It can take 300 years for natural processes to replace 1 inch of eroded soil; unfortunately, soil in agricultural areas typically erodes at a faster rate. Wind and water erode soil naturally, but human activities, such as farming, forestry and construction, accelerate the process. Soil erosion reaches beyond the land, as it also affects water and air.
Several components of erosion affect the soil's suitability for growing plants, ultimately reducing crop yield on agricultural lands. Erosion mainly impacts topsoil, which contains many of the nutrients and organic materials necessary for plant growth. Erosion may also compact topsoil and increase the amount of clay present; both these changes reduce the amount of water available to plants. A shallower topsoil depth can also limit root development. A combination of these factors leads to reduced crop yields on eroded soils: a study in Indiana found a 9 to 34 percent yield reduction in corn grown on severely eroded soil.
When water causes soil erosion, it often carries soil particles into surface waters, such as lakes and rivers. The nutrients necessary for plant growth can have detrimental effects on aquatic ecosystems; for example, excess phosphorous causes overgrowth of algae in ponds and lakes. The eroded soil carries along agricultural chemicals, such as fertilizers and pesticides, further deteriorating water quality. Water erosion can also result in sedimentation -- the deposition of large amounts of eroded soil in one place. Sedimentation causes downstream flooding, damages the landscape and even buries plants.
Soil particles account for as much as half of the dust in the air. This contributes to air pollution, since chemicals may be carried along with the soil. Soil dust in air has also been linked to human respiratory and skin diseases. Wind can deposit a blanket of soil particles over the land; deposited silt and clay particles sometimes improve soil quality, but sand deposits tend to be nutrient poor and highly acidic, decreasing soil quality.
The 1930s Dust Bowl in the North American prairies dramatically demonstrated the economic consequences of soil erosion. Drought coupled with a lack of conservation practices led to widespread dust storms and crop failures. Many Midwest families lost farms and businesses, due in part to the effects of severe soil erosion. Though the country no longer sees such harmful results, the United States still loses upwards of 40 billion dollars per year due to soil erosion. The economic effects of soil erosion are harsher in developing countries, where subsistence-level farmers cannot afford to implement conservation practices and the poverty level has a direct link with soil erosion.