The relative importance of degree of cloddiness and stability of clods varies with conditions. If the field subject to erosion is small, the amount of abrasion is also small, and the amount of erosion is influenced primarily by the proportion of erodible fractions. If the field is large, the stability of clods is the more important factor. In such a case, if the clods lack mechanical stability, the presence of even a small amount of loose, erodible particles on the surface is usually enough to disintegrate the clods by abrasion from particles moving in saltation and ultimately to cause intense erosion over the whole field.

A ROUGH GROUND SURFACE is more resistant to wind erosion than a smooth one: A rough surface slows down wind velocity and tends to trap whatever particles might have been dislodged from its more exposed parts.

Vegetation and vegetative residues act as a protective cover as well as a rough surface does. Generally they have a greater capacity to trap the moving soil than does a ridged or a cloddy soil surface. Thus, in addition to providing roughness, cover in itself is likewise important in reducing erosion by wind. The two factors therefore are inseparable in many cases.

The first addition of vegetative cover reduces the amount of erosion most. Each addition is proportionately less effective. How much vegetative material is required to protect the soil from wind depends primarily on surface roughness, degree of cloddiness, size of field, and strength of the wind. Often one-fourth ton an acre (air-dry basis) of wheat stubble is sufficient on slightly erodible soil, 1 ton on moderately susceptible soil, and at least 4 tons on the most erodible dune material.

Tall stubble is more effective against wind than an equal weight of short stubble because it reduces the surface velocity of the wind more.

Stubble is more effective than straw because the crowns are heavier and less subject to removal by wind. Sorghum stubble is less effective than an equal weight and height of wheat stubble because it has less protective surface.

Grass makes one of the best covers because it has a relatively great protective surface above and below the ground and because it is well anchored.

THE BASIC FACTORS that influence clod structure, stability of structure, and erodibility by wind are varied.

Soil texture that is, the relative proportion of sand, silt, and clay is one factor. The coarsest and often the finest textured soils are more erodible than the medium-textured soils because they have a less well developed clod structure. The coarse-textured soils lack sufficient silt and clay to bind the erodible sand grains. The fine-textured soils have too much clay, which, especially under freezing and thawing, causes the soil clods to disintegrate into a finely granulated, erodible condition. The cloddiest and least erodible soils have a clay content of 20 to 30 percent and a silt content as high as possible. Silt ranging from 0.005 to 0.01 millimeter produces the greatest degree of cloddiness and resistance to erosion by wind.

The water-stable particles greater than about 1 millimeter in diameter, whether they are single grains or aggregates, are highly resistant to movement and abrasion by wind erosion.

Many dryland soils contain relatively small amounts of these particles but contain a considerable proportion of the fine, water-stable particles smaller than 0.02 millimeter in diameter. These fine particles are partly responsible for the formation of clods large enough to resist the force of wind.

Free calcium carbonate, or lime, in excess of 1 percent and not above 10 percent by weight, in soil reduces cloddiness and mechanical stability of clods and increases erodibility by wind. These influences are amplified in soils containing a high percentage of decomposed organic matter.

The organic matter has a variable effect on soil structure and erodibility, according to its degree of decomposition. Undecomposed vegetative matter is highly effective as a soil cover and binder. Decomposing vegetative matter increases soil aggregation and reduces erodibility. Decomposed organic matter, which usually gives the soil a characteristic black color, often facilitates erosion by wind.

In a sense, the decomposed organic matter, or humus, induces aggregation in that it causes the soil to aggregate into small granules. In dryland regions especially subject to the effects of freezing and thawing, the granules generally are too small to resist the force of wind. Some soils with much organic matter are coarsely granulated and not readily eroded by wind except under exposure to extreme influences of freezing and thawing.

THE EFFECTS of wind erosion are serious and extensive. Many farmers have lost entire crops, and the fertility of the soil has been greatly reduced. But those effects are less serious than the loss of fine soil fractions (silt, clay, and organic matter) gradually sorted from the sands and carried to distant places, where they may be of little or no use to anyone.

The wind acts on some soils like a sieve that removes the fine fractions and retains the coarse ones. The coarse mechanical fractions are infertile and are no more than a skeleton of the soil. The remaining sand often is much more erodible than was the original soil and usually is a serious threat to better surrounding lands. Extensive areas of aeolian (wind-deposited) soils in this country do not show this type of sorting action. All of the material to the depth of plowing is known to have been removed in some instances from these soils in a single season. Crop yields in some areas have been much lower for many years after serious erosion by wind occurred. Sometimes the land seems to have been injured permanently.

Wind erosion is bad in other ways. Railways and highways have been buried under drifted soil. Traffic accidents have been common during severe soil drifting. Insects and weed seeds are blown far and wide with drifted soil. Fences, hedges, and shelterbelts have been buried and ruined. Shrubs and trees may be smothered under the accumulated soil. Farmsteads may be blocked and farm buildings ruined by soil drifts. Duststorms are disagreeable and sometimes unbearable to the farm family. People in towns and cities also suffer.

THE DECISIVE FACTOR of duststorms is depletion of vegetative cover on the land.

One of the methods of wind erosion control obviously should be to protect the soil by vegetation and crop residues. But this should be only one of the methods.

All of the major factors that influence wind erosion can be modified by various practices.

The principles of the control are fourfold: Protect the soil surface with a cover (the best of which is vegetation); roughen the surface to slow down wind velocity and trap drifting soil; produce stable soil aggregates large enough to resist the force of wind; and place barriers, such as crop strips, ridges, or shelterbelts, in the path of the wind to trap the drifting soil and keep it from spreading.

The principles are applicable everywhere, but their importance varies with the conditions.