Soil Erosion - the battle to keep ourselves from destroying the source of all food, begins with saving the soil. To read about how some civilizations have not been able to do this go to Erosion of Civilizations
You do not miss the soil until it is gone. The USA is not adequately protecting the soil it has. This resource will also be destroyed if we do not take care of it. See these maps of the soil loss and sediment delivered to streams in the USA. Soil Loss & Sediment Delivered
Problems caused by soil Erosion
1. Loss of valuable topsoil. If agriculture land costs an average of $1000/acre
for the upper 4 feet of soil, or $20.83/inch,($1000 ÷ 48 in.). However,
topsoil is considered to be ten times more valuable than the subsoil. When soil
is removed from a field it includes: the soil particles, nutrients, water,
water holding capacity, and SOM.
2. Damage due to deposition of soil from up-slope by burying more valuable land
with less valuable soil.
3. Damage to fields because gully erosion is reducing the field size and taking
land out of production .
4. Pollution due to off site or Non-Point Pollution. It is estimated that
non-point pollution causes 3 to 18 Billion dollars per year or $50.00 per acre
if one-half the farmland is causing the pollution .
Non-Point Pollution=sediment, nutrients, and pesticides.
5. Erosion causes a steady but slow productivity decline. For example: if a
soil lost 2.5 inches it would have a 5-15% decline in productivity and if the
soil lost five inches lost the decline in productivity would be 10-35% .
The average daily amount of soil being transported downstream by the Mississippi River at Winona is about 302,000 tons per year or 827 tons/day
Causes of Soil Erosion
Impact of
Raindrops fall at 20 m.p.h.. The impact of the raindrop breaks apart the soil aggregate. The individual sand, silt and clay particles are now not held in an aggregate due to beating of rain drops. These particles fill the soil pores and reduce infiltration. The larger the raindrop, the greater the energy released at impact and thus the more destruction of the soil aggregates. After the surface pores are filled with sand, silt or clay, overland surface flow of water begins due to the lowering of infiltration rates. Once the rate of falling rain is faster than infiltration, runoff takes place.
Erosion Types
Sheet Erosion - thin film of water over the entire
field moving down-slope Another view of sheet
erosion
Rill Erosion - collection of sheet erosion water into channels (
rills) that erode the bottom and side of the rill.
Gully erosion - increasing size of rills eventually lead to a gully
or a channel too large for crossing by farm equipment.
Another view of a large gully. Large Gully
Water Erosion in Southeast Minnesota - 2003. These pictures by Gyles Randall UM-Waseca, show the problems that still occur today in Minnesota regarding water erosion. The end result is the increase in sediment load in the tributaries of the Mississippi like the Root River. Click on the following pictures to see the soil erosion. Rill Erosion- Start of Gully- Sheet Erosion - Head of Large Gully - Deep Gully-Wide Gully in Corn Field-Root River Delivering Sediment to the Mississippi.
Wind Erosion Wind erosion results in soil movement by three processes:
Saltation - fine and medium sand-sized particles are lifted a short distance
into the air, dislodging more soil as they fall back to the ground
Suspension - very fine soil particles are lifted from the surface by the impact
of saltation and carried high into the air, remaining suspended in air for long
distances
Surface Creep - the movement of large soil particles along the surface of the
soil after being loosened by the impact of saltating particles
View this diagram to review the three processes of wind erosion. Wind Erosion Processes
Wind Erosion Laboratory Web PageUSDA WIND EROSION LAB
Predicting Soil Loss
The universal soil loss equation (USLE) is used to determine how much soil is being eroded from the land. The factors you need to use in this equation are listed in your lab manual.
A revision of the USLE is now being used by the NRCS. For information about this, go to RUSLE at Purdue University.
For the time being, we will continue to use the USLE, which is:
A=R×K×LS×C×P
The rainfall factor (R) is an erosion index that measures the intensity
and quantity of rainfall. See Maps for
information on R for Minnesota and USA.
The K factor, or soil erodibility factor, is the erosion rate per unit of R for a specific soil in cultivated fallow on 9% slopes that are 72.6 ft. long. While all areas don't meet these requirements, the K factors can be established for any soil and are found in the Soil Survey, Table 15 or 16.
The K
factor is dependent on how easily soil can be broken down by the impact of rain
drops. In the lab, we do a simple test of aggregate stability by placing peds
from an A and an E horizon on a small sieve and slowly moving them in and out
of water.
The LS factor combines both the length and steepness of slope and is the ratio of erosion on a slope of 9% that is 72 ft. long. The chart in your lab manual allows you to interpret this for various gradients or lengths of slopes. For example, a 600 ft. slope with a 10% gradient would have an LS factor of 3.5. The steeper the slope the greater the erosion. The worst erosion is between 10 and 25% slope. This graph shows that sustainable soils have slopes less than 5%. Slope and Erosion Graph
The
C factor is the crop management factor and is the ratio of soil
loss compared to fallow (bare, exposed) soil. Some examples of C factors for
various cropping systems are:
C=1.0 for continuous corn or soybeans,
conventional tillage
.30=continuous corn conservation tillage
.20=corn - oats
.08=corn - oats - pasture
.009=pasture poor
.006=pasture good
.005=woodland
See this diagram for a look at the C factor.
The bottle demonstration in the lab (seen below) shows the influence of cover on soil erosion.
The P factor is the erosion control factor. If a farmer plows up and down the slope of a hill, P=1. When a farmer plows around the hill or follows the slope contour of the hill, P is reduced. The values for contouring and strip contouring are listed in the lab manual. Fields that have strips of different crops that are not on the contour also fall under this practice.
This aerial view of southeast Minnesota shows the use of strip cropping which reduces the P factor.
Sample Problem using the USLE:
For a southeastern Minnesota (R=150) field with a crop rotation of
corn-soybeans, no conservation tillage, K=.24.
The slope length is 300 ft., with a slope of 2-6 % (assume an average of 4%).
LS=.64;
P=1;
C=1
Therefore, A=150×0.24×0.64×1×1=23 tons/acre/year and since
T=5 tons/acre this soil has 18 tons / acre too much soil erosion . In
order to reduce this you would need to change C or P factors.
Soil Erosion: Increasing or Decreasing ?
"In 1985, the U.S. Congress, with strong support from the environmental community, created the Conservation Reserve Program (CRP) to reduce soil erosion and control overproduction of basic commodities. By 1990 there were some 14 million hectares (35 million acres) of highly erodible land in permanent vegetative cover under 10-year contracts. Under this program, farmers were paid to plant fragile cropland to grass or trees." This has been the most effective way to reduce soil erosion along with conservation tillage practices. From : World Watch Institute- Conserving and Rebuilding Soils
The increase in corn production with the building of ethanol plants has been dramatic. In 1995 there were 65 million acres of corn planted. This acerage increased to70 million in 1999, 78 million in 2006, and 90,000 million in 2007. The impact on soil erosion from this increase will be determined in the future. For more information on corn and erosion go to Corn & Erosion or on cover crops and soil erosion go to Cover Crops and Conservation Tillage for Soil Erosion Control on Cropland
According to the National Corn Growers Association : "Corn growers are the best stewards of the land because we need to take care of the land in order to survive,” said Bill Chase, NCGA chairman of the Production and Stewardship Action Team. “This report indicates what we have known all along: because of new tillage systems, increased conversation methods and more efficient farming practices, we are caring for the land better than we ever have before and will continue to do so to ensure following generations are able to feed America.”
Another concern dealing with soil erosion is the difference in land stewardship between farmers who rent land and farmers who own their land. According to Energize America the majority of the best soil in the nation is rented and has the highest erosion rates. More than half the best farmland in the United States is rented: 65% in Iowa, 74% in Minnesota, 84% in Illinois, and 86% in Indiana. Owners seeking short-term profits have far less incentive than farmers who work their land to preserve soil and water. Often the areas with the highest erosion rates, are the same areas with the highest percentage of rented farmland.
"Declines in soil erosion result primarily from investments in conservation measures that include terraces, strip cropping, crop rotations, windbreaks, and switching to conservation tillage (reduced tillage and no-till cultivation). By 1994 no-till farming techniques were practiced on about 12% of row crop production. Mulch tillage (in which crop residue is left on the soil surface) and ridge tillage (in which crop residue is collected in valleys alongside ridges of soil that are planted) were practiced on another 26% of planted crops in the United States." From: "World Agriculture & Environment" by Jason Clay
© Terence H. Cooper & Regents of the University of Minnesota, 2007. The University of Minnesota is an equal opportunity educator and employer.
