Soil Conservation
Soil Erosion
Soil erosion is a major concern around the globe. Soil erosion involves the detachment and transport of soil particles. Detached particles are transported by water and wind. Soil erosion is both naturally-occurring (geologic erosion) and influenced by humans (accelerated erosion). In agriculture, soil erosion refers to the loss of a field’s topsoil by the natural physical forces of water and wind or through forces associated with farming activities such as tillage. The rate of soil erosion depends on many factors, including the soil’s makeup, vegetation, and the intensity of wind and rain. Climate is also a major driver of erosion. Changes in rainfall and water levels can shift soil, extreme fluctuations in temperature can make topsoil more vulnerable to erosion, and prolonged droughts can prevent plants from growing, leaving soil further exposed.
Water Erosion
Water erosion is a threat to soil productivity on most annual crop land. It is caused by rain pounding on an inadequately protected soil surface and by the action of runoff when water flows have enough energy to cut into the soil. Soil erosion by water occurs when bare-sloped soil surface is exposed to rainfall, and the rainfall intensity exceeds the rate of soil intake, or infiltration rate, leading to soil-surface runoff. Soil erosion can occur in two stages: (1) detachment of soil particles by raindrop impact, splash, or flowing water; and (2) transport of detached particles by splash or flowing water.
Mechanics of Water Erosion
Water erosion is generally recognized in several different forms. Splash erosion is the first stage in the erosion process that is caused by rain. Raindrops basically “bombard” the exposed and bare land, moving its particles and destroying the structure of the top layer. Eventually, it causes the formation of surface crusts, negatively affects soil infiltration ability, and eventually results in runoff formation. Sheet erosion by water occurs when the rainfall intensity is greater than the soil infiltration ability and results in the loss of the finest soil particles that contain nutrients and organic matter.
Factors Affecting Water Erosion
Water erosion is affected by precipitation patterns, soil texture, slope, surface roughness, and vegetative cover. Rainfall quantity, intensity and duration influence the extent of water erosion. Intense rainstorms of more than 1 inch per hour (2.5cm/hr) exceed most soils’ capacity to absorb water, creating runoff conditions which lead to water erosion on unprotected fields. In general, the most severe erosion occurs when rains are of relatively short duration, but high intensity versus if the same volume of rain was to fall on the same site over the course of a week in several intermittent showers. Long, low intensity storms can also be highly erosive due to saturated soil conditions causing increased runoff. Soil properties affecting water erosion include those that influence infiltration and soil stability, such as texture, organic matter content, soil structure, and tilth.
Impact of Water Erosion
The effect of soil loss depends on the type and depth of the topsoil that has been washed away. As topsoil is lost, the ability of the remaining soil to hold nutrients and moisture is diminished—which can greatly reduce crop emergence, growth, and yield. Some seriously eroded soils are not usable for crop production at all. Erosion can also reduce the soil’s ability to absorb water, which can result in flooding and create large areas of standing water. If areas remain flooded during the planting season, it can delay or impede the planting of new crops.
Wind Erosion
Wind erosion is the natural process of transportation and deposition of lighter, less dense soil particles such as organic matter, clay, and silt particles by the wind. It is a common phenomenon occurring mostly in arid and semiarid climates. Wind erosion damages land and natural vegetation by removing soil from one place and depositing it in another. One dramatic example of soil erosion is the dust picked up by winds just south of Africa’s Sahara Desert—the Sahel region of transition from desert to savannah—traveling some 3,000 miles to South America and the Caribbean, and occasionally reaching the southeastern United States. In drier regions of North America, millions of tons of soil are lost to wind erosion annually.
Mechanics of Wind Erosion
The mechanism of wind erosion is quite different from water erosion. The drier the soil the more effect wind will have on dislodging soil particles and carrying them away causing significant damage to the air and water quality. Moving air has energy that can detach and transport soil particles. Detachment occurs when the energy exerted by wind exceeds the forces keeping the soil particles in place, such as weight and “cohesion.” Detachment can also occur via the impact of particles already in motion dislodging other particles.
Factors Affecting Wind Erosion
Soil properties that affect erodibility include texture, moisture, and aggregation. Silt and fine sand particles are most prone to erosion due to their smaller mass than larger particles, and less cohesiveness between particles than fine, clay-like particles. Moisture increases cohesive forces between particles, making them more difficult to dislodge. Aggregation reduces erosion by binding potentially erodible particles together into larger particles which resist detachment and transport. Aggregate stability (how well the aggregate is bound) will also affect its erodibility and is related to chemical and organic compounds in the soil. When surface soils contain a wide range in particle sizes, wind erosion preferentially removes the finer particles, leaving the larger particles behind.
Impact of Wind Erosion
Like water erosion, wind erosion removes the best soil first—the topsoil rich in organic matter. When wind causes soil to become airborne, the blowing soil can sandblast delicate leaves and stems or even bury plants and seeds, resulting in decreased crop yields. Plants vary in their tolerance with small grains being relatively tolerant of abrasion. Corn, soybeans ,and mature alfalfa have a moderate tolerance, vegetables have a low to very low tolerance, and seedling alfalfa and sugar beets have a very low tolerance to abrasion.
Tillage Erosion
Tillage erosion is the progressive downslope movement of soil by tillage causing soil loss on hilltops (knolls) and soil accumulation at the base of slopes (depressions). Landscapes that are very topographically complex (with many short, steep, diverging slopes) are more susceptible to tillage erosion. Visual evidence of tillage erosion includes: loss of organic rich topsoil and exposure of subsoil at the summit of ridges and knolls; and undercutting of field boundaries, such as fence lines, on the downslope side and burial on the upslope side. Tillage erosion is probably the major cause of topsoil loss and subsoil exposure in our environment reducing crop productivity and increases field variability.
Managing Soil Erosion
In order to strike a balance between agricultural output and soil conservation, soil erosion control becomes a very essential component. Key points for managing any type of soil erosion are to reduce the erosivity of the eroding agent, decrease the soil’s susceptibility to erosion, and prevent particle transport. Using best management practices that focus on these key points will be most effective for managing erosion. Management strategies can be implemented by using one of two general strategies, or a combination of both.
Conservation Tillage
While there are numerous conservation tillage systems in use today as discussed in Chapter 17, all have in common that they leave significant amounts of organic residues on the soil surface after harvesting and planting. Conservation tillage maintains residue on the soil surface which makes it less susceptible to soil erosion than practices that remove excess residue. For example, a field with 20 percent cover will have a 50 percent reduction in soil loss compared to a bare field.
Strip Cropping
Strip cropping is a method of farming used when a slope is too steep or too long, or otherwise, when one does not have an alternative method of preventing soil erosion. Crops of the strips vary in their root/shoot characteristics and cultural requirements. It alternates strips of closely sown crops such as hay, wheat, or other small grains with strips of row crops, such as corn, soybeans, cotton, or sugar beets. Strip cropping helps to stop soil erosion by creating natural dams for water, helping to preserve the strength of the soil.
Contour Tillage and Planting
Contour tillage and planting follows the contours of hills and slopes, rather than orienting crop rows up and down a slope. It is a technique that was popularized during the New Deal and afterwards in response to soil erosion. Contour tillage furrows run crosswise to the slope, slowing runoff and allowing the soil to absorb rainfall rather than wash away. Rainfall ponds behind the small ridges created by tillage, providing more time for water to infiltrate and impeding downhill flow.
Cover Crops
Cover crops are a great tool that farmers can use to minimize soil movement off of field. Active roots in the soil hold the soil from water erosion while above ground growth shields soil movement from wind erosion. Covering the soil also protects the soil from rainfall splatter. For instance, implementing cover crops into continuous corn and corn-soybean rotations may reduce soil erodibility (soil’s susceptibility to erosion) by improving soil properties such as soil organic matter, water infiltration, and aggregate stability.
Shelterbelts
Trees, shrubs, hedgerows, and ground plants help control wind erosion, reduce the drying effects of wind on soil and plants, and help prevent the abrasive action rapidly moving soil particles have on young tender seedlings. Shelterbelts, also known as windbreaks, protect soil for a distance of about 10 times the height of the trees by shortening the field, reducing wind velocity, and capturing blowing soil.
Conservation Buffer Strips
Off-site damage from water erosion can be managed through the use of conservation buffer strips. These strips, or buffer zones, are widths of vegetation that reduce off-field transport and deposition of sediment and other pollutants by reducing water flow velocity and trapping sediment. Placed at the bottom of a field or just above the tail water ditch in an irrigated system, conservation buffer strips have been shown to be effective for reducing sedimentation in runoff water.
Grass Waterways
Grassed waterways are broad, shallow, saucer-shaped channels, seeded to grass or other suitable vegetation, designed to move surface water across farmland without causing soil erosion. The key component of this control is the vegetative cover in the waterway, which slows the speed of water flow in the watercourse and serves as a physical filter that removes sediment (and sediment bound nutrients) from water flow.
Irrigation
Care should be taken when irrigating land which is susceptible to erosion. Always adjust application rates to suit field and soil conditions so that the risk of surface run-off is minimized. Water droplet size is an important factor. The larger the droplet size, the greater its erosivity. If a large droplet size is combined with an excessive application rate, soil particles can be detached and carried away by surface water run-off.
Managed Grazing
The main factors influencing erosion on grazed lands are type of grazing systems, stocking rate, forage type and growth patterns, and soil characteristics. Overgrazing leaves little vegetative cover to protect the soil from erosion, and heavy animal traffic can compound the problem by compacting the soil. Grass cover generally protects the soil better than legumes.
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