EROSION - ITS EFFECT ON SOIL PROPERTIES, PRODUCTIVITY AND PROFIT
L. W. Murdock and W.W. Frye
Department of Agronomy
Almost all people recognize that erosion
is harmful, but few realize how harmful. Certainly, there are costs to
society for clean-up and repair from soil and its constituents polluting
the water and air. However, the greatest costs are borne by the landowner
and producer. Erosion results in higher fertilizer and fuel requirements,
and lower yields. The benefits of erosion control are sometimes obscure,
but the costs of erosion are real.
Erosion reduces productivity by modifying
soil properties and is more harmful to soils that: 1) are shallow, 2) have
poor quality subsoils, and 3) do not have a thick topsoil. Any combination
of these characteristics greatly increases the damage from erosion. Deep
soils with more excellent subsoil properties are virtually unaffected by
soil erosion. More fertilizer would be required to replace the fertility
loss, but the production potential would change little, if any. However,
most soils have some undesirable properties which lower production, since
erosion causes more subsoil to be incorporated into the plow layer.
Erosion removes a field's original
topsoil, causing the subsoil to mix with the remaining topsoil during annual
plowing. In mature soils (like many of those in Kentucky), this subsoil
material has more clay, less organic matter, lower available water-holding
capacity and lower fertility status. Also, the soil structure is likely
to be coarser, less stable and subject to more damage by rainfall impact,
tillage or traffic. As more and more of the subsoil is mixed into the topsoil
by tillage, it adds more and more of these characteristics to the topsoil.
The clay content in the topsoil of
two soils studied in Kentucky increased from 25 to 31 percent in a Maury
soil and from 20 to 25 percent in a Crider soil as a result of moderate
erosion. This increased clay content also increases fuel, chemical and
Soil erosion removes the lighter and
more easily dislodged particles. This means that organic matter is one
of the more easily erodable constituents. Moderate erosion of a Crider
soil in Western Kentucky was found to have reduced the topsoil organic
matter content from 1.7 to 1.5 percent.
Soil density also increases with erosion.
This increases problems associated with tillage, tilth and seedbed preparation.
Plant Available Water
The most limiting factor on crop production
in Kentucky is probably water. Erosion usually reduces the plant available
water holding capacity of a soil. In a study of Kentucky Maury and Crider
soils, the plant available water holding capacity decreased from 29 to
24 percent in the Maury soil and from 24 to 20 percent in the Crider soil
because of erosion.
Two things cause this reduction in
plant available water holding capacity --a shallower soil and more undesirable
soil properties. As the clay increases and organic matter decreases, the
amount of water a soil can make available to the plant decreases. This
is probably the single most important effect of erosion on soil productivity.
Soil erosion removes the part of the
soil which is usually richest in plant nutrients. Erosion on soils which
are highly fertile, naturally or by fertilizer addition, will result in
greater fertility losses. All nutrients are lost during erosion but the
most economically significant losses will probably be that of nitrogen,
phosphorus, potassium and lime.
If the soil is naturally low in fertility,
then erosion will cause a loss of the added nutrients and increase the
fertility requirements of the soil. A Crider soil at Princeton, Kentucky,
which is naturally low in fertility, was studied to determine the effect
of moderate erosion. Although the site had been highly fertilized for years,
erosion caused a difference in the fertility status of the soil (Table
Table 1. Effect of Moderate Erosion on the Fertility Status and Requirement
for Corn of a Crider Soil.
More fertilizer and lime would be required
to raise and maintain the soil test when erosion is allowed to occur. The
greater the erosion, the more fertility that would be lost. It has been
estimated that the value of plant available nutrients lost from a highly
fertile soil in Kentucky could range from $3 to $14 per acre (considering
1981 fertilizer prices and a soil loss of 14 tons per acre). It is difficult
to put a real value on the soil nutrients lost due to erosion. It depends
on the soil fertility level and the availability of the nutrients in the
soil. Regardless, the loss can be considerable and should not be ignored.
Fortunately, the fertility can be replaced, but the cost must be recognized
and taken into account.
A number of soil properties are affected
by soil erosion. Fertility, organic matter, rooting depth, soil tilth and
available water holding capacity are reduced and soil texture is changed.
Collectively, these properties affect the yield potential of the soil.
In most cases, the potential productivity of the soil will be decreased
as erosion occurs. The yield decrease is usually so slight from year to
year that the farmer may not realize that it is happening. Therefore, productivity
loss due to soil erosion is usually not considered to be a financial loss.
As small annual losses accumulate with time, productivity is substantially
reduced. Also, crop yields may actually continue to increase during and
after excessive soil erosion because of advanced technology such as fertilizer
management, irrigation, improved varieties and pest control. This concept
is presented in Figure 1. The yield potential
of most soils decreases with erosion, but actual yield may continue to
rise on moderately-eroded soils. Yields do not rise as rapidly or as high
as on similar soils with none to slight erosion. Another way to view this
concept is that more technological inputs are necessary for crop yields
on eroded soils to equal those on uneroded soils. And more technological
inputs usually mean more financial inputs as well.
The corn production of a Crider soil
decreased over three years from 125 to 100 bushels per acre after moderate
erosion had taken place. Both lower available water supplying capacity
and lower fertility status probably contributed to the lower yields. However,
water supplying capacity was probably the greatest limitation since fertilizer
additions were based on 150 bushels per acre yield goals.
Many of the soils in Kentucky have
limited rooting volumes due to underlying fragipans, rocks or claypans.
Loss of surface soil by erosion further decreases the already limiting
volume of soil favorable for root growth and available water storage. Studies
on a fragipan soil (Zanesville) at Princeton indicate that as the soil
depth to the fragipan horizon decreases to less than 2 feet, the corn yield
will decrease about 5 bushels per acre for each inch of soil lost by erosion.
This effect is cumulative and should be considered in situations where
erosion is allowed to occur on a shallow soil.
After erosion has occurred and the
productivity of the soil has been decreased, can it be restored? Two practices
which would probably be most effective in restoring productivity would
be 1) high fertility amendments and 2) the planting of a sod crop to prevent
further erosion and increase organic matter. In an experiment at Lexington,
moderate erosion caused the corn yields of a Maury soil to be reduced from
99 to 87 bushels per acre. The erosion had taken place 60 years prior to
the study and the soil was in a very high fertility state, indicating that
the effects of erosion are long-lasting. Neither low intensity use nor
high fertility amendments restored the soil to its original production
potential after being damaged by erosion. This is not to say that the productivity
of eroded soils can not be improved through management; however, the production
capacity of some eroded soils may never be restored to the level of that
soil in its uneroded state.
What could moderate erosion cost a
producer over the period of a lifetime? It would depend greatly on the
soil and its fertility status, but it would not be unusual to lose 17 tons
of soil per acre per year from a sloping field in continuous cultivation
in Kentucky. If this much soil were lost for 50 years, then 5 inches of
soil would be removed. To maintain the soil in a high fertility status,
it might cost $10 per acre per year for additional fertilizers. In addition,
the annual loss in productivity could be as much as 20 bushels of corn
per acre after the 50 years of erosion. Again the losses could be higher
or lower. Considering these two factors, the annual loss after 50 years
would be $70 per acre per year, if corn is considered to be $3 a bushel.
Certainly this is a very arbitrary figure but could be considered realistic
for many cases.
Erosion reduces the productivity of
many soils by affecting the soil properties and depth. This reduction is
substantial and long-lasting, and can not be reversed over a lifetime,
even with conservation management, low intensity use or high fertility
amendments. The most important yield-limiting effect of erosion is probably
the decrease in plant available water holding capacity. The soil fertility
status may be significantly decreased by erosion but can be replenished
by additions of lime and fertilizer, although with greater expense and
lower efficiency than for uneroded soil. The overall effect of erosion
is an economic loss which accumulates with time as erosion continues. The
cost of erosion, over time, will probably outweigh any costs associated
with erosion control practices.