Soil Erosion The dust bowl of the 1930s was caused, in part, by conversion of prairie land to cropland in ways that left soil vulnerable to erosion. Soil erosion is the removal of soil by water or wind. Soil erosion is often worse when land is plowed and left barren between plantings. When no roots are left to hold soil in place, it is easily washed away. And when soil is badly eroded, organic matter and minerals that make it fertile are often carried away with the soil. In parts of the world with dry climates, a combination of farming, overgrazing, seasonal drought, and climate change can turn farmland into desert. This process is called desertification, and it is what happened to the Great Plains in the 1930s. Roughly 40 percent of Earth's land is considered at risk for desertification. Figure 6–6 shows vulnerable areas in North and South America.

Deforestation, or loss of forests, can also have a negative effect on soil quality. Healthy forests not only provide wood, but also hold soil in place, protect the quality of fresh water supplies, absorb carbon dioxide, and help moderate local climate. Unfortunately, more than half of the world's old-growth forests (forests that had never been cut) have already been lost to deforestation. In some temperate areas, such as the Eastern United States, forests can regrow after cutting. But it takes centuries for succession to produce mature, old-growth forests. In some places, such as in parts of the tropics, forests don't grow back at all after logging. This is why old-growth forests are usually considered nonrenewable resources.

Deforestation can lead to severe erosion, especially on mountainsides. Grazing or plowing after deforestation can permanently change local soils and microclimates in ways that prevent the regrowth of trees. Tropical rain forests, for example, look lush and rich, so you might assume they would grow back after logging. Unfortunately, topsoil in these forests is generally thin, and organic matter decomposes rapidly under high heat and humidity. When tropical rain forests are cleared for timber or for agriculture, their soil is typically useful for just a few years. After that the areas become wastelands, the harsh conditions there preventing regrowth.

In Your Notebook Describe the relationship between agriculture and soil quality.

FIGURE 6–6 Desertification Risk The U.S. Department of Agriculture assigns desertification risk categories based on soil type and climate. Interpret Visuals Find your approximate location on the map. What category of desertification risk is your area in?

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