matter management practices we
discussed in the earlier chapters all reduce erosion. We'll also briefly
cover other important practices for keeping erosion to a minimum.
Reduced Tillage
Transition to tillage systems that increase surface cover (chapter
15) is probably the single most effective and economic approach
to reducing erosion. Restricted and no-till regimes succeed in many
cropping systems by providing similar or even better economic returns
than conventional tillage, while providing excellent erosion control.
Maintenance of residues on the soil surface and the lack of soil
loosening by tillage greatly reduce dispersion of surface aggregates
by raindrops and runoff waters. The effects of wind on surface soil
are also greatly reduced by leaving crop stubble on untilled soil
and anchoring the soil with roots. These measures facilitate infiltration
of precipitation where it falls, thereby reducing runoff and increasing
plant water availability.
In cases where
tillage is necessary, reducing its intensity and leaving some residue
on the surface slows down the loss of soil organic matter and aggregation.
Less tillage promotes higher infiltration rates and reduces runoff and
erosion. Leaving a rougher soil surface by eliminating secondary tillage
passes and packers that crush natural soil aggregates may significantly
reduce runoff and erosion losses by preventing surface sealing after intense
rain.
Reducing or eliminating
tillage also diminishes tillage erosion and keeps soil from being moved
downhill. The gradual losses of soil from upslope areas exposes denser
subsoil and may in many cases further aggravate runoff and erosion. It
is, however, possible to gradually reverse the effects of tillage erosion
caused by using a moldboard plow. Because the plow moves soil forward
and to the side, topsoil can be gradually moved back up the slope, if
plowing is performed diagonally to the slope in the uphill direction,
with the soil being thrown 45 degrees to the front/right of its original
location. Of course, this approach may not give good soil inversion during
plowing and does not address water and wind erosion concerns.
Significance of Soil Residues
Reduced-tillage and no-tillage practices result
in less soil disturbance and leave significant quantities
of residue on the surface. Surface residues are important
because they intercept raindrops and can also slow down water
running over the surface. The amount of residue on the surface
may be close to zero for the moldboard plow while continuous
no-till planting may leave 90 percent or more of the surface
covered. Other reduced-tillage systems, such as chiseling
and disking (as a primary tillage operation), typically leave
more than 30 percent of the surface covered. |
Adding Organic
Materials
Maintaining good soil organic matter levels helps keep topsoil in place.
A soil with more organic matter usually has better tilth and less surface
crusting. This means that more water is able to infiltrate into the soil
instead of running off the field, taking soil with it. When you build
up organic matter, you help control erosion by making it easier for rainfall
to enter the soil.
Adding organic
materials regularly to soils also results in larger and more stable soil
aggregates. Larger aggregates are not eroded by wind or water as easily
as smaller ones. Surface residue mulches provide both physical protection
of the soil surface from raindrop impact, as well as food for large numbers
of earthworms.
The adoption rate
for no-till practices is lower for livestock-based farms than for grain
and fiber farms. Manures may need to be incorporated into the soil for
best use of nitrogen, protection from runoff, and odor control. Also,
the severe compaction sometimes resulting from use of heavy liquid manure
spreaders on very moist soils may need to be lessened by tillage. Direct
injection of liquid organic materials in a zone or no-till system is generally
an option but requires additional equipment investments.
Cover Crops
Cover crops decrease erosion and increase water infiltration in
a number of ways. Cover crops add organic residues to the soil and
help maintain tilth and organic matter levels. Cover crops frequently
grow during seasons when the soil is especially susceptible to erosion,
such as the early spring. Their roots help to bind soil and hold
it in place. Because raindrops lose most of their energy when they
hit leaves and drip to the ground, less soil crusting occurs. Cover
crops are especially effective in reducing erosion if they are cut
and mulched, rather than incorporated. See chapter
10 for more information about cover crops.
Perennial Rotation
Crops
Grass and legume forage crops can help lessen erosion because they maintain
a cover on most of the soil surface for the whole year. Their extensive
root systems hold soil in place. Ideally, such rotations are combined
with reduced and no-tillage practices for the annual crops. Permanent
sod is a very good choice for steep soils or other soils that erode easily.
Other Practices
and Structures For Soil Conservation
Diversion ditches are frequently helpful for channeling water away from
the field without flowing over the entire area. Grassed waterways for
diversion ditches and other field water channels do not reduce erosion
from all of the field, but they do keep sediments on the field and reduce
scouring of the channels. Grassed waterways help prevent surface water
pollution by filtering sediments out of runoff before they reach a stream
or pond.
Tilling and planting
along the contour is a simple practice that helps control erosion. When
you work along the contour, instead of up and down slope, wheel tracks
and depressions caused by the plow, harrow, or planter will retain runoff
water in small puddles and allow it to slowly infiltrate. This approach
is not very effective when dealing with steep erodible lands and also
does not reduce tillage erosion.
Alternating strips
of row crops and perennial forages along the contour, referred to as strip
cropping, is an effective way of reducing erosion losses. In this system,
erosion from the row crop is not allowed to worsen over long, unprotected
slopes because the sediments are filtered out of runoff when the water
reaches the sod of the forage crop. This conservation system is generally
effective in fields with moderate erosion potential, and on farms with
use for both row and sod crops (for example, dairy farms). Research indicates
that crop yields may be slightly higher when crops are grown in strips,
rather than in the entire field. The increase in yield is probably due
to better use of light and soil where the different strips meet.
Terracing soil
in hilly regions is an expensive practice, but one which results in a
more gradual slope and greatly reduced erosion. Well-constructed and maintained
structures can last a long time, frequently making the high initial investment
worthwhile.
Wind erosion is
reduced by most of the same practices that reduce water erosion reduced
tillage or no-till, cover cropping, and perennial rotation crops. In addition,
practices that increase roughness of the soil surface diminish the effects
of wind erosion. The resulting increase in turbulent air movement near
the land surface reduces the wind's shear and its ability to sweep up
soil material. Therefore, fields subjected to wind erosion may be rough-tilled.
Also, tree shelterbelts planted at regular distances perpendicular to
the main wind direction act as windbreaks and are very effective in reducing
wind erosion losses.
Sources
American Society of Agricultural Engineers. 1985. Erosion and
Soil Productivity. Proceedings of the national symposium on
erosion and soil productivity, December 1011, 1984, New Orleans,
Louisiana. American Society of Agricultural Engineers Publication
8-85. St. Joseph, MI.
Edwards, W.M. 1992. Soil structure: Processes and
management. pp. 714. In Soil Management for Sustainability
(R. Lal and F.J. Pierce, eds.). Soil and Water Conservation Society.
Ankeny, IA. This is the reference for the Ohio experiment on the
monitoring of runoff.
Lal, R., and F.J. Pierce (eds.). 1991. Soil Management
for Sustainability. Soil and Water Conservation Society. Ankeny,
IA.
Ontario Ministry of Agriculture, Food, and Rural Affairs.
1997. Soil Management. Best Management Practices Series.
Available from the Ontario Federation of Agriculture, Toronto, Ontario
(Canada).
Reganold, J.P., L.F. Elliott, and Y.L. Unger. 1987.
Long-term effects of organic and conventional farming on soil erosion.
Nature 330:370372. This is the reference for the Washington
State study of erosion.
Smith, P.R. and M.A. Smith. 1998. Strip intercropping
corn and alfalfa. Journal of Production Agriculture 10:345353.
Soil and Water Conservation Society. 1991. Crop
Residue Management for Conservation. Proceedings of a national
conference, August 89, 1991, Lexington, KY. Soil and Water
Conservation Society. Ankeny, IA.
United States Department of Agriculture. 1989. The
Second RCA Appraisal: Soil Water, and Related Resources on Nonfederal
Land in the United States, Analysis of Conditions and Trends.
Government Printing Office. Washington, DC.
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