 |
 |
VI. SOIL EROSION AND COMPACTION
Soil Erosion by Wind Next Section>>
Subsections:Soil erosion processes | Factors effecting wind erosion | Controlling wind erosion
Wind erosion is caused by a strong, turbulent wind blowing across an unprotected soil surface that is smooth, bare, loose, dry, and finely granulated. Soil particles start to move when the force of the wind overcomes gravity. The wind speed required to start movement depends on the size and weight of soil particles. For loose sand, the wind speed that initiates soil movement is about 13 miles per hour, measured at a height of 1 foot above the ground surface.
Soil usually starts blowing on exposed knolls or hilltops, in tracks or paths made by implements or animals, and in corners or turn rows where excessive turning and cultivation have pulverized the surface soil. After soil particles start to move, the wind carries some of them a great distance.
Once blowing begins, soil flow is zero at the windward edge of an eroding field, but the rate increases leeward (downwind) until it reaches the maximum that a given wind can carry. The distance downwind at which the maximum rate of flow occurs varies with soil erodibility. The more erodible the soil, the greater the rate of erosion and the shorter the distance to maximum soil movement.
back to topMajor factors that affect the amount of erosion are soil cloddiness, surface roughness, wind speed, soil moisture, field size, and vegetative cover. A discussion of each follows.
The cloddiness of a given soil largely indicates whether the wind will erode it. Soil clods prevent wind erosion because they are large enough to resist the forces of the wind and because they shelter other erodible materials. Clods form during tillage. Their firmness and stability vary with soil type and depend on soil moisture, compaction, organic matter, clay content, and possibly other factors. Clods are broken down by weathering, tillage, implement and animal traffic, and abrasion by wind-driven soil particles. Weathering includes freezing and thawing, wetting and drying, and raindrop impact. Sandy loams, loamy sands, and sands form the weakest aggregates or clods and thus are most susceptible to erosion. These soils have low silt, clay, and organic-matter content. The least wind-erodible soils are the loams, silt loams, clay loams, and silty clay loams.
Surface roughness also affects erosion. Ridges and depressions formed by tillage alter wind speed by absorbing and deflecting part of the wind energy away from erodible soil. Effective ridges must be nearly perpendicular to the direction of prevailing winds. Rough surfaces also trap moving particles. This reduces abrasion and the normal build-up of eroding materials downwind.
Wind speed is also important. The rate of erosion caused by a 30-mile-per-hour wind is more than three times that of a 20-mile-per-hour wind.
Wind erosion decreases as soil moisture increases. For example, dry soil erodes about one-and-one-third times more than soil with barely enough moisture to keep plants alive.
Field size affects the distance the wind blows without encountering a barrier. The rate of soil loss increases rapidly with distance downwind from the point in the field where the wind erosion process begins. Barriers, such as a windbreak of trees, on the windward side of the field provide shelter from the wind and reduce the area of the field subject to wind erosion.
Good vegetative cover on the land is the most permanent and effective way to control wind erosion. Living or dead vegetative matter protects the soil surface from wind by reducing wind velocity at the soil surface and by preventing much of the direct wind force from reaching erodible soil particles. It also reduces rates of erosion by trapping soil particles.
back to topThere are four major principles of wind erosion control:
1. Establish and maintain vegetation or vegetative residues.
2. Reduce field widths.
3. Produce, or bring to the soil surface, aggregates or clods.
4. Roughen or ridge the land surface perpendicular to the prevailing wind direction.
Wind erosion can be controlled by several different practices or combinations of practices. Conservation tillage, such as no-till and ridge-till planting, which leaves significant amounts of crop residue on the surface, is the most easily adopted control method for most of Indiana’s cropland. The surface residue decreases wind velocity at ground level and traps dislodged soil particles. Conservation tillage may be used with other wind erosion practices to provide increased protection. A surface mulch can effectively control erosion only if sufficient mulch is retained on the surface; thus, conservation tillage is better adapted to some situations and some crops than others. For example, soybeans produce relatively small amounts of crop residue.
Cover crops may be used in certain situations. These crops must be seeded early enough for the cover crop to reach sufficient height to provide adequate winter and early spring protection.
Alternate strips of row crops and close growing crops control wind erosion by halting the erosion process before movement of eroded soil particles can reach serious levels. As nearly as possible, the strips should be at right angles to the prevailing wind direction. Strips parallel to the wind direction do not protect against soil erosion.
Windbreaks are also placed at right angles to the prevailing wind direction in April. Prevailing wind direction in Indiana is mainly from the west. The most common wind barrier is the single-row tree windbreak. As a general rule, a windbreak protects for a distance downwind of approximately 10 to 12 (or more) times the height of the windbreak, depending on soil conditions and supporting practices. Windbreaks may be combined with other conservation practices to provide increased effectiveness.
back to top |
|