Several powerful forces are constantly at work on earth. Because of the force of gravity, every object on the earth tends to move toward the center of the earth. Gravity continuously pulls down every particle including soil and bedrock on the surface of the earth.
After a prolonged season of heavy rain, the soil and loose rock grow slippery. Gravity can then pull these down. Soil that is suddenly pulled down constitutes a landslide. Soil that is pulled down very slowly is referred to a soil creep. Rocks and soil are moved from higher to lower levels.
The entire process of moving rocks and soil particles from one place to another is called erosion. The movers of soil and stones are called agents of erosion. The most common agents of erosion are running water and wind.
Running water does the most of the work of erosion. After a heavy rainstorm, water rolls off the land and carries away soil, especially in areas where the land has insufficient plant cover to protect the soil. Water that runs across the land surface is called surface runoff. Some soil consists of water and tightly packed earth, and waster easily flows off this surface. However in the water can soak into the soil as fast as it falls on the surface, there can be no surface runoff.
A river is an example of running water that carries away materials by rolling, sliding, and bouncing the heavier particles. Fine, light materials are carried by suspension. The suspended particles give the water a cloudy or muddy appearance. Mush rock material is transported by being dissolved in the water as it flows to the sea.
When you dust furniture at home, you are wiping off soil that has been dropped there by the air. Wind can blow clouds of dust from a dry, freshly lowed field. Soil may be carried across thousands of kilometers during a dust storm. This is one cause of erosion in desert areas. Sand particles may collect piles called dunes in dessert areas.
The faster the wind blows the more soil it can carry. As the wind slows down, the larger particles of soil are dropped first. When it slows down some more, the finer particles are dropped. Large amounts of fertile topsoil are carried away by the wind.
How can we prevent erosion?
A Rice terrace is one type of contour farming. Crops are planted on sloping land so that each row runs around the slope at the same level. This method of farming helps prevent soil erosion on the mountain sides.
On the grasslands, contour farming is often practiced to conserve rainwater. Ditches are built on the contours of grassy slopes to hold the water that will otherwise run off. More water will thus soak onto the soil where it will be available to cultivate plants in dry weather.
Strip cropping is used to prevent the washing down of soil after slopes have been smoothed and planted. The farmer plants a row crop such as corn in one strip, and another strip he plants another kind of crop.
Other methods used to keep the soil fertile and productive. Plants use minerals from the soil. Then when the plants are removed, the minerals they took from the soil go with them. To replace these minerals, it is necessary to add fertilizers to the soil.
Some plants remove more of one kind of mineral. To prevent the continuous loss of the same mineral from the soil, farmers do not plant the same crops to maintain soil fertility. This practice is called crop rotation, meaning one type of crop is planted for a year and another type the next year.
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