Lesson Objectives

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

Vocabulary

• leaf litter

Introduction

Soil is an extremely important natural resource, one that is not always recognized or appreciated. Soil loss can lead to tragedy, as it did during the Dust Bowl of the 1930s, and can degrade farmland permanently. There are many ways to prevent soil erosion and, at least in the developed nations, those practices are increasingly followed.

The Dust Bowl

The 1930s were a terrible time in U.S. history. That was the time of the Great Depression and the Dust Bowl. The Dust Bowl was a portion of the prairie where farmers were forced from their lands as the soil was blown away in storms that came to be known as black blizzards.

The Dust Bowl was caused by bad farming practices in a region that was not naturally suited for farming (see Figure below). The problem began during World War I, when grain was needed to feed the troops and farmers were paid a good wage for growing it. The Great Plains is naturally grassland and soil erosion is minimal. Why do you think the soil did not erode very much while the region was grassland?

Initially, rains fell and the land was productive. But in 1931, the rains stopped. Without grasses to hold the soil in place, wind picked up the dirt and blew it across the region. Muddy rain even fell out of the atmosphere as far away as the eastern United States (Figure below). Farmers that were forced off their land migrated to southern California and other places along the west coast where they met with poverty of a different sort. John Steinbeck’s The Grapes of Wrath deals with this difficult time in American history.

We learned many lessons from the Dust Bowl storms. Modern practices encourage farmers to keep the soil covered so that it is not exposed and vulnerable to erosion. Massive irrigation projects, with water coming from groundwater sources, including the Ogallala Aquifer, keep crops healthy and soil in place. Still, some of the region’s topsoil erodes away each year.

Causes of Soil Erosion

The agents of soil erosion are the same as the agents of all types of erosion: water, wind, ice, or gravity. Running water is the leading cause of soil erosion, because water is abundant and has a lot of power. Wind is also a leading cause of soil erosion because wind can pick up soil and blow it far away.

Activities that remove vegetation, disturb the ground, or allow the ground to dry are activities that increase erosion. What are some human activities that increase the likelihood that soil will be eroded?

Farming

Agriculture is probably the most significant activity that accelerates soil erosion because of the amount of land that is farmed and how much farming practices disturb the ground (Figure below). Farmers remove native vegetation and then plow the land to plant new seeds. Because most crops grow only in spring and summer, the land lies fallow during the winter. Of course, winter is also the stormy season in many locations so wind and rain are available to wash soil away. Tractor tires make deep grooves, which are natural pathways for water. Fine soil is blown away by wind.

The soil that is most likely to erode is the nutrient-rich topsoil, which degrades the farmland.

Grazing

Grazing animals (Figure below) wander over large areas of pasture or natural grasslands eating grasses and shrubs. Grazers expose soil by removing the plant cover for an area and they also churn up the ground with their hooves. If too many animals graze the same land area, the animals’ hooves pull plants out by their roots. A land is overgrazed if too many animals are living there.

Logging and Mining

Logging removes trees that protect the ground from soil erosion. The tree roots hold the soil together and the tree canopy protects the soil from hard falling rain. Logging results in the loss of leaf litter, or dead leaves, bark, and branches on the forest floor. Leaf litter plays an important role in protecting forest soils from erosion (Figure below).

Much of the world’s original forests have been logged. Many of the tropical forests that remain are currently the site of logging because North America and Europe have already harvested many of their trees (Figure below). Soils eroded from logged forests clog rivers and lakes, fill estuaries, and bury coral reefs.

Surface mining disturbs the land (Figure below) and leaves the soil vulnerable to erosion.

Construction

Constructing buildings and roads churns up the ground and exposes soil to erosion. In some locations, native landscapes, such as forest and grassland, are cleared exposing the surface to erosion (in some locations the land that will be built on is farmland). Near construction sites, dirt, picked up by the wind, is often in the air. Completed construction can also contribute to erosion (Figure below).

Recreational Activities

Recreational activities may accelerate soil erosion. Off-road vehicles disturb the landscape and the area eventually develops bare spots where no plants can grow. In some delicate habitats, even hikers’ boots can disturb the ground so it’s important to stay on the trail (Figure below).

Soil erosion is as natural as any other type of erosion, but human activities have greatly accelerated soil erosion. In some locations soil erosion may occur about 10 times faster than its natural rate. Since Europeans have arrived in the United States, about one-third of the nation’s topsoil has eroded away.

Preventing Soil Erosion

Soil is a natural resource that is vitally important for sustaining natural habitats and for growing food. Although soil is a renewable resource, it is renewed slowly, taking hundreds or thousands of years for a good fertile soil to develop.

Most of the best land for farming is already being cultivated. With human populations continuing to grow, it is extremely important to protect our soil resources. The rate of topsoil loss in the United States and other developed countries has decreased recently as better farming practices have been adopted. Unfortunately, in developing nations, soil is often not protected.

Table below shows some steps that we can take to prevent erosion. Some are things that can be done by farmers or developers. Others are things that individual homeowners or community members can implement locally.

Lesson Summary

• Soil erosion is a natural process that has been greatly accelerated by human activities.
• Activities that accelerate erosion include agriculture, grazing, logging, mining, development, and recreation.
• Soil is an important natural resource and should be protected as any natural resource.
• Many practices can be adopted to slow down or prevent soil erosion.

Review Questions

1. 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?

2. Discuss five ways human activity has accelerated soil erosion.

3. How do urban areas contribute to soil erosion?

4. What is the connection between poverty and soil erosion in developing countries?

5. What is one way you can prevent soil erosion when you are hiking?

6. 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?

7. What can people in developed nations do to reduce the likelihood of bad environmental practices being used in developing countries, particularly activities that increase soil erosion?

Further Reading / Supplemental Links

People who lived during the Dust Bowl talk about their experiences, the Ganzel Group http://www.livinghistoryfarm.org/farminginthe30s/water_02.html

Video of the Dust Bowl http://www.weru.ksu.edu/vids/dust002.mpg

Points to Consider

• Why should soil be considered a renewable resource? Why should it be considered a non-renewable resource?
• Could humans live without soil?
• What can you do to help to conserve soil?