Soil Erosion
© 2007 Donald G. McGahan (aka soilman) All Rights Reserved
A nation that destroys its soils, destroys itself! (F.D. Roosevelt, 1937)
Soil erosion is the most serious problem affecting the sustainable production of food and fiber. About 80% of the world's agricultural lands suffer moderate to severe erosion; an additional 10% suffer slight to moderate erosion. It is estimated that 54% of U.S. rangelands are over-grazed resulting in enhanced soil erosion and decreased productivity. Worldwide, about 12 million hectares of arable land are destroyed and abandoned annually because of non-sustainable farming practices. Total cultivated land is currently 1.5 billion hectares.
Worldwide, about 75 billion metric tons (Mg) of soil are eroded annually:
- 2/3 eroded by water & 1/3 eroded by wind
The annual cost of erosion is estimated at about $44 billion in the U.S.; about $400 billion worldwide (1995).
Depending on soil forming factors, soils can sustain a loss of 1-10 Mg/ha/yr without losing significant productivity. A 1 mm (1/32 inch) thickness of soil over one hectare weighs approximately 12 Mg.
What is soil erosion?
Classes of Erosion
- Geologic erosion
- Erosion due to natural processes that are not affected by human activities (e.g., erosion of mountains, cutting of valleys). Normal rates of geologic erosion are on the order of 0.2-0.5 Mg/ha/yr. ℹ
- Accelerated erosion
- Increased erosion rates above geologic erosion rates due to human activities (e.g., grazing and farming practices).
Types of Erosion
- Water Erosion
- Due to surface runoff.
Although gully erosion looks more catastrophic, far more total soil is lost by the less obvious sheet and rill erosion.
Factors affecting a soil's water erodibility:
- Texture of topsoil
- silt and very fine sand are most erodible.
- Structure of topsoil
- Soil organic matter concentrations
- increase soil structure and water infiltration.
- Infiltration/permeability rate
- Slope percent and slope length
- Soil cover (e.g., vegetation and crop residues)
Wind Erosion involves two processes: (1) detachment and (2) transportation.
- Saltation
- movement of soil particles by a short series of bounces along the surface of the ground (50-80%).
- Suspension
- movement of particles parallel to the ground surface and upward (3-38%).
- Surface Creep
- rolling or sliding of particles along the soil surface (7-25%).
Factors affecting a soils wind erodibility:
- Texture of topsoil
- very fine sand and silt particles are most susceptible
- Structure of topsoil
- Aggregating agents such as clays, oxides, hydroxides, and organic matter promote structure.
- Climate
- wind velocity and soil moisture content
- Soil roughness
- Width of unprotected soil
- Soil cover
- e.g., vegetation and crop residue.
- Landslide
- instantaneous movement of a mass of earth
- Soil creep
- slow and persistent movement of soil
Why should anyone care about soil erosion?
Soil Productivity
- Erosion removes the best part of the soil (topsoil); results in loss of organic matter, plant nutrients and soil organisms, and decreases the soil thickness which reduces water and nutrient holding capacity.
- Requires the use of fertilizers, pesticides and irrigation to offset productivity decrease. About 10% of energy used in U.S. agriculture is spent to offset the productivity loss due to soil erosion.
Environmental
- Air Pollution.
- Fugitive dust.
- PM10 & PM2.5
- particulate matter less than 10 and 2 microns, respectively.
- PM2.5 is believed to have the greatest human health risk.
- Sand-blasting and death of plants.
- Detrimental deposition
- Fence rows.
- Hedge rows.
- PM10 & PM2.5
- Fugitive dust.
- Water pollution.
- Sediments.
- Nutrients.
- Pesticides.
- Pathogens.
- Detrimental deposition.
- Non-point source pollution.
- Sediment Load behind dams decreases water storage capacity.
- Sedimenting navigable waterways impeding commerce .
- Sediments.
What can be done?
Soil Conservation Practices
Water
- Vegetation (establish permanent cover or seasonal cover).
- Human-made structures
- check dams, riprap, mulching.
- Conservation tillage (minimum tillage, no-till).
- Special practices (terraces, strip cropping, grassed waterways).
Wind
- Mulch
- Windbreaks
- Conservation tillage