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ABSTRACT
A study of structure and erodibility of soil by wind was carried out on wind eroded and residual soil materials in a condition usually existing in the field and after sampling, cultivating, and dry sieving.
Field structure and erodibility of soils varied greatly with as little as 1 inch of simulated rainfall. In soils undisturbed by cultivation after rain, four distinct phases of structure were found, all of which possess different degrees of erodibility by wind. These phases are the primary aggregates (water-stable aggregates), the secondary aggregates (granules and clods), the surface crust, and the consolidated soil material between the secondary aggregates.
The abrasive action of wind erosion was shown to be one of the most serious aspects of erodibility by wind.
The mechanical stability, that is, the resistance of a soil to breakdown by mechanical forces such as cultivation or sieving, was found to vary directly with the resistance of the soil to abrasion by wind-blown sand. Mechanical stability was greatest for drift particles (sand grains and water-stable aggregates mostly), less for the secondary aggregates, followed in order by the surface crust, the consolidated materials between the secondary aggregates, and lastly the consolidated materials, if any, which held drifted particles together after they were wetted and dried.
The amount of dispersed fine silt was found to be a primary factor influencing the formation of the surface crust and the consolidation of the soil body after it was wetted and dried. A fraction of clay size, being much less dispersible than silt, was found to be the primary factor of secondary aggregate formation. Water-dispersible silt and clay were responsible in large measure for the resistance of the soil to erosion by wind. Although the amount of erosion was limited to some degree by the presence of water-stable aggregates too large to be moved by wind, it was found that cultivated dryland soils lack these aggregates in sufficient amounts. The resistance of these soils to wind action was found to depend primarily on their ability to form secondary aggregates, or clods.
The secondary aggregates preserved their identity below the surface even after repeated wetting and drying in the field. It was concluded, therefore, that the physical condition of the soil is indicated as well or better by methods such as dry sieving, which primarily measure the state of the secondary aggregates, than by methods that measure the state of the primary aggregates.
1 Contribution No. 472, Department of Agronomy, Kansas Agricultural Experiment Station, Manhattan, Kansas, and the Soil Conservation Service, U. S. Department of Agriculture. Cooperative investigations in the mechanics of wind erosion. Presented before Division I, Soil Science Society of America, Cincinnati, Ohio, November 20, 1952.
2 Professor of soils, Kansas Agricultural Experiment Station, and Agent, Soil Conservation Service, U.S.D.A.
Received for publication December 8, 1952.
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