Soil Degradation and Conservation Guiding Question: How do erosion, desertification, and soil pollution affect the productivity of soil?
• Describe some practices that can lead to soil erosion and some that can prevent it. • Identify the causes and effects of desertification. • Discuss the activities of U.S. and international agricultural organizations. • Explain how irrigation and pesticide use can cause soil pollution.
Reading Strategy Before you read, preview Figure 7. As you read the photo captions, take notes on how each method helps to conserve soil. Vocabulary soil degradation, intercropping, crop rotation, cover crop, shelterbelt, tilling, terracing, contour farming, overgrazing, desertification, irrigation, salinization, pesticide.
Scientists’ studies and the experiences of farmers have shown
12.2 LESSON PLAN PREVIEW
Real World Students discuss how global desertification can have local impacts. Differentiated Instruction Advanced learners research to learn more about international soil conservation programs. Inquiry Students model salinization of soil.
12.2 RESOURCES Modeling Lab, Combating Erosion • Lesson 12.2 Worksheets • Lesson 12.2 Assessment • Chapter 12 Overview Presentation GUIDING QUESTION FOCUS Make a two-column table on the board. In the left column, ask volunteers to list ways that humans can damage soils. In the right column, have volunteers list ways that soil damage can affect humans. At the conclusion of the lesson, have students revise the lists to reflect what they have learned.
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that the most productive soil, or the type of soil most plants grow best in, is loam with a neutral pH that is workable, contains nutrients, and holds water. Human activities can cause erosion, desertification, and pollution and make soil less productive. The deterioration of the soil characteristics needed for plant growth or other ecosystem services is called soil degradation. Soil degradation results in major ecosystem changes. It also makes farming more challenging, so it could make it more difficult to feed Earth’s growing human population in the future.
Erosion Certain farming, ranching, and forestry practices can erode soil, but other practices can protect it. Erosion and deposition are natural processes that, in the long run, can help create soil. Flowing water can deposit sediment eroded from other areas into river valleys and deltas, producing rich and productive soils. This is why floodplains are excellent for farming and why preventing floods can decrease long-term farming productivity. But erosion can be a problem because it usually occurs much more quickly than soil is formed. Furthermore, erosion tends to remove topsoil, the most fertile soil layer. And erosion can be gradual and hard to detect. In many parts of the world now, scientists and farmers are carefully measuring soil depth in hopes of identifying areas in danger of serious erosion before they become too badly damaged. Today, human activities cause more erosion than natural events. More than 19 billion hectares (47 billion acres) of the world’s croplands now suffer from erosion and other forms of soil degradation resulting from human activities. People make fertile soils vulnerable to erosion in several ways, including leaving soil bare after harvests, overgrazing rangelands, and clearing forests on steep slopes or with large clear-cuts.
Farming Practices Plant communities, including crops, protect soil from erosion. Plants slow wind and water, and their roots hold soil in place and absorb water. After fields are harvested, and there is no plant cover protecting the soil, wind and water can erode soil, especially if the land is sloped. Erosion rates in the United States are now declining, thanks to soil conservation measures such as those in Figure 7.
Figure 7 Soil Conservation Techniques Intercropping
Crop Rotation Intercropping is the planting of different crops in mixed arrangements. Intercropping helps slow erosion by providing more plant cover than a single crop does. Intercropping also reduces a field’s vulnerability to insects and disease that specialize in certain crops because different crops are planted in the same field. When a nitrogenfixing crop is planted, intercropping can also replenish the soil’s fertility.
In crop rotation, farmers alternate crops grown in a field. Crop rotation can return nutrients to the soil, break disease and pest cycles, and prevent the erosion that can come from letting fields lie fallow, or unplanted. In a practice similar to crop rotation, farmers plant crops to reduce erosion after a field has been harvested and before the next season’s planting. These cover crops help prevent erosion and often limit nitrogen loss, because they are often nitrogen-fixing crops such as alfalfa.
Shelterbelts A widespread technique for reducing wind erosion is to establish shelterbelts, or windbreaks. These are rows of trees or other tall, perennial plants that are planted along the edges of fields to slow the wind. Fast-growing trees such as poplars are often used in shelterbelts. Conservation Tillage Tilling is the turning-over of soil before planting. It creates more pores for air and water but makes soil more susceptible to erosion. To help conserve soil, farmers use no-till or reduced tillage methods. To plant using the no-till method, a tractor pulls a drill that cuts furrows through weeds and crop remains and into the topsoil. Seeds and fertilizer are dropped into the furrows, and they are closed. Reducedtillage agriculture disturbs the soil surface slightly more than no-till does. By maintaining organic matter in soil, these techniques can improve soil quality and reduce erosion. They, however, often require substantial use of weed-killers (because weeds remain in fields) and fertilizers (because weeds use soil nutrients). Terracing
Terracing minimizes erosion on steep hillsides. Terracing transforms steep slopes into a series of steps like a staircase, enabling farmers to cultivate slopes without losing huge amounts of soil to water erosion. Terracing is labor-intensive but is probably the only sustainable way to farm mountains.
Contour Farming Water running down a hillside erodes soil. Contour farming reduces erosion on gently sloping hillsides. Contour farming consists of plowing sideways across a hillside, perpendicular to the hill’s slope. The plowed furrows follow the contours of the land, and the downhill side of each furrow acts as a dam that catches soil before it is carried away.
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Ranching Practices The raising and grazing of livestock, animals
raised to be used on a farm or sold at a profit, also affects soils and ecosystems. When sheep, goats, cattle, or other livestock graze on open rangelands, they feed primarily on grasses. As long as livestock populations stay within a range’s carrying capacity and do not eat grasses faster than the grasses can grow back, grazing may be sustainable. However, when too many animals eat too much of the plant cover, impeding regrowth, the result is overgrazing.
Reading Checkpoint Compacted soil, damaged soil structure, and removal of native grass Figure 8 Overgrazing In the flowchart below, you can see how overgrazing can set in motion a positive feedback cycle that degrades soils and ecosystems. The effects of overgrazing can be dramatic, as shown in this photo along a fence line separating a grassy, ungrazed plot (left) from a shrubby, overgrazed plot (right).
Rangeland scientists have shown that overgrazing causes or aggravates several soil problems. Some of these problems give rise to positive feedback cycles that increase damage to soils, natural communities, and the land’s productivity (Figure 8). When livestock remove too much plant cover, more soil is exposed and made vulnerable to erosion. Soil erosion makes it difficult for vegetation to regrow, allowing yet more erosion. Moreover, non-native weedy plants may invade exposed soils. Livestock often avoid tough non-native plants, leaving the plants to reproduce and outcompete the native plants the livestock eat, further decreasing useful grazing land.
▶ Effects of Overgrazing
ccording to the flowchart in Figure 8, which problems does A overgrazing directly cause?
Range managers in the United States do their best to assess the carrying capacity of rangelands. They inform ranchers, or livestock owners, of the limits so that they rotate their herds from site to site. Range managers also can establish and enforce grazing limits on publicly owned land. U.S. ranchers have traditionally had little incentive to limit grazing, since most of their grazing has taken place on public lands leased from the government, not on their own lands. The U.S. government has also heavily subsidized grazing. These two situations have led to extensive overgrazing and resulting environmental problems on many public lands in the American West. Today, however, increasing numbers of ranchers are working cooperatively with government agencies, environmental scientists, and even environmental advocates to find ways to graze their animals more sustainably and safeguard the health of grasslands.
▶ Prevention of Overgrazing
Decreases pores for water
Compacts soil and damages structure
Removes native grass
Decreases pores for air
Exposes bare topsoil
Decreases grass growth and survival
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Allows invasive species to outcompete native species because of altered environment Leads to wind and water erosion
Forestry Practices Forestry can also have substantial impacts on soils. Forestry practices have been altered over the years to try to minimize damage to soils, as you may recall. Nevertheless, some current methods, such as clear-cutting, can lead to severe erosion, particularly on steep slopes. Other logging methods, such as selective systems, tend to lead to less erosion.
Desertification Desertification reduces productivity of arid lands. Soil degradation is especially severe in arid environments, where desertification is a concern. Desertification is a loss of more than 10 percent of productivity due to erosion, soil compaction, forest removal, overgrazing, drought, salt buildup, climate change, depletion of water sources, and other factors. Severe desertification can enlarge existing deserts and create new ones in once-fertile regions. This process has occurred in areas of the Middle East that have been inhabited, farmed, and grazed for thousands of years—including the Fertile Crescent region, where agriculture began more than 10,000 years ago. The Fertile Crescent is not so fertile anymore. Arid and semiarid lands are prone to desertification because they get too little precipitation to meet growing human demands for water. Declines in soil quality in these areas have endangered the food supply and the well-being of more than 1 billion people. In the affected lands, most degradation has been caused by wind and water erosion, as you can see in Figure 9.
Global Desertification By some estimates, desertification affects
one third of Earth’s land area, costing people tens of billions of dollars in crop income each year. China alone loses $6.5 billion annually from desertification. In the western parts of China, desert areas are expanding and combining because of overgrazing from more than 400 million goats, sheep, and cattle. In Kenya, overgrazing and deforestation fueled by rapid population growth has left 80 percent of the land vulnerable to desertification. In an ever-intensifying cycle, soil degradation forces ranchers to crowd their animals onto less-productive land and farmers to keep planting in poor soils, both of which worsen desertification. A 2007 United Nations report estimated that desertification, worsened by climate change, could displace 50 million people in 10 years. The report suggested that industrialized nations fund reforestation projects in dryland areas of the developing world. This would slow desertification while gaining these nations carbon credits in emissions trading programs. It would be worth their cost and effort: Desertification knows no national boundaries. In recent years, gigantic dust storms from desertified land in China have blown across the Pacific Ocean to North America, and dust storms from Africa’s Sahara have blown across the Atlantic Ocean to the Caribbean Sea. Reading Checkpoint
List five possible causes of desertification.
Reading Checkpoint Students may list any five of the following or other reasonable answers: erosion, soil compaction, forest removal, overgrazing, drought, salt buildup, climate change, depletion of water sources Figure 9 Soil Degradation Soil degradation on drylands is due primarily to erosion by wind and water. (Percentages add up to more than 100 percent because of rounding.)
Causes of Soil Degradation Soil structure problems (3.4%)
Chemical problems (9.7%)
Wind erosion (41.8%) Water erosion (45.2%)
Data from U.N. Environment Programme. 2002. Tackling land degradation and desertification. Washington and Rome: Global Environment Facility and International Fund for Agricultural Development.
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Figure 10 Dust Bowl In the 1930s, drought combined with poor agricultural practices brought devastation to millions of U.S. farmers in the southern Great Plains. The photo shows towering clouds of dust approaching houses near Stratford, Texas, in a 1935 dust storm. The map shows the Dust Bowl region, with darker colors indicating the areas most affected. BIG QUESTION How can we balance our growing demand for food with our need to protect the environment? Application Have students discuss in small groups why people should keep in mind the lessons of the Dust Bowl as they seek new ways to keep up with the growing demand for food.
The Dust Bowl Massive dust storms have also occurred in the
United States. During the Dust Bowl of the 1930s, desertification shook American agriculture and society as a whole to their very roots. Prior to large-scale farming on North America’s Great Plains, the native prairie grasses there held the soil in place. In the late 1800s and early 1900s, many settlers arrived in Oklahoma, Texas, Kansas, New Mexico, and Colorado hoping to make a living as farmers. Between 1879 and 1929, the farmed area of the Great Plains increased by 700 percent. Farmers grew abundant wheat, and ranchers grazed thousands of cattle, sometimes on unsuitable land. Both types of agriculture contributed to erosion by removing the grasses that had conserved the soil and by breaking down the soil structure. Then in the early 1930s, a drought occurred, aggravating that erosion. The region’s strong winds began to erode millions of tons of topsoil, which would blow around in huge dust clouds (Figure 10). Dust storms traveled up to 2000 kilometers (1200 miles) across the continent, blackening rain and snow as far away as New York. Some areas lost as much as 10 centimeters (4 inches) of topsoil in a few years. The most-affected region, the southern Great Plains, became known as the Dust Bowl, a term now also used for the event itself. The Dust Bowl forced thousands of farmers off their land.
Soil Conservation Policies U.S. and international agricultural organizations promote soil conservation. In response to the devastation of the Dust Bowl, the U.S. government, along with state and local governments, increased support for soil conservation research. In the United States, the Natural Resources Conservation Service works through county conservation districts to promote soil conservation and conservation of other natural resources. Various United Nations programs have similar responsibilities elsewhere.
U.S. Policies In 1935, the U.S. Congress passed the Soil Conservation
Act, establishing the Soil Conservation Service (SCS). The SCS began to work with farmers to develop conservation plans for their farms. 362 Lesson 2
In 1994, the SCS was renamed the Natural Resources Conservation Service, and its responsibilities were expanded to include water quality protection and pollution control. Recently, the U.S. Congress has enacted provisions promoting soil conservation through the farm bills it passes every 5 to 6 years. Many of these provisions require farmers to adopt soil conservation plans before they can receive government subsidies. The Conservation Reserve Program (CRP), established in the 1985 farm bill, pays farmers to stop cultivating cropland that erodes easily and to instead place it in conservation reserves planted with grasses and trees. Land under the CRP now covers an area nearly the size of Iowa, and the U.S. Department of Agriculture (USDA) estimates that each dollar invested in this program saves nearly 1 ton of topsoil. Besides reducing erosion, the CRP generates income for farmers, improves water quality, and provides habitat for wildlife. Congress reauthorized and expanded the CRP in the farm bills of 1996, 2002, and 2008.
International Programs Internationally, the United Nations pro-
motes soil conservation and sustainable agriculture through its Food and Agriculture Organization (FAO). The FAO’s Farmer-Centered Agricultural Resource Management Program (FARM) supports creative approaches to resource management challenges in many developing nations. Rather than relying on government control of farming practices, FARM calls upon local leaders to educate and encourage local farmers.
Soil Pollution Irrigation and pesticide use can improve soil productivity in the short term, but they can pollute soil in the long term. Erosion is not the only threat to the health of soils. You might think that watering crops and protecting them from pests would improve soil health, but that is not always the case.
Irrigation The providing of water other than precipitation to crops is
known as irrigation. Some crops, such as rice and cotton, require large amounts of water, whereas others, such as beans and wheat, require relatively little. By irrigating crops, people have managed to turn previously dry and unproductive regions into fertile farmland.
Figure 11 Salinization In this cross-section of salinized soil, the white crust is salt.
Irrigation is not without consequences. Too much, or carelessly timed, irrigation can result in waterlogged crops. A more frequent problem is salinization, the buildup of salts in upper soil horizons. In dry areas where precipitation is minimal and evaporation rates are high, water evaporating from the soil’s A horizon may pull water up from lower horizons by capillary action. As this water rises through the soil, it carries dissolved salts. When the water evaporates at the surface, those salts remain, as you can see in Figure 11. Irrigation water also usually contains some dissolved salts, so it adds salt to the soil, increasing the salinization.
▶ Salinization Causes
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Salinization currently decreases harvests on 20 percent of all irrigated cropland worldwide, costing farmers $11 billion in crop income each year, so it is a very expensive problem. And it is easier to prevent than to correct. The best way to prevent salinization is to avoid planting crops that require a great deal of water in dry areas. A second way is to irrigate with water that is low in salt content. A third way is to irrigate efficiently, supplying no more water than the crop requires and supplying it as close to the roots as possible. Drip irrigation systems that target water directly at plant roots are one option. Less water evaporates, which means less salt accumulates in the topsoil. Drip irrigation also conserves water and dramatically decreases erosion. It might seem that the remedy is to stop irrigating and wait for rain to flush salts from the soil. But remember where irrigation is needed. Salinization generally becomes a problem only in dry areas where precipitation is too little to even water crops, never mind to flush salt from the soil. A better option is to plant salt-tolerant plants, such as barley, that can be used as food or pasture. Another option is to bring in large quantities of less-salty water with which to flush the soil. However, too much water can waterlog crops and wash out soil nutrients.
▶ Salinization Solutions
Lesson 2 Assessment 1. Sample answers: Eroding practice: leaving soil bare after harvests; conserving practice: using cover crops to prevent erosion and limit nitrogen loss 2. Sample answer: Overgrazing and planting in poor soil further degrade soil, which can lead to desertification. 3. Answers will vary but should include the effects of the CRP as described in the text. 4. Irrigation causes salinization, which makes soil too salty for many plants; pesticides can linger in soil, causing harm to humans and other organisms. 5. Answers will vary but should be supported by content in the text.
Pesticides Chemicals that kill organisms that attack or compete with
plants we value are called pesticides. Pesticides may kill plant, animal, fungal, bacterial, or viral pests. (Chemicals that kill other plants are sometimes called herbicides.) Some pesticides are toxic to humans. Toxic pesticides and the chemicals they break down into may remain in soil for long periods of time, basically poisoning it. And they can filter through the soil into the groundwater and evaporate into the air. Although pesticides may increase the the amount of a crop produced in a given area in the short term, they can be hazardous to humans and other animals in the long term. And broad-spectrum pesticides kill a wide variety of insects, some of which may be helpful to soil. So pesticides can be hazardous to soil health as well. You will read more about pesticides in the next lesson.
2 1. Review Describe one farming practice that can erode soil and one farming practice that can conserve soil. 2. Relate Cause and Effect Explain how overgrazing and planting in poor soil can cause a cycle of desertification. 3. Communicate In your own words, write one paragraph about the effects of the Conservation Reserve Program. 4. Explain How can irrigation and pesticides cause soil pollution?
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You are a land manager with the U.S. Bureau of Land Management and have just been put in charge of 200,000 hectares (500,000 acres) of public lands that have been degraded by decades of overgrazing and poor management. Soil is eroding. Invasive weeds are replacing native grasses. Environmentalists want to end grazing on the land. Ranchers want grazing to continue, but they are concerned about the land’s condition. How would you assess the land’s condition and begin restoring its soil and vegetation? Would you allow grazing, and if so, would you set limits on it?