High School Earth Science/Loss of Soils

Have you ever seen muddy rain or snow falling from the sky? Can you imagine what it might be like if the water that came down as rain and snow was muddy and brown? In May 1934, a huge wind storm picked up and blew away massive amounts of topsoil from the Central United States (Figure 19.1). The wind carried the soil eastward to Chicago. Some of the soil then fell down to the ground like a snowstorm made of mud. The rest of it continued blowing eastward, and reached all the way to New York and Washington, D.C. That winter, states like New York and Vermont actually had red snow because of all the dusty soil in the air.

A little less than one year later, in April 1935, another such storm happened (Figure 19.2). It was called a Black Blizzard. It made the day turn dark as night; people could not see right in front of them because of all the soil blown up by the wind storm. The storm caused tremendous damage and led to many people leaving the central United States to find other places to live. Many people became sick from breathing the soil in the air.

These storms are sometimes called the Dust Bowl storms. They continued on and off until about 1940. They are extreme examples of soil erosion, which is the process of moving soil from one place to another. Soil erosion is a serious problem because it takes away a valuable resource that we need to grow food. Several factors contributed to the Dust Bowl storms. First, farmers in the Central United States had plowed grasslands there to grow food crops. They left the crop fields bare in the winter months. This left the soil exposed to wind. Secondly, a long drought in the 1930s left the exposed soil especially dry. When the spring winds began blowing, the dry exposed soil was easily picked up and blown away.

We learned many lessons from the Dust Bowl storms. Today, we encourage farming practices that keep the soil covered even during the winter, so that it is not exposed and vulnerable to erosion. We have also learned of ways to prevent erosion in cities and towns as well as on farmlands. In this lesson, you will learn about some human activities that lead to erosion. You will then learn some of the specific ways we can prevent soil erosion.

Lesson Objectives

Explain how human actions accelerate soil erosion.
Describe ways that we can prevent soil erosion.

Causes of Soil Erosion

Soil erosion occurs when water, wind, ice, or gravity moves soil from one place to another. Running water is the leading cause of erosion, since it can easily take soil with it as the water flows downhill or moves across the land. Wind is the next leading cause of erosion. Just as in the Dust Bowl storms of the 1930s, wind can blow soil many hundreds of kilometers away. Soil is especially vulnerable to erosion if it is bare or exposed. Plants therefore serve a tremendous role in preventing soil erosion. If the soil is covered with plants, erosion is slowed down. But when soil is bare, the rate of erosion speeds up tremendously. What are some human activities that leave the soil exposed and speed up erosion? We speed up erosion through the following actions:

Agriculture
Grazing animals
Logging and mining
Construction
Recreational activities, like driving vehicles off-road or hiking

Figure 19.3: The bare areas of farmland are especially vulnerable to erosion.

Agriculture is probably the most significant human action that accelerates, or speeds up, erosion (Figure 19.3). We first plow the land to plant fields of crops. This takes away the natural vegetative cover of an area and replaces it with rows of crop plants mixed with bare areas. It also creates an area where there may not be anything growing in the winter, because in most areas, food crops only grow in the spring and summer. The bare areas of a field are very susceptible to erosion. Without anything growing on them, the soil is easily picked up and carried away. The fields also experience more erosion in the winter if no plants are growing on them and they are just left as bare soil. In addition, farmers sometimes make deep grooves in the land with their tractor tires. These grooves act like small channels that give running water a path. This speeds up erosion from water.

Some parts of the world use an agricultural practice called slash and burn. This involves cutting and burning forests to create fields and pastures. It is one of the worldwide leading causes of excessive soil erosion. It is most commonly practiced in developing countries in tropical areas of the world, as people create more land for agriculture.

Grazing animals are animals that live on large areas of grassland (Figure 19.4). They wander over the area and eat grasses and shrubs. They can remove large amounts of the plant cover for an area. If too many animals graze the same land area, once the tips of grasses and shrubs have been eaten, they will use their hooves to pull plants out by their roots.

Figure 19.4: Grazing animals can cause erosion if they are allowed to overgraze and remove too much or all of the vegetation in a pasture.

When an area is logged, large areas of trees are cut down and removed for human use (Figure 19.5). When the trees are taken away, the land is left exposed to erosion. Even more importantly, logging results in the loss of leaf litter, or dead leaves, bark, and branches on the forest floor. Leaf litter decreases because no trees are left to drop leaves or other plant parts to the ground. The leaf litter plays an important role in protecting forest soils from erosion.

Figure 19.5: Logging exposes large areas of land to erosion.

Mining is another activity that speeds up erosion (Figure 19.6). When we mine we are digging in the Earth for mineral resources, like copper or silver. The huge holes dug by mining operations leave large amounts of ground exposed. In addition, most of the rock removed when mining is not actually the precious mineral, but tailings, or unwanted rock that is left next to the mine after the valuable minerals are removed. These tailings are usually piled up next to a mine, and are easily eroded downhill.

Figure 19.6: This large coal mining pit in Germany, and other mines like it, are major causes of erosion.

Constructing human buildings and roads also causes much soil erosion. This development involves changing forest and grassland into cities, buildings, roads, neighborhoods, and other human-made features. Any time we remove natural vegetation, we make the soil more susceptible to erosion. In addition, features like roads, sidewalks, and parking lots do not let water run through them into the ground because they are hard and impermeable (Figure 19.7). Since the water cannot enter the ground, it then runs over the ground faster than usual. This can speed up water erosion.

Figure 19.7: Urban areas and parking lots result in less water entering the ground. Therefore, more water runs over the land and quickly forms channels that can speed up erosion.

Humans also cause erosion through recreational activities, like hiking and riding off-road vehicles. An even greater amount of erosion occurs when people drive off-road vehicles over an area. The area eventually develops bare spots where no plants can grow. Erosion becomes a serious problem in these areas.

Human-caused Erosion

Some erosion is a natural process and has always happened on Earth. However, human activities like those discussed above, have accelerated soil erosion, which may occur about 10 times faster than its natural rate. As the human population grows, we increase our impact on soil erosion. In order to support Earth's human population, we need to create more and more farmland, we develop more areas and build more cities, and we use much more of the land for recreation. Human population growth can lead to degradation of the natural environment.

Human impact on erosion differs throughout the world. In developed countries like the United States, we have learned good agricultural practices that greatly slow down agriculture's impact on erosion. However, we still experience much erosion from the development of urban areas and construction of new cities. In developing countries, many people are very poor and just want to be able to grow food and make a simple living. They carry out slash and burn agriculture because it quickly gives them land to grow food crops on. Poverty is a big contributing factor to environmental problems like soil erosion in developing countries.

Preventing Soil Erosion

Soil is a renewable, natural resource necessary for growing food. However, it renews itself slowly: it can take hundreds or thousands of years to replenish lost soil. When we lose valuable soil, we also lose an important natural resource. Many of the farmers affected by the Dust Bowl storms of the 1930s lost their homes because they could no longer grow crops and earn money to live, once their topsoil had all blown away. While agriculture can cause erosion, it is also necessary for human life. We have learned many good agricultural practices that reduce erosion, instead of speeding it up.

Table 19.1 shows some steps that we can take to prevent erosion. Which of these things can you do in your own personal life? Can you think of any other steps we can take to slow down erosion? Notice that many of the things listed here involve ways that we use the land. Land use always requires humans to make choices.

Table 19.1: Erosion Prevention Strategies
Source of Erosion	Strategies for Prevention
Agriculture	Leave leaf litter on the ground in the winter Grow cover crops, special crops grown in the winter to cover the soil Plant tall trees around fields to buffer the effects of wind Drive tractors as little as possible Use drip irrigation that puts small amounts of water in the ground frequently Avoid watering crops with sprinklers that make big water drops on the ground Keep fields as flat as possible to avoid soil eroding down hill
Grazing Animals	Move animals throughout the year, so they don't consume all the vegetation in one spot Keep animals away from stream banks, where hills are especially prone to erosion
Logging and Mining	Reduce the amount of land that we log and mine Reduce the number of roads that are built to access logging areas Avoid logging and mining on steep lands Cut only small areas at one time and quickly replant logged areas with new seedlings
Development	Reduce the amount of land that we turn into cities, urban areas, parking lots, etc. Keep as much "green space" in cities as possible, such as strips of trees where plants can grow Invest in and use new technologies for parking lots that make them permeable to water in order to reduce runoff of water
Recreational Activities	Avoid using off-road vehicles on hilly lands Stay on designated trails
Building Construction	Avoid building on steep hills Grade surrounding land to distribute water rather than collecting it in one place Where water collects, drain to creeks and rivers Landscape with plants that minimize erosion

Lesson Summary

Soil erosion is a natural process, but human activities have greatly accelerated soil erosion.
We accelerate erosion through agriculture, grazing, logging and mining, development, and recreation.
Soil is an important natural resource necessary for plant growth and should be kept safe from erosion as much as possible.
There are many ways that we can slow down or prevent erosion, but practicing these involves making decisions about how we use land resources. It also requires striking a balance between economic needs and the needs of the environment.

Review Questions

Many farmers harvest their crops in the fall and then let the leftover plant material stay on the ground over winter. How does this help prevent erosion?
List five ways human activity has accelerated soil erosion.
How do urban areas contribute to soil erosion?
What is the connection between poverty and soil erosion in developing countries?
What is one way you can prevent soil erosion when you are hiking?
You often see stone barriers or cage-like materials set up along coastal shores and river banks. How do you think these serve to prevent erosion? Why are areas like this prone to erosion?
How can your own activities affect the environment, especially soil erosion?
What can we do to help solve environmental problems in developing countries? What responsibility do you have to help solve this problem?

Vocabulary

cover crop: A special crop grown by a farmer in the wintertime to reduce soil erosion. Cover crops often also add nitrogen to the soil.
development: The construction of new buildings, roads, and other human-made features in a previously natural place.
impermeable: Not allowing water to flow through it.
leaf litter: Dead leaves, branches, bark, and other plant parts that accumulate on the floor of a forest.
pasture: Land that is used for grazing animals.
topsoil: The very important top few inches of soil, where much of the nutrients are found necessary for plant growth; Part of the A horizon

Points to Consider

Is soil a renewable resource or a nonrenewable resource? Explain the ways it could be either.
Could humans live without soil?
What could you do to help to conserve soil?

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