What is soil erosion? What are the effects of soil erosion and what can we do to conserve the soil? Importance of soil for life.
Soil is a mixture of sand or clay, or both, together with humus. Sand and clay come from rocks that have been broken up and slowly ground to bits. Humus is made of decayed plant and animal materials. Scientists estimate that from 500 to 1000 years are needed to form a layer of good soil 1 inch deep. Yet in many parts of our country, soil is being lost from the land faster than it can be made.
If a cubic yard of soil is taken from an acre of land each week, you can hardly notice the loss. But if this goes on for thirty years, about a foot of soil will be removed from the entire surface. In some places, we are losing soil this fast or even faster. Fortunately, however, soil erosion is occurring more slowly in most places. Even so, 200,000 acres of good soil are washed into rivers every year. Most of our food comes directly or indirectly from green plants which grow in the soil. So this loss of soil is a serious matter.
In most places, the average depth of the soil is from 3 to 6 feet. But plants grow well only in the upper soil, or topsoil. This is usually darker and looser than the soil a few feet below the surface. When our country was first settled, it had an average of 9 inches of fertile topsoil. Now, just a few hundred years later, the average is less than 7 inches. Some places have no topsoil at all. Beneath the topsoil is a deeper, lighter-colored soil, or subsoil. Ordinarily, plants do not grow well in the subsoil because it contains little or no humus.
Wind erosion is usually more destructive in dry regions. But there is erosion by water wherever it rims off the land. Water causes two different kinds of erosion. One kind is called sheet erosion because water wears away the topsoil in thin layers. The other kind is called gully erosion because water running rapidly off the soil digs gullies, or little valleys, in it. Gully erosion usually occurs in the subsoil after sheet erosion has removed practically all of the topsoil.
Scientists have found that there, is not just one reason for soil erosion. Several different things affect it such as: (1) the amount of rain that falls and when it falls, (2) the slope of the land, (3) the type of soil, (4) how the soil is managed, and (5) when and where the wind blows. You can easily understand how the amount of rainfall affects soil erosion. In different parts of our country, the rainfall varies from as much as 61 inches a year to as little as 7 inches. You would naturally expect more erosion where the most rain falls in a year. However, a large amount of rain well distributed throughout the year causes much less erosion than heavy rains for short periods.
The slope of the land also has much to do with soil erosion. You know that water runs downhill because of the pull of gravity. If the slope is steep, the water runs faster than if the slope is gentle.
The faster water moves, the more soil it can carry. Water gains speed as it runs downhill. The amount of water is also greater toward the bottom of the slope. So even on gently sloping land, loose topsoil may be carried down to lower levels and finally into streams.
Another thing that affects soil erosion is the type of soil. Though there are many different kinds of soil, these can be classified into three main types. Sandy soil is loose, coarse soil that is mostly sand with little clay and some humus. Clay soil is tightly packed, fine soil that is mostly clay with some humus and little sand. Loam is about half sand mixed with equal parts of clay and humus. Other types in between the main ones are sandy loam and clay loam.
Coarse soils, such as sandy soil and sandy loam, absorb water more easily than most other types. During light rains, these soils are not likely to suffer from so much erosion as finer soils. Water sinks in quickly instead of running off and carrying soil with it. But sand can hold only half as much water as clay and one eighth as much as humus. So if a coarse soil is nearly all sand, much erosion may occur during heavy rains. Clay soil and clay loam do not absorb water quickly. They are made of such fine particles that they pack together tightly and form a hard surface. Rain falling on these soils runs off easily and carries much soil with it before it can sink into the ground.
How the soil is managed also has an effect on erosion. Bare or thinly covered soil is much more easily eroded than soil that is well covered with plants. Corn, cotton, potatoes, tobacco, and truck crops are planted in rows. The soil between the rows is left bare with no roots to hold it. Even on gently sloping land, much erosion usually occurs when row crops are grown there. When the rows run up and down hillsides, a great deal of erosion takes place with every rain. The spaces between the rows form channels in which the water can run downhill faster.
Some farmers plow their land in the fall after the crops are harvested. Then the soil is exposed to erosion until new crops are planted and begin to grow. Plowing up grasslands and cutting down or burning off forests also increase erosion. Grass has many fine roots that form sod and hold the soil. The roots of trees hold the soil, too. When the leaves fall, they cover the soil and hold moisture. As they decay, they add humus to the soil.
When and where the wind blows is still another thing that affects soil erosion. When the soil is covered with plants, wind can do little damage to it. Neither can wind carry away soil when it is moist. But whenever bare soil becomes dry, it drifts in the wind almost like snow. Then a few windstorms can carry away all the fertile topsoil from a field or even an entire farm. Wind does most of its damage in regions where there is little rainfall.
The plants that we need and use must have soil to grow in. But every rainstorm and every dust storm carries away some of our valuable soil. To control this erosion by water and wind, we can use what has been learned about the conditions that increase erosion. Even though we cannot change most of these conditions, we can use the land in ways that conserve the soil.
Whenever water runs downhill, it carries some soil with it. The steeper the slope, the more soil is carried away. To reduce this loss of soil, a farmer must get the water to sink into the ground or run to lower levels without taking too much soil along. One way to do this is to plow the soil and plant the crops so as to fit the contour of the land, or the shape of its surface. This method of plowing and planting is called contour farming. Instead of running up and down hillsides, the furrows and rows run across the slopes of the hills. So they form many little dams that hold back the water when it rains. Then much of the water sinks into the cground instead of rushing downhill and washing away the soil.
Another method of controlling soil erosion is known as strip cropping. Instead of planting an entire field with just one crop, different crops are planted side by side in strips. As in contour farming, the strips run across the slopes and not up and down. Strips of row crops such as corn, potatoes, and tobacco have strips of sod crops such as grass, wheat, soybeans, or the like in between. Most of the soil that is washed down from the bare spaces between the row crops is stopped by the thick growth of sod crops in the next strip below.
The following year, the crops in the strips can be rotated. For example, corn can be planted where there was grass the year before, while soybeans are grown where there was corn. Nearly 60 tons of topsoil have been lost in one year from an acre of farm land that was planted in corn. But only 8 tons were lost in one year from an acre on which the crops were rotated. So strip cropping, together with crop rotation, helps conserve the soil.
Soil erosion can be reduced by still another method called terracing. Sloping fields are divided into strips by low ridges, called terraces, that run across the slopes. Above each ridge is a shallow ditch. The ditch slopes just a little toward one end, where it empties into a larger ditch. When a field has been terraced, water can run straight downhill only a short distance before it is stopped. Then the water runs into a ditch and flows slowly across the slope and off the field. So it does not gain enough speed to do much damage. As the water flows slowly along the ditch, it drops most of the soil that was washed off the land. Also, more of the water has a chance to sink into the soil.
Probably the simplest method of preventing erosion is to use plants to cover and protect the soil. There are various ways of doing this. Instead of plowing their fields in the fall and then leaving them bare, many farmers plant winter cover crops such as rye or wheat. These crops hold the soil during the season when there is much rain. Another way to protect the soil is to cut up the stalks, stubble, and other plant remains left after the harvest and scatter them over the fields. This plant material forms a loose covering, or mulch, that helps hold the soil. Planting grass in drainage ditches also reduces erosion. Grass holds back the water until it flows so slowly that very little erosion occurs. The roots also hold the soil in place. Trees, shrubs, and vines control erosion, too.
Farmers often leave steep slopes for pastures or wood lots. There is too much soil erosion on such slopes to grow other crops. But grass or trees protect the soil and provide a useful crop as well. In dry regions, trees are used to control wind erosion. Along the edge of a farm toward the direction from which the wind usually blows, trees are planted close together in rows to form what is called a shelter belt. This slows down the wind and keeps it from making a clean sweep over the fields. For added protection, shelter belts are often planted around all the edges of the farm.