Soil erosion is the deterioration of soil by the physical movement of soil particles from a given site. Wind, water, glacial ice, animals and humans and their tools may be agents of erosion.
It is vegetation that keeps soil (in its natural state) from eroding. Undisturbed soil is usually covered by a canopy of shrubs and trees, by dead and decaying leaves, or by a mat of grass, herbs, mosses or lichen. Whatever the plant cover, it protects the soil when the rain falls or the wind blows. Root systems hold the soil together. The roots of native grasses extend several metres into the ground and tie down the soil even in a drought.
The two most important agents of soil erosion are wind and water; but in most instances these are important only after man, animals, insects, diseases or fire have removed or depleted the natural vegetation. Wind erosion is most commonly a problem in (seasonally) dry, windy regions, with a smooth, flat terrain, whereas water erosion is more common in (seasonally) wet regions with a sloping or hilly or mountainous terrain. Both result in a loss of topsoil, rich in humus, and lead to a decline in long-term productivity. With the destruction in soil structure, eroded land is even more susceptible to erosion.
With its covering of vegetation stripped away, soil is vulnerable to damage. Whether the plant cover is disturbed by cultivation, grazing, deforestation, burning, or bulldozing, once the soil is laid bare to the erosive action of wind and water, the slow rate of natural erosion is greatly accelerated. Losses of soil take place much faster than new soil can be created, and a kind of deficit spending of topsoil begins.
Accelerated erosion describes the speeding up of erosive processes. Accelerated erosion is directly or indirectly caused by human activity. It may be associated with very rapid environmental change, such as clearfelling, or chronic and long-term land misuse, such as overgrazing. It is the most serious form of soil degradation because the rate of soil loss is so rapid that surface soil may be blown or washed away right down to bedrock. Urbanization also causes accelerated erosion because concrete and tarmac are impermeable to water and the surface runoff is magnified in adjacent areas, such as roadside verges and steep gullies.
Climate change is becoming a significant causal factor in soil erosion because there is an increased incidence of extreme events (severe storms, high and low rainfall, high winds, etc.
Nearly every semi-arid area with cultivation or long-continued grazing, every hill land with moderate to dense settlement in humid, temperate and subtropical climates, and all cultivated or grazed hill lands in the Mediterranean climate areas, suffer to some degree from such erosion. Thus recognized problems of erosion are found in such culturally diverse areas as southern China, the Indian plateau, South Australia, South Africa, the former Soviet Union, Spain, the south-eastern and mid-western USA, and Central America.
Around the world, erosion is increasingly pushing marginal farmland out of productive use. The former Soviet Union had 120 million hectares of harvested grain in 1977; by 1994, that amount had shrunk to 92 million hectare. Every year, even as it adds 360,000 people, the Indonesian island of Java loses 50 hectares of productive land, enough to feed annual population increases. Around the world, 6 million hectares of what was once useful land becomes desert every year.
In the USA, soil erodes at an average rate of 9 to 12 tonnes per acre per year. Between 1972 and 1976 the USA grain area expanded by some 24%. Soil erosion increased far more. By 1976, USA farmers were losing an estimated six tons of soil for every ton of grain they produced. Almost half of America's cropland is losing soil faster than it can be replaced. A 1982 survey measured the annual loss at 3.1 billion tons, a loss approaching that during the Dust Bowl years.
In developing countries erosion rates per acre are twice as high as the USA, partly because population pressure forces land to be more intensively farmed. Worldwide, farmers are losing an estimated 24 billion tonnes of topsoil each year, roughly the amount of topsoil covering all of Australia's wheatlands. New topsoil is constantly being formed, of course, but at nature's leisurely pace of about 1.5 tonnes per acre per year (3.4 tonnes per hectare). The difference between creation and loss represents an annual loss of 7.5 to 10 tonnes per acre (16.8 to 23.5 tonnes per hectare) worldwide. Erosion has now reached unprecedented proportions and, should it continue at the current rate, one-third of the world's arable land will be depleted within the next 20 years. Perhaps the grimmest soil erosion report in recent years came in 1978 when a dispatch from the USA Embassy in Addis Ababa indicated that an estimated 1 billion tonnes of topsoil were washing down from Ethiopia's highlands each year, presaging the recurrent famines that have followed. In parts of northern Ethiopia, as for cropland degradation throughout the Third World, there is simply not enough topsoil left to sustain even subsistence-level farming.
According to a 1991 report, nearly two-thirds of Haiti's 6 million people depended on agriculture for a living. Over the years they have burnt trees and bushes to make room for crops – and to produce charcoal, which they sell. Farmers have exhausted the land by planting too many crops too often. The lack of trees was wearing away the topsoil, which means that even fewer things would grow in the years ahead. With two percent of its forests left, more than one-third of the country was so affected by topsoil loss as to be unreclaimable for farming. The amount of arable land was declining by as much as 3% a year. Soil washed into the streets of Port-au-Prince had to be cleared with bulldozers in the rainy season.