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Chapter 5. Protecting Cropland: Losing Soil and Fertility
Soil erosion is not new. It is as old as the earth itself. But with the advent of agriculture, the acceleration of soil erosion on mismanaged land increased to the point where soil loss often exceeded new soil formation. Once this threshold is crossed, the inherent fertility of the land begins to fall.
As soil accumulated over millennia, it provided a medium in which plants could grow. Plants protected the soil from erosion. The biological fertility of the earth is due to the accumulation of topsoil over long stretches of geologic time—the product of a mutually beneficial relationship between plants and soil. But as the human enterprise expanded, soil erosion began to exceed new soil formation in more and more areas, slowly thinning the layer of topsoil that had built up over time. Each year the world’s farmers are challenged to feed another 70 million or more people but with less topsoil than the year before. 6
Erosion of soil by water and wind reduces the fertility of rangeland and cropland. For the rangelands that support the nearly 3.1 billion head of cattle, sheep, and goats in our custody, the threat comes from the overgrazing that destroys vegetation, leaving the land vulnerable to erosion. Rangelands, located mostly in semiarid regions of the world, are particularly vulnerable to wind erosion. 7
In farming, erosion comes from plowing land that is steeply sloping or too dry to support adequate soil protection with ground cover. Steeply sloping land that is not protected by terraces, by perennial crops, or some other way loses soil when it rains heavily. Thus the land hunger that drives farmers up mountainsides fuels erosion. Land that is excessively dry, usually receiving below 25 centimeters (10 inches) of rain a year, is highly vulnerable to wind erosion once vegetation, typically grass, is cleared for cropping or by overgrazing. Under cultivation, this soil often begins to blow away. 8
In the United States, wind erosion is common in the semiarid Great Plains, where the country’s wheat production is concentrated. In the U.S. Corn Belt, where most of the country’s corn and soybeans are grown, the principal erosion threat is from water. This is particularly true in the states with rolling land and plentiful rainfall, such as Iowa and Missouri. 9
Land degradation from both water and wind erosion in the world’s vulnerable drylands is extensive, affecting some 900 million hectares (see Table 5–1), an area substantially larger than the world’s grainlands (some 670 million hectares). Two thirds of this damaged land is in Africa and Asia, including the Middle East. These also are the world’s two most populous regions. And they are where fully two thirds of the 3 billion people expected to be added to world population by 2050 will live. If more people translate into more livestock, as historically has been the case, the damage will spread to still more land. 10
The enormous twentieth-century expansion in world food production pushed agriculture onto highly vulnerable land in many countries. The overplowing of the U.S. Great Plains during the late nineteenth and early twentieth century, for example, led to the 1930s Dust Bowl. This was a tragic era in U.S. history—one that forced hundreds of thousands of farm families to leave the Great Plains. Many migrated to California in search of a new life, a movement immortalized in John Steinbeck’s The Grapes of Wrath. 11
Three decades later, history repeated itself in the Soviet Union. The Virgin Lands Project, a huge effort to convert grassland into grainland between 1954 and 1960, led to the plowing of an area for wheat that exceeded the wheatland in Canada and Australia combined. Initially this resulted in an impressive expansion in Soviet grain production, but the success was short-lived as a dust bowl developed there too. 12
Kazakhstan, at the center of the Virgin Lands Project, saw its grainland area peak and begin to decline around 1980. After reaching a historical high of just over 25 million hectares, it shrank to barely half that size—13 million hectares. Even on the remaining land, however, the average wheat yield is only 1.1 tons per hectare, a far cry from the nearly 7 tons per hectare that farmers get in France, Western Europe’s leading wheat producer and exporter. This precipitous drop in Kazakhstan’s grain harvest illustrates the price that other countries will have to pay for overplowing and overgrazing. 13
In the closing decades of the twentieth century, yet another dust bowl—perhaps the biggest of all—began developing in China. As described in Chapter 8, it is the result of overgrazing, overplowing, overcutting of trees, and overpumping of aquifers, all of which make the land in northern and western China more vulnerable to erosion. 14
Africa, too, is suffering from heavy losses of topsoil as a result of wind erosion. Andrew Goudie, Professor of Geography at Oxford University, reports that dust storms originating over the Sahara—once so rare—are now commonplace. He estimates they have increased tenfold during the last half-century. Among the countries most affected by topsoil loss via dust storms are Niger, Chad, northern Nigeria, and Burkino Faso. In Mauritania, in Africa’s far west, the number of dust storms jumped from 2 a year in the early 1960s to 80 a year today. 15
The Bodélé Depression in Chad is the source of an estimated 1.3 billion tons of dust a year, up tenfold from 1947, when measurements began. Dust storms leaving Africa travel westward across the Atlantic, depositing so much dust in the Caribbean that they damage coral reefs there. When the dust is carried northward and deposited on Greenland, it reduces the reflectivity of the ice, leading to greater heat absorption and accelerated ice melting. The 2–3 billion tons of fine soil particles that leave Africa each year in dust storms are slowly draining the continent of its fertility and, hence, its biological productivity. 16
Dust storms and sand storms are a regular feature of life in the Middle East as well. The Sistan Basin on the border of Afghanistan and Iran is now a common source of dust storms in that region. Once a fertile complex of lakes and marshes fed by the Helmand River, which originates in the highlands of eastern Afghanistan, the area has become largely a desert as the river has been drained dry by the increasing water withdrawals by Afghan farmers for irrigation. 17
The bottom line is that the accelerating loss of topsoil from wind and water erosion is slowly but surely reducing the earth’s inherent biological productivity. Unless governments, farmers, and herders can mobilize to reverse this trend, feeding 70 million more people each year will become progressively more difficult.
|Table 5-1. Soil Degradation by Region in Susceptible Drylands, 1990s|
|Source: See endnote 10.|
6. United Nations, op. cit. note 5.
7. Livestock data from U.N. Food and Agriculture Organization (FAO), FAOSTATStatisticsDatabase, at apps.fao.org, updated 24 May 2004.
8. Yang Youlin, Victor Squires, and Lu Qi, eds., Global Alarm: Dust and Sandstorms from the World’s Drylands (Bangkok: Secretariat of the U.N. Convention to Combat Desertification, September 2002), p. 319.
9. U.S. Department of Agriculture (USDA), Summary Report: 1997 Natural Resources Inventory (Washington, DC: 1999, rev. 2000), pp. 46–51.
10. Table 5–1 from U.N. Environment Programme (UNEP), cited in Global Environment Fund–International Fund for Agricultural Development Partnership, Tackling Land Degradation and Desertification (Washington, DC: July 2002); grainland from USDA, Production, Supply, and Distribution, electronic database, at www.fas.usda.gov/psd, updated 13 August 2004; population from United Nations, op. cit. note 5.
11. John Steinbeck, The Grapes of Wrath (New York: Penguin Books, 1992).
12. FAO, The State of Food and Agriculture 1995 (Rome: 1995), p. 175.
13. USDA, op. cit. note 10.
14. U.S. Embassy, Grapes of Wrath in Inner Mongolia (Beijing: May 2001).
15. Brown, op. cit. note 1.
17. UNEP, Afghanistan: Post-Conflict Environmental Assessment (Geneva: 2003), pp. 50–60; NASA, “Dust Storm Over
Southern Asia,” Earth Observatory Newsroom, at earthobservatory.nasa.gov/Newsroom, 6 May 2004.
Copyright © 2004 Earth Policy Institute