Lester R. Brown
Chapter 3. Eroding Soils and Shrinking Cropland: Soil Erosion: Wind and Water
The thin layer of topsoil that covers much of the earth's land surface is the foundation of civilization. Today perhaps a third or more of that foundation, the world's cropland, is losing topsoil through erosion faster than new soil is forming, thereby reducing the land's inherent productivity. Where losses are heavy, productive land turns into wasteland or desert.5
Some early civilizations, such as that of the Mayans in the lowlands of what is now Guatemala, which flourished from the sixth century B.C. to the ninth century A.D., may well have declined because soil erosion undermined the food supply.6
Over long periods of geological time, new soil formation exceeded soil erosion, forming a fertile layer of topsoil over much of the earth. But in recent decades, soil erosion has accelerated, often outpacing the creation of new soil. This loss of soil can be seen in the dust bowls that form as wind erosion soars out of control. Among those that stand out are the U.S. Dust Bowl in the Great Plains during the 1930s, the Soviet dust bowl in the Virgin Lands in the 1960s, the huge dust bowl that is forming in northwest China, and the dust storms that come out of Africa, crossing the Atlantic with the prevailing winds. Each of these is associated with a well-developed pattern of agricultural expansion onto marginal land followed by retrenchment as the soil begins to disappear.7
The erosion of soil can be seen in the silting of reservoirs and from the air in muddy, silt-laden rivers flowing into the sea. Pakistan's two large reservoirs, Mangla and Tarbela, which store Indus River water for that country's vast irrigation network, are losing roughly 1 percent of their storage capacity each year as they slowly fill with silt from their deforested watersheds. And Pakistan is not alone. In varying degrees, reservoirs are plagued with siltation spurred by deforestation and farming, yielding a lose-lose situation where the loss of soil also reduces the supply of irrigation water.8
As soils erode, land productivity falls. An analysis of several studies on the effect of soil erosion on crop yields in the United States concluded that for each 1 inch of topsoil lost, wheat and corn yields declined by 6 percent. A 1982 USDA Natural Resource Inventory, which measured the loss of topsoil from U.S. cropland at 3.1 billion tons per year, found that excess erosion was concentrated on a small share of the land. It set the stage for the landmark 1985 Conservation Reserve Program.9
Ethiopia, a mountainous country with highly erodible soils on steeply sloping land, is losing an estimated 1 billion tons of topsoil a year. This is one reason why Ethiopia always seems to be on the verge of famine, never able to accumulate enough grain reserves to provide a meaningful measure of food security.10
India is thought to be losing 4.7 billion tons of topsoil a year, mostly through water erosion. Its monsoonal climate, with the concentration of rainfall during a few months of the year, leaves its exposed soils vulnerable to erosion.11
In neighboring Nepal, a government report estimated annual soil nutrient loss from erosion at 1.3 million tons—on top of the 500,000 tons of soil nutrients removed through harvesting of crops. Of this total loss of 1.8 million tons, only 300,000 tons are being replaced through the use of organic and mineral fertilizers.12
In China, plowing excesses became common in several provinces as agriculture was pushed northward into the pastoral zone between 1987 and 1996. In Inner Mongolia (Nei Monggol), for example, the cultivated area increased by 1.1 million hectares, or 22 percent, during this period. Other provinces that expanded their cultivated area by 3 percent or more during this nine-year span include Heilongjiang, Hunan, Tibet (Xizang), Qinghai, and Xinjiang. Severe wind erosion of soil on this newly plowed land made it clear that the only sustainable use of much of it was grazing. As a result, Chinese agriculture is now engaged in a strategic withdrawal in these provinces, pulling back to only the land that will sustain crop production.13
Since the health of people is closely related to the health of the soil on which they depend, most hunger is found in the mountains and the hills and on semiarid farmlands with marginal rainfall. Little hunger is found on well-watered agricultural plains. In the absence of conservation practices, marginal soils tend to marginalize the people who depend on them. Richard Bilsborrow, an economist/demographer at the University of North Carolina, notes: "Three quarters of the poorest 20 percent in Latin America live on marginal lands. Fifty-seven percent of Asia's poor and 51 percent of Africa's also inhabit marginal lands. Not just the lack of land, but also its quality, contributes to world poverty."14
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