Spring rains come with unexpected quantities and force, causing significant amounts of soil erosion to unprotected cropland. Spring is the most critical time for soil erosion because of degraded crop residue, tillage in preparation for planting, and lack of crop canopy. Residue cover is not only good for preventing soil erosion, but it will cut down sediment transport to water bodies and contribute to the improvement of water quality.
Raindrop splash and displacement of soil particles. Source: USDA Natural Resources Conservation Service.
Why is rainfall so destructive to bare cropland? In a normal rainfall, raindrops range in size from 1 to 7 millimeters in diameter and hit the ground going as fast as 20 miles per hour (see photo). The impact of millions of raindrops hitting the bare soil surface can be incredible, dislodging soil particles and splashing them 3 to 5 feet away (Figures 1 and 2). A heavy rainstorm may splash as much as 90 tons of soil per acre. However, the majority of the soil splashed is not immediately lost from the field. Most of the splashed soil particles don't leave the field; they clog surface pores, which in turn reduces water infiltration, increases water runoff, and increases soil erosion.
Figure 1. Sequential profile of a raindrop splash pattern. (Source: Environmental Soil Physics, Hillel)
Figure 2. Raindrop splash pattern from a sloped landscape. Source: Environmental Soil Physics, Hillel)
After a rainfall event, soil crusting is a significant problem, particularly on soils with low residue cover. The surface crust is caused by a breakdown of soil aggregates due to raindrop impact. The raindrop splash detaches particles that fill soil pores. When rapid drying occurs, a hard crust layer can form in the top 2 inches of the soil. Soil crusting is troublesome when it develops prior to seedling emergence. Additionally, soil crusts create conditions that are extremely conducive to soil erosion during following rainfall events.
The use of a well-designed conservation system can limit exposed soil and rainsplash erosion. An effective conservation system also depends on the planning, observation, and timing of operations. Spring is a good time to make observations and develop a new, more comprehensive conservation system.
Conservation systems to reduce raindrops' effect
Tillage and cropping management systems are critical components for reducing raindrop impact on soil particles due to the availability of crop residue to protect the soil surface. Excessive tillage can damage soil structure, leading to increased soil sealing and soil erosion. Conservation systems promote soil aggregates, infiltration, and soil tilth. Additionally, the improved soil structure of no-tillage and other conservation tillage systems stands up better against raindrops. A conservation system that includes high amounts of crop residue such as corn or fall cover crop traditionally provide abundant residue cover to protect the soil surface from spring rains.
Farmers are encouraged to assess residue cover since last fall's harvest and ask themselves the following questions: Was surface residue enough to prevent soil erosion? Is the surface residue cover distributed evenly across the field? Is there enough residue cover left after winter decomposition? If these questions can be answered no, then fall tillage passes and fall manure or anhydrous application need to be considered based on the amount of residue and the residue distribution in the field. Remember that spring is the best time to evaluate conservation systems for their impact on improving soil and water quality.
Options for adjusting spring field operations
With spring weather and the most susceptible field conditions for water erosion here, what options remain before planting? Farmers should consider the effect of any additional tillage on remaining crop residue. If residue cover should fall below 30 percent, adjust your field operations to minimize potential soil erosion due to early spring rain. Options for steep slope areas include cover crops, permanent vegetation, strip cropping, and planting on the contour, all of which can reduce the speed of water runoff and slow soil erosion. If soil crusting occurs, consider using a rotary hoe to allow seedling emergence to occur unrestricted. The faster the crop is growing, the sooner a crop canopy will develop; a partial crop canopy is better than none at all.
Conservation structures such as terraces, grassed waterways, and field buffers are good components of a conservation system, which help in slowing water flow, settling out sediments, and directing water away from the field to a suitable outlet.
Remember that field observations in the spring can help in developing a more comprehensive conservation plan that greatly improves soil and water quality.
This article originally appeared on pages 57-58 of the IC-494 (8) -- May 2, 2005 issue.
Soil erosion problems are not only limited to water erosion in Iowa, although it is the dominant one due to high rainfall events and their significant impacts on sediment transport to lakes and streams in the state. However, wind erosion at this time of the year can be very significant and contribute to serious topsoil loss given the high winds experienced in the state during recent weeks. Soil loss by wind erosion may not be physically noticeable on the field, but it can be significant in terms of its effects on air, soil, and water quality over time.
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. Soil that is left unprotected due to tillage operations is prone to wind erosion, especially on flat, dry fields. Wind erosion is caused by strong winds that physically move lighter, less dense soil particles such as organic matter, clay, and silt particles. Very fine particles can simply be suspended in the airstream and carried long distances. Slightly larger soil particles may hop along the surface. Still larger particles are rolled along the soil surface. Loose soil particles can drift along, bombarding and dislodging still more particles with the same effect as sandblasting.
These particles are the most fertile part of the soil, therefore lowering soil productivity (see Iowa State University Extension publication PM 1870, Soil Erosion, Crop Productivity and Cultural Practices). Lost soil productivity has been masked over the years by improved crop varieties and increased fertilization. Thus, wind erosion reduces potential soil productivity and increases economic costs. In addition to reduced soil productivity, wind erosion can reduce seedling survival and growth, increase soil crusting, and increase the susceptibility of plants to disease pathogens.
Wind erosion across a field in north central Iowa.
The most important factor in minimizing both wind and water erosion is residue cover. The clean tillage observed in many areas in Iowa not only accelerates wind erosion but also leads to other field management problems such as soil surface crusting and poor plant emergence. Fields with extensive tillage have shown a great deal of wind erosion, resulting in soil crusting problems this time of the year.
Wind erosion prevention
- Maintain surface residue cover throughout the year. The benefit of crop residue is to protect the soil surface not only from water erosion but also from high winds by reducing soil evaporation, keeping the soil structure in place, reducing soil crusting, and promoting a good soil environment for better plant emergence.
- Reduce tillage operations. Each tillage operation causes loss of soil moisture and residue. The use of subsurface tillage tools and other reduced tillage practices will control many of the weeds without destroying a large amount of the residue.
- Consider strip cropping. Using strip cropping is effective to prevent severe wind erosion. Strip cropping provides a good protective cover of growing plants or residues.
- Establish windbreaks. Windbreaks can slow wind a distance of ten times the windbreak height. Additionally, windbreaks provide habitat for wildlife.
It is very important to remember that by keeping good residue cover, all forms of soil erosion can be minimized given the unpredictable weather conditions of high winds and precipitation experienced during the early spring through the planting season. Leaving good crop residue is an important management decision that has both economic and environmental impacts.
This article originally appeared on pages Page 1-2 of the IC-494(11) -- May 23, 2005 issue.
Print this fact sheet
by T.A. James, R.L. Croissant and G. Peterson1 (8/09)
Quick Facts…
- Vegetative barriers reduce erosion from wind by reducing unsheltered distance across fields.
- Barriers can protect young, sensitive, high-value crops from damage by blowing soil particles.
- Combinations of barriers and ridges can reduce residue requirements in conservation compliance plans.
- Fall-planted cover crops can prevent soil erosion by wind after harvest of low residue crops such as beans, beets, or potatoes. Covering the soil surface with crop residues is the most
effective means to control soil erosion.
Soil Erosion by Wind
Blowing soil can be an unpleasant nuisance, a serious safety hazard, or a costly disaster depending on one’s perspective and the intensity and duration of the windstorm. The public response to blowing soil has led to such legislation as the Colorado Dust Blowing Law and provisions of the 1985 Food Security Act.
| Figure 1: The wind erosion process. |
Blowing soil or soil erosion by wind is a complex process. It involves detachment, transport, sorting, abrasion, avalanching, and deposition of soil particles. Turbulent winds above a threshold velocity (13 miles per hour at one foot above the ground) blowing over erodible soils can cause erosion or blowing. At selected locations and dates in Colorado, average wind speeds may exceed 5 to 9 miles per hour.
Wind transports soil particles in three ways:
Saltation. Individual particles are lifted off the soil surface by wind; then they return and the impact dislodges other particles. Fifty percent to 80 percent of total transport is by saltation.
Suspension. Dislodged particles, small enough to remain airborne for an extended period of time, are as visible as dust but generally make up less than 20 percent of the total soil transported.
Surface Creep. Sand-sized particles are set in motion by saltation. These sand size particles creep slowly along the surface. Up to 25 percent of total transport may be from surface creep. (Figure 1).
Barriers
Three primary means to control or reduce erosion or damage from wind are available.
- Decrease the distance across a field that receives no shelter from the wind.
- Form ridges with appropriate tillage equipment on the soil surface at right angles to the prevailing erosive winds.
- Protect the soil surface with a covering of plant residue or growing plants.
Unsheltered distance across a field is reduced by installing vegetative barriers at right angles to the prevailing erosive winds. These barriers reduce the soil transported by:
- providing a stable barrier to stop saltation of soil particles,
- trapping soil particles creeping along the ground, and
- reducing wind speeds below the threshold velocity along the ground for a distance of ten times (10x) the barrier height. This horizontal distance is sheltered from the wind, reducing the distance in a field where wind exceeds the threshold velocity. Table 1 illustrates the effect of installing barriers in a cropland field.
| Table 1: Conservation compliance systems providing equivalent soil loss1 in Lincoln County, Colorado. | |||
| Loamy fine sand | 2,640 | 500 | 0 |
| 330 | 350 | 8 | |
| Silt loam | 2,640 | 265 | 0 |
| 660 | 150 | 4 | |
| 1The predicted soil loss equals 14 tons annually per acre. 2The barriers consisted of seven rows of grain sorghum. Residues were present during the critical period, November through April. |
| Figure 2: Vegetative barrier of sorghum in field. |
Vegetative barriers (Figure 2) can consist of perennial plants or annual plants or a combination. Taller plants provide more protection than shorter plants when used at the same spacing. For example, a barrier of tall wheatgrass 2.5 feet high will provide a stable barrier as well as reducing the unsheltered distance by 25 feet (2.5 ft. x 10).
When annual plants such as corn, sorghum, or millet are used for barriers, the plants must be in place during the critical erosion period in order to be effective. In Colorado, the critical erosion period from wind is generally from November through April. Consequently, such annual barriers need to be planted in May or June and left undisturbed through the following April.
| Figure 3: Ridging as an effective barrier. Here a deep furrow drill creates ideal ridges. |
Ridges
Ridging the soil surface can also be an effective practice. Ridges absorb and deflect wind energy and trap moving soil particles. Ridges with an ideal height to spacing ratio of 1:4 are most effective when constructed at right angles to the prevailing erosive wind. Ridges 4 inches high spaced 16 inches apart will provide optimum protection if oriented correctly.
Soil texture determines, to a great extent, the effectiveness and persistence of ridging. Silt loams, loams, and clay loams consolidate well with tillage and hold ridges against wind, rain, and snow impacts. Sandy loams, loamy sands, and coarse sands are easy to till into ridges but the ridges
deteriorate quickly.
Cover Crops
Cover crops work well in irrigated fields where moisture is not a limiting factor. Wheat or rye can be fall-planted after harvest of low residue crops, such as sugar beets, potatoes, or vegetables, if sufficient growing degree days remain to obtain germination and growth before winter freeze-up. Using close drill spacings of 6 to 7 inches and/or doubling the seeding rate will provide soil surface cover as rapidly as possible.
Early planting of winter wheat can offset insufficient surface residues if favorable growing conditions exist. However, unfavorable fall growing conditions can eliminate any advantage
to early plantings.
Crop Residues
| Figure 4: Corn stubble is used as cover for winter wheat seeding. |
Using previous crop residues to protect the soil surface is the single most effective practice for controlling erosion from wind.
Crop residues vary in their effectiveness. Fine-strawed, upright stubble in rows perpendicular to the wind are more effective than large, randomly distributed stalks lying flat on the soil surface. Relationships have been developed to indicate the relative effectiveness of most types of crop residues. Figure 5 compares the effectiveness of corn and wheat residues.
Erosion control systems can be developed from a combination of practices to manage residues, ridge the soil surface, and reduce the unsheltered distance across fields.
The calculations to assess the effectiveness of any set of practices is laborious. However, there is software available at USDA SCS field offices in each Colorado county that will perform the evaluations.
The best way to estimate the amount of vegetative cover on a field is to use one of several methods:
Clip residue from 1 to 2 square yards on at least three different places, and weigh dry matter.
- Use a 100-foot tape, counting the number of intersects on each foot. Thirty-five intersects on the foot markers will indicate 35 percent residue cover. Additional calculations provide an estimate of residue weight.
- Various crop residue is compared to flat crop residue using flat small grain equivalents (wheat straw), with stubble 10 inches long, as the base.
| Figure 5: Flat small grain equivalents for winter wheat and corn residues. |
Other residue configurations are visible when examining Figure 5. The standard “Flat winter Wheat” with 10-inch long stubble is compared with “Standing winter wheat” residue having stubble 10 inches long. From the graph, it is estimated that it takes 6,000 pounds of flat corn stalks to equal 1,000 pounds of flat small grain equivalent (SGE) per acre.
Agricultural producers planning for Conservation Compliance requirements are encouraged to contact the local SCS office for assistance.
1Originally written by T.A. James, agronomist, USDA Soil Conservation Service, Lakewood,
Colorado; and R.L. Croissant, Colorado State University Extension specialist, soil and crop sciences. 8/94. Reviewed by G. Peterson, department head, soil and crop sciences, Colorado State University. 8/09.
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