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Worldwide, 26.4 billion tons of soil is lost each year. This rate is 10 times higher than the natural rate.
Even though erosion is a natural phenominon, human interference into natural systems have created erosion that is much higher than the average geological erosion rate. Erosion is a threat to sustained agricultural production. Soil erosion is a process of moving soil by water or wind - when the soil particles are detached and transported to a different location. This is a natural process that has occurred for eons of time. Water, wind, ice, and gravity are involved in moving soil materials. Humans, however, have often caused accelerated erosion by our manipulation of the soil for agriculture or contruction use. When soils are left bare even for a short period (such as when fields are tilled for planting), the soil can be picked up and moved. These bare soil drops are exposed to the energy of the raindrops and wind. The finer particles of soil are eroded first, taking with them most of the natural fertility and production potential. However, with plant cover, the roots bind the soil particles together and lesson erosion.
Erosion involved three processes: detachment (from the ground), transportation (via water or wind), and deposition. The deposition is often in places we don't want the soil such as streams, lakes, reservoirs, or deltas. And, of concern is that the topsoil is often the most fertile and when it erodes away, the subsoil is less productive.
Water is powerful! Water erosion is caused by two main forces - raindrop impact and flowing water. Raindrops can both destroy soil aggregates and transport soil small distances. Then, flowing water transports these detached particles down hill. The size of the particles transported increases with the kinetic energy of the water. The deeper the water is, the larger the particles that move. These particles move away from the field, and end up in streams and waterways.
Three types of water erosion can occur, sheet, rill, and gully.
Sheet erosion: This erosion is the hardest to see, as a uniform soil layer is removed from an area over the surface.
Rill erosion: This type of erosion starts as water flowing over the soil surface concentrates into small streams, creating channels of water flow.
Gully erosion: If the rill erosion is not kept under control, it creates gullies, which are deeper and wider cuts. They are really problematic on cropland, since tractors and tillage implements cannot get across them.
Soil can fill the air when it is dry and not anchored by vegetation. Think back to the Dust Bowl of the 1930s, which is one of the biggest environmental disasters in United States history. The inversion of soils burying resides left the surface bare. When the rains didn't come, the conditions were ready for disaster. The winds came and lifted the bare soils into the sky over 10,000 feet two to three times a week. This ended after ten years. The development of irrigation and cover decreased erosion by the time the 1950s came along. The less amount of precipitation in the Western great plains still creates more wind erosion, because they get less precipitation and has lower amounts of vegetation. There are also less trees and they are flatter. The soils are lower in organic matter, which acts as a glue to hold soil clumps together.
Dust blown by the wind from an Iowa field that was not planted to grass to prevent soil erosion in 1890. Credit: National Archives and Records Administration.
Sands actually start the wind erosion process. They are round and get lifted into the air like plane wings. These moving particles knock the clay and silt loose into the air, where they can go several thousand kilometers. Like water erosion, there are three different types of wind erosion, surface creep, saltation, and suspension.
Surface Creep: When the wind speed at the soil surface exceeds 13 miles per hour (21 kilometers per hour), soil particles start to roll along the soil surface.
Saltation: As the wind speeds pick up, the surface particles start leaping off the surface into the air, this process is saltation.
Suspension: After the saltation begins, the particles hit the surface and knock loose other particles, knocking them into the air. These include smaller sand particles and clays. They then are lifted into the atmosphere and can be transported long distances until the wind speed decreases.
Wise use of our soils requires us to minimize erosion. The best way to control water erosion involve slowing down the flow of water and limiting soil detachment. The best way to prevent this is to keep the soil covered with either growing plants or residues from past crops. These absorb the energy from raindrops and slow the rate of water flow over the surface, allowing more time to have water infiltrate through the soil. Tilliage is very important in agriculture, but there are several best management practices that can help reduce erosion. Using cover crops, filter strips, contour farming and conservation tillage, and riparian buffers can help reduce or eliminate soil movement. Minimum and no till systems help with both types of erosion.
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