Over much of the landscape, soils have been highly disturbed. Soils have been buried under fill; overturned and moved about by excavations, surface mining, and road construction; and exposed to erosion by wind and water. Upper horizons have been mixed by agricultural plowing and tillage, and compacted by heavy machinery and trampling.

Soil protected by vegetation maintains its integrity. Vegetation breaks the force of the wind and disperses raindrops, breaking their force. Rain trickles slowly through the litter, infiltrating the soil. If rainfall exceeds the soil's capacity to absorb it, the excess runs across the surface, but vegetation slows its movement.

Stripped of its protective vegetation and litter by plowing, logging, grazing, road building, and urban and suburban construction, soil is highly vulnerable to soil erosion--the carrying away of particles by wind and water faster than new soil can form. (Soil forms at the rate of about l metric ton per hectare per year.) Loss of the upper layers of humus-charged, granular absorptive topsoil exposes the humus-deficient, less stable, less absorptive, and erodible layers beneath. If the subsoil is clay, it absorbs water so slowly that heavy rains produce a highly erosive and rapid runoff.

Soil compaction intensifies the problem. Heavy machinery from large agricultural tractors to construction equipment compacts soil over large areas of ground. Trampling on lawns and playing fields, concentrated use of pathways and hiking and riding trails through woods and fields, and off-road use of all-terrain vehicles compact the soil on other sites. Soil compaction occurs when any weight pushes the soil particles together and reduces the size of the pores. Greatest compaction occurs under wet and moist conditions. Moist soil particles easily slide over one another. Compacted soil cannot absorb water, so the water flows across the surface.

Rain falling on bare ground hammers the soil's surface, removing lightweight organic matter, breaking down soil aggregates, and forming a seal on the surface. Unable to infiltrate the soil, the water moves across the surface as runoff, carrying soil particles with it. The least conspicuous type of soil erosion is sheet erosion. It is a more or less even removal of soil over a field. Soil compaction can increase sheet erosion. When runoff concentrates in small channels or rills instead of moving evenly over a sloping land, its cutting force increases. Rill erosion channels water rapidly downslope. On areas where concentrated water cuts the same rill long enough or where runoff concentrates in sufficient volume to cut deeply into the soil, highly destructive gullies result. Gully erosion often begins in wheel ruts made by off-road vehicles in fields and forests, on logging roads and skid trails, and on livestock and hiking trails.

Bare soil, finely divided, loose, and dry, as it often is after tillage, is ripe for wind erosion. Wind picks up fine particles of dust and carries them as dust clouds. Drastic erosion occurred in the Great Plains during the droughty Dust Bowl days of the 1930s. Wind erosion is increasing worldwide today, especially in arid and semiarid regions. Often dust particles are lifted high in the atmosphere and carried for hundreds and even thousands of kilometers. Wind erosion exposes roots of plants or covers vegetation with drifting debris. In the Great Plains wind erosion exceeds water erosion.

Erosion by wind and water ruins land. Worldwide, about 12 million hectares of arable land are destroyed and abandoned annually because of soil mismanagement. Such land is usually so degraded that natural vegetation has difficulty returning. Erosion becomes progressively worse unless extreme measures are taken to restore vegetation.

Effects of erosion are felt both on and off site. Erosion on agricultural and forest lands reduces organic matter and increases clay content. It reduces water-holding capacity of the soil, intensifying drought conditions in dry weather and flooding in wet weather. Erosion degrades soil structure and reduces plant nutrients and plant rooting depths, depressing crop yields. It also reduces the diversity and abundance of soil organisms, essential to soil productivity and water infiltration. The loss of 2.5 cm of topsoil reduces corn and wheat yields by 6 percent. Costly, energy-demanding chemical fertilizers mask the ruinous effects of soil erosion on the inherent fertility of the soil. In the United states, the economic cost of soil erosion amounts to close to $27 billion annually, and the environmental costs are $17 billion.

The costs of off-site effects may be more than twice as high. Soil eroded by wind and water has to go somewhere. Erosion carries sediments into rivers, reducing light penetration and clogging navigation. Sediment fills reservoirs and hydroelectric dams, shortening their lives and polluting the water. Wind-born soil contributes significantly to air pollution, illness, and damage to machinery.

All forms of soil erosion destroy the integrity of ecosystems and ecological cycles. They raise the cost of food, fostering hunger and famine. For humans, the ecological consequences could well be social disorder and degradation of life. There is a true saying, "Poor soils make poor people."

From R.L. Smith & T.M. Smith, Elements of Ecology. Copyright © 2000 by Allyn & Bacon (http://www.abacon.com). Reprinted by permission. Use of this material without written permission from the publisher is prohibited.