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6.2: Erosion

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    25021
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    Soil loss during agricultural production is mainly caused by water, wind and tillage. Additionally, landslides (gravitational erosion) may occur on very steep slopes. While water erosion and landslides occur under extremely wet soil conditions, wind erosion is a concern with very dry soil. Tillage erosion occurs on fields that are either steep or have undulating topography. Erosion is the result of the combination of an erosive force (water, wind or gravity), a susceptible soil and several management- or landscape-related factors. A soil’s inherent susceptibility to erosion (its erodibility) is primarily a function of its texture (generally, silts more so than sands and clays), its aggregation (the strength and size of aggregates, related to the amount of organic matter and clay), and soil water conditions. Many management practices can reduce soil erosion, although different types of erosion have different solutions.

    Water Erosion

    Water erosion is especially severe on bare, sloping land when intense rainfall rates cause runoff. The water flowing over the soil surface concentrates into tiny streamlets, which detach the saturated soil and transport the particles downhill. Runoff water gains more energy as it moves down the slope, scouring away more soil and also carrying more agricultural chemicals and nutrients, which end up in streams, lakes and estuaries (Figure 6.2). Erosion can involve broad areas in fields where small depths of soil are removed all the way to deep gullies that leave scars in the landscape.

    contaminated stream in Venezuela
    Figure 6.2. Left: Water erosion on clean-tilled soil in Bulgaria. Topsoil has been lost in the background field. Right: A stream in Guarico, Venezuela, contaminated with dispersed sediment.
    water erosion on clean tilled soil
    Figure 6.2. Left: Water erosion on clean-tilled soil in Bulgaria. Topsoil has been lost in the background field. Right: A stream in Guarico, Venezuela, contaminated with dispersed sediment
    Figure 6.2. Left: A stream in Guarico, Venezuela, contaminated with dispersed sediment. Right: Water erosion on clean-tilled soil in Bulgaria. Topsoil has been lost in the background field.

    Soil erosion is of greatest concern when the surface is unprotected and directly exposed to the destructive energy of raindrops and wind (Figure 6.2). The erosion process leads to a decrease in soil organic matter and aggregation, which in turn promotes further erosion. Thus, a vicious cycle begins. Soil is degraded because the most fertile part of the soil, the surface layer enriched in organic matter, is removed by erosion. Erosion also selectively removes the more easily transported finer soil mineral particles, clays, which help store nutrients and organic matter and stabilize soil aggregates. Severely eroded soils, therefore, have less favorable physical, chemical and biological characteristics, leading to a reduced ability to sustain crops and an increased potential for harmful environmental impacts.

    The lower infiltration capacity of eroded soils reduces the amount of water that is available to plants and the amount that percolates through the soil into underground aquifers, while increasing the potential for flooding. This reduction in underground water recharge results in streams drying up during drought periods. Watersheds with degraded soils thus experience lower stream flow during dry seasons and increased flooding during times of high rainfall, undesirable in both cases. In fact, we surmise that the trend of increased flooding in many areas is not only the result of changed weather patterns but also compounded by gradual soil degradation.

    Wind Erosion

    Dust bowl photograph with large blooms of dark soot in the air
    Figure 6.3. Drought and poor soil health created wind and water erosion during the Dust Bowl. Photo by USDA.

    The photograph of wind erosion from the Dust Bowl era (Figure 6.3) provides a graphic illustration of land degradation. Wind erosion can occur when soil is dry and loose, the surface is bare and smooth, and the landscape has few physical barriers to wind. The wind tends to roll and sweep larger soil particles along the soil surface, which will dislodge other soil particles and increase overall soil detachment. The smaller soil particles (very fine sand and silt) are lighter and will go into suspension in the atmosphere. They can be transported over great distances, sometimes across continents and oceans. Wind erosion affects soil quality through the loss of topsoil rich in organic matter and can cause crop damage from abrasion (Figure 6.4). In addition, wind erosion affects air quality, which is a serious concern for nearby communities. During the Dust Bowl, soil was blown all the way from the central part of the continent to New York and Washington, making East Coast residents directly aware of the environmental disaster occurring in the middle of the continent.

    The ability of wind to erode a soil depends on how that soil has been managed, because strong aggregation makes it less susceptible to dispersion and transportation. In addition, many soil-building practices like no-till, mulching and the use of cover crops protect the soil surface from both wind and water erosion.

    Soil And Water Conservation In Historical Times
    soil water in historical times

    Some ancient farming civilizations recognized soil erosion as a problem and developed effective methods for runoff and erosion control. Ancient terracing practices are apparent in various parts of the world, notably in the Andean region of South America and in Southeast Asia. Other cultures, like in pre-Columbian America, did not till the fields and effectively controlled erosion using mulching and intercropping. Some ancient desert civilizations, such as the Anasazi in the southwestern United States (600–1200 A.D.), retained runoff water and eroded silt from upper parts of the landscape with check dams to grow crops in downhill depressions (see the picture of a now forested site). For most agricultural areas of the world today, erosion still causes extensive damage (including the spread of deserts) and remains the greatest threat to agricultural sustainability and water quality.

    wheat plant wind erosion damage
    Figure 6.4. Wind erosion damaged young wheat plants through abrasion. Photo by USDA Wind Erosion Research Unit.
    Landslides in central america
    Figure 6.5. Sustained rains from Hurricane Mitch in 1998 caused super-saturated soils and landslides in Central America. Photo by Benjamin Zaitchik.

    Landslides

    Landslides occur on steep slopes when the soils have become supersaturated from prolonged rains. They are especially of concern in mountainous countries where high population pressure resulted in farming on steep hillsides (Figure 6.5). The sustained rains saturate the soil, especially in landscape positions that concentrate water from upslope areas. This has two effects: It increases the weight of the soil mass (all pores are filled with water), and it decreases the cohesion of the soil (see the compaction of wet soil in Figure 6.12, right) and thereby its ability to resist the force of gravity. Agricultural areas are more susceptible than forests because they lack large, deep tree roots that can hold soil material together and may be without living vegetation for a portion of the year. Pastures on steep lands, common in many mountainous areas, typically have shallow-rooted grasses and may readily experience slumping. With certain soil types, landslides can become liquefied and turn into mudslides.

    Tillage Erosion

    impacts of tillage erosion on soils
    Figure 6.6. Effects of tillage erosion on soils. Photo by Ron Nichols, USDA-NRCS.

    Tillage promotes water and wind erosion by breaking down aggregates and exposing soil to the elements. But it can also cause erosion by routinely moving soil down the slope to lower areas of the field, which becomes an increasing problem with more intensive mechanized tillage. In complex topographies—such as seen in Figure 6.6—tillage erosion ultimately removes surface soil from knolls and deposits it in depressions (swales) at the bottom of slopes. What causes tillage erosion? Basically, when soil is moved by a plow or harrow on sloping land it causes more soil to move into the downslope than the upslope direction, resulting in net downslope transport. As an analogy, when throwing a ball upwards or downwards on a hillside it will go a farther distance in the down direction. Soil is similarly thrown farther downslope when tilling in the downslope direction than is thrown uphill when tilling in the upslope direction (Figure 6.7a). Over many years this has the cumulative effect of moving a lot of soil down the slope. Also, downslope tillage (with gravity) typically occurs at greater speeds than when traveling uphill (against gravity), making the situation even worse.

    causes of erosion
    Figure 6.7. Three causes of erosion resulting from tilling soils on slopes. Illustration by Vic Kulihin.

    Tillage along the contour also results in downslope soil movement. Soil lifted by a tillage tool comes to rest at a slightly lower position on the slope (Figure 6.7b). A more serious situation occurs when using a moldboard plow along the contour. Moldboard plowing is often performed by throwing the soil to the side and down the slope, as this inverts the soil better than by trying to turn the furrow up the slope (Figure 6.7c).

    One unique feature of tillage erosion compared to wind, water and gravitational erosion is that it is unrelated to extreme weather events and occurs gradually with every tillage operation. Tillage erosion makes field management more challenging as it results in lower crop productivity on the knolls and hillsides, and higher productivity in the swales. However, it does not generally result in offsite damage because the soil is merely moved from higher to lower positions within a field. But it is another reason to reduce tillage on sloping fields.


    This page titled 6.2: Erosion is shared under a CC BY-NC 4.0 license and was authored, remixed, and/or curated by Fred Magdoff & Harold van Es (Sustainable Agriculture Research and Education (SARE) program) via source content that was edited to the style and standards of the LibreTexts platform.