Soil erosion

In almost every part of the world modern agriculture has led to severe soil erosion in the wake of deforestation, ploughing up of grasslands and the cultivation of steep slopes. Soil erosion has led to dust storms, flooding, loss of fertility and the abandonment of cultivation. Soil degradation now affects about a third of the world’s land surface and a third of the world’s cropland is losing soil faster than it is created. In India nearly 800,000 square kilometres of land is affected by soil erosion, Haiti has no quality topsoil left, and in Turkey about three-quarters of the land is affected, half of it seriously. The United Nations Environment Programme estimates that since 1945 human activities have degraded 2 billion hectares of land, of which 430 million hectares have been irreversibly destroyed.

The experience of the United States provides a particularly clear illustration of the problem of soil erosion. To the early settlers the amount of land available seemed in exhaustible and they paid little attention to maintaining soil quality. Cultivation was extensive and it was easy to abandon ruined land and move to new areas. The worst problems were associated with the cultivation of tobacco and cotton – the former requires 11 times the nitrogen and 36 times the phosphorus of a food crop and therefore exhausts the soil very quickly. Tobacco farmers found that a second crop on new land was the best but that after a couple more seasons maize or wheat was all that could be grown. This could only be done for a few years before the soil was completely exhausted. Land was abandoned and the ruined soil was easily eroded away by the wind and rain. Settlers moved steadily westwards from the Tidewater to the Piedmont region, clearing the forests to find new land. Within less than a century from the first settlement in 1607 Virginia was suffering from severe flooding brought on by deforestation. In the 18^th century Georgia had the same experience with soil erosion gullies over 40 metres deep in places. Continued cotton growing had the same impact on the soil and this was one of the main pressures behind the continued westward movement of the frontier, cotton cultivation and slavery. By 1817 in North Carolina the amount of abandoned land equalled the area under cultivation.

Until the latter half of the 19^th century the Great Plains area was avoided by settlers because the ploughs they had available could not break up the tough, compacted grass.
The development of the heavy steel plough allowed farmers to move into an area that had previously supported large herds of bison and the native Americans who hunted them.
The Great Plains were ploughed up to grow wheat even though this was a marginal area for cereal cultivation – the climate was semi-arid with only about 50 centimetres of rain a year and the thin topsoil was only held together by the grass.
Despite the experience of previous centuries (and in defiance of all ecological principles) the US Bureau of Soils claimed in 1909 that: ‘the soil is the one indestructible, immutable, asset that the nation possesses. It is the one resource that cannot be exhausted; that cannot be used up.’

When they made that claim the early stages of one of the world’s greatest ecological disasters were being enacted on the Great Plains. The largest territory, Oklahoma, was opened for settlement in 1889. In the next 40 years about 10 million hectares of virgin land were ploughed up and cultivated with drought-resistant wheat. After 1919, when Russian wheat exports ceased, American production expanded. Another 1.25 million hectares were ploughed up and output rose to 2½ times the 1914 level. However, the history of American over-exploitation of the land was about to meet its nemesis.

In the early 1930s one of the periodic droughts that regularly affect the Great Plains occurred. The loose, fragile and dry soil, which had lost its protective grass cover, was blown away in the high winds. Huge dust storms were created across a region covering west Kansas, south-east Colorado, north-west Oklahoma, north Texas, north-east New Mexico and parts of Nebraska and the Dakotas. The first big storm in May 1934 picked up about 350 million tonnes of topsoil and deposited it over the eastern United States (an estimated 12 million tonnes fell on Chicago alone) and dust was even detected on ships 400 kilometres out into the Atlantic. In March 1935, over one million hectares of wheat were destroyed by dust storms and by 1938 over 4 million hectares of land had lost the top 12 centimetres of soil and more than 5 million hectares the top 6 centimetres. The social and economic effects were terrible (and immortalised in John Steinbeck’s novel The Grapes of Wrath) – respiratory diseases across the region rose by a quarter and the infant mortality rate by a third. By 1938, when 850 million tonnes of soil were still being lost every year over 3½ million people had abandoned farms in the area. Oklahoma lost almost a fifth of its population – in some counties almost half the population left. Better conservation practices improved the situation in the 1940s but the periodic droughts returned in 1952-57 when wind erosion affected twice the area damaged in the 1930s and again in the 1970s when another 4 million hectares suffered from soil erosion.

The results of a survey published in 1938, revealed an alarming situation. In total an area as big as South Carolina had been eroded away, an area the size of Oklahoma and Alabama had been seriously damaged and the amount of sand and gravel washed down rivers would have covered an area as big as Maryland.
The situation worsened in the second half of the 20^th century. By the 1970s a third of the topsoil of the United States had been lost and 50 million hectares of cropland had been either ruined or made very marginal for cultivation.
Another 430,000 square kilometres were suffering unacceptable rates of erosion.
Topsoil was still being eroded away at the rate of 5 billion tonnes a year – 6 times the rate a century earlier.
This widespread devastation was the result of extensive and continual monocropping on marginal land and the practice of regarding the soil as no more than a medium to prop up plants while large quantities of artificial fertilisers, herbicides and pesticides were poured on to the crops.
A similar disaster took place in the Soviet Union in the 1950s and 1960s as part of the ‘virgin lands’ programme, when 40 million hectares of land were ploughed up.
Yields peaked in 1956 and then fell steadily. Practices such as deep ploughing and leaving the soil bare during fallow periods led to severe soil erosion following the severe drought of 1963.
Within a decade almost half the newly ploughed-up land had been severely affected by erosion. From the mid-1960s, on average another million hectares of land were abandoned every 4 years.
Most of Australia consists of desert or highly marginal land and the remaining land is easily eroded if ploughed or over-grazed.
By the mid-1980s over half of all Australia’s agricultural land was affected by soil erosion.
In China the situation was bad in the 19^th century but worsened considerably in the 20^th. In just 20 years in the middle of the century over 8 million hectares of agricultural land was lost to soil erosion.
Over ⅓ of what was once arable land has now been abandoned. The dust caused by spring ploughing in China can now be detected thousands of miles away in Hawaii.
A major secondary problem caused by soil erosion is that rivers carry away large amounts of the soil and this causes river beds to rise (increasing the risk of flooding), it silts up any downstream dams and eventually extends deltas at river mouths.
Almost an eighth of the world’s population now lives in areas affected by flooding caused by deforestation and soil erosion in the Himalayan region.
Another factor in increasing the rates of deforestation and soil erosion has been over-grazing by pastoral groups who live in the marginal areas of Africa, the Near East and Central Asia.
The rising number of farmers forced pastoral groups away from the better land and into ever more marginal areas.
Aid programmes designed to turn the pastoralists into sedentary herders have worsened the situation by concentrating the destructive effects of grazing.
The most extreme form of soil erosion and loss is desertification, which now affects the south-western parts of the United States, northern Mexico, north Africa, the Sahel, large parts of southern Africa, parts of Australia and an increasing amount of land in China.
The best estimate is that the world’s deserts are expanding by about 70,000 square kilometres a year.
Throughout the 20^th century the Sahara moved steadily northwards at about 100,000 hectares a year and the speed of advance in the south, particularly in Sudan, was far greater.
About 7000 million people (just over 10% of the world’s population) live in the arid and the semi-arid areas of the world threatened by the encroachment of deserts.
In some areas the figure is far higher – in Kenya a third of the population is affected by desertification.

A Green History of the World Part 14

Soil erosion

Water

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