Airborne dust and its significance to soils

Movement of mineral dust by the atmosphere has been underway for ages, but efforts to assess its magnitude and significance have come largely in the last century. Attempts to identify and understand effects of the dust on soils are even more recent, mostly in the last 30 years. Airborne movements have several forms. Dust is transported by large storms that can be spectacular. Dust is also moved by smaller pulses and in a rather continuous flux. Most movement is within continents or from continents to nearby oceans, but fine dust (line silt and clay) is carried from one continent to another. Prime sources of the dust are the deserts and semi-arid regions of the world. For example, large quantities of dust are carried out of the Sahara every year. Quantities of dust carried by individual pulses, from the small ones to the massive storms, extend from a few thousand to 150 million metric tons. During the course of a year, the total quantities can be large. One example is an estimate that on the average 770 million metric tons originating in the western United States are carried 2300 km each year. That amount would add roughly 3000 kg ha’ yr’ to soils between the Rocky Mountains and the Mississippi River. Small pulses and the near-continuous flux add between 100 and 200 kg ha’ yr-’ to soils of the eastern United States, on the average. Comparable quantities are being added to soils in other parts of the world as well. Airborne accessions of dust have a variety of effects in soil development and on soils after they have been formed. Dust dominates processes of genesis in a few soils, affects horizon differentiation appreciably in many more, is a primary source of constituents essential to some horizons and layers in a number of soils, provides the bulk of the material for silty surface horizons in many soils, and can contribute supplies of nutrient elements for plants. Examples of all such effects are covered in this review. First among these effects to be recognized even though not understood were the silty surface layers that failed to match underlying horizons. Later recognized was airborne dust as a source of calcium carbonate for calcic horizons. That the dust could add significant quantities of nutrient elements as, for example, south of the Sahara, came into the picture even later. The beneficial effects of airborne dust to many soils raises an interesting question. Is the dust more, or less, useful in its new location than it was in its source area? An adequate answer to that question would provide an improved basis for assessing the effects of wind erosion. ’ Director (retired), Soil Classification and Correlation, U.S. Department of Agriculture. 0016-7061/95/%09.50