In his 2005 best-selling book Collapse: How Societies Choose to Fail or Succeed, Jared Diamond describes how soil depletion has contributed to the fall of ancient civilizations, such as the Maya in Central America. Today, damage to soils, either through depletion of nutrients or physical loss of soil, continues to be a major global environmental concern (Figure 7.12).

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Erosion is a natural land-forming process that moves soil from one place to another. However, human activities can accelerate it—to devastating effect. In undisturbed temperate forest, soils are formed at a rate of about 1 ton per hectare per year, which is 100 to 1,000 times greater than the natural erosion rate. However, when people cut forests or churn up soils during agriculture or road-building, rates of soil loss skyrocket. Over the past 150 years, erosion has removed more than 50% of the topsoil from the prairies of Iowa, which include some of the most productive farmland in North America. Overall, soil losses in North America and Europe average approximately 17 metric tons per hectare (ha) per year. Soil losses in the croplands of South America, Asia, and Africa are even greater, around 30 to 40 tons per hectare annually.

Losses of topsoil to erosion can be broken down into losses of mineral particles, organic matter, or nutrients. In one landmark 1992 study, Cornell University researchers reported that the 17 metric tons of topsoil lost per hectare in the United States contain an average of approximately 14.5 metric tons of mineral soil, 2 metric tons of organic matter, and nearly 0.5 metric ton of inorganic nutrients (Figure 7.13).

Those nutrients include potassium, phosphorus, and nitrogen, which are critical to agriculture and to maintaining soil fertility. Most replacement potassium and phosphorus comes from mining activities, which have a significant impact on the environment and require substantial amounts of fossil fuels. In contrast, replacement nitrogen is derived from the atmosphere in a process that also requires substantial use of fossil fuels (through industrial synthesis). The losses of soil organic matter can also reduce farm productivity because the crumblike structure of soil organic matter is key to promoting the infiltration of water, aeration, retention of nutrients, and resistance to erosion.

Soil erosion is a potential problem associated with farming and ranching, and each has unique challenges. Conventional-tillage agriculture involves tilling a field to break up soil clumps and smooth the soil surface before planting, as well as weeding using specialized machinery. Although conventional tillage can be very effective for seeding crops and controlling weeds, it exposes large areas of bare soil to wind and water, often resulting in erosion of topsoil (Figure 7.14).

Grazing by livestock causes erosion in the same way as conventional tillage, by reducing plant cover and disturbing topsoil. Livestock are generally heavy animals that consume a lot of plant material and compress soils with their hooves. Overgrazed rangelands may lose up to 100 tons of soil per hectare per year. Rangelands in hot, dry regions are particularly sensitive to erosion because they naturally support sparse plant cover, which can be easily overgrazed. Severe erosion can occur during torrential rains and flash floods (Figure 7.15).

The impacts of overgrazing in arid and semiarid rangelands commonly lead to desertification, a process of degradation of once fertile lands to a desertlike condition of reduced plant cover and primary production. Desertification is a major problem in central Asia, much of China, and of northern Africa, particularly in the Sahel region (Figure 7.16).

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