Soil erosion definition: During heavy rainfall, look at the soil. You can see that when larger droplets fall on the soil, smaller holes are formed in the soil, and water becomes turbid. The mud-mix water flows downward. Thus, soil erosion occurs with mud-mix water during rainfall. Again, look at the stormy air or cyclone. You can observe that soil particles are swept away with wind from one location to other. These soil particles will hit your eyes and face if you are outside. Now probably you can say that transportation of surface soil due to various reasons is called soil erosion.
By the processes of soil erosion eroded, the soil is deposited on the lowest slope. The main causes of soil erosion are rainfall, cyclone, river flow, deforestation, cultivation, hilly slopes, etc. Due to continuous rainfall, soil loses its water-holding capacity, and then the excess water carries away some surface soil particles towards the lower slope.
By the current of water, surface soil becomes loose and moves to be deposited on the lower slope. Similarly, the wind brings away the soil from the cultivated land in the form of dust. River flow breaks the bank of the river and brings away the soil, and creates ‘char’ region. When men clear the forest to cultivate crops, the land becomes open. Again, livestock also grazes. Thus soil erosion occurs. Similarly, by clearing hilly forests, crops are cultivated on hilly slopes. By the current of rainwater, surface soil falls on the valley.
Types of soil erosion
There are two types of soil erosion:
- Natural soil erosion
- Soil erosion by men.
Natural soil erosion:
Nature exerts a strong influence on soil erosion. This type of erosion has started from the beginning of the earth. Sometimes a strip of sand land or char rises out of a river & bed due to natural soil erosion. Many areas of the earth become unfertile in consequence of this type of erosion. Naturally, soil erosion occurs continuously, but we cannot realize it.
Wind and rainfall are the principal reasons among the natural ones. These factors bring soil particles from one location to other towards the direction of their flow. Probably for that reason, this type of soil erosion is called usual soil erosion. Natural soil erosion is also considered as part of the soil formation process. There is a balance between soil formation and soil erosion. But agricultural activities fall into a hazardous condition due to continued soil erosion.
Classification of natural soil erosion
Soil erosion is mainly divided into two classes. These are:
- Soil erosion due to rainfall and
- Soil erosion due to wind
Soil erosion due to rainfall: Large scale soil erosion occurs in Bangladesh due
to rainfall. This soil erosion may be divided into the following classes;
- Sheet erosion
- Rill erosion
- Channel erosion
- Riverbank erosion
These soil erosions are discussed below.
Sheet erosion: When rain or irrigation water flows on the soil surface from the higher slope to the lower slope, the upper soft and fertile soil particles are cut and moves away in the form of a thin sheet. This is called sheet erosion. Soil erosion due to rainfall is not visible. But after several years, it is realized that soil fertility has decreased. The cause of decreasing this fertility is not but sheet erosion.
Rill erosion: Rill soil erosion is the second step of sheet erosion. Due to heavy rainfall, several long narrow lines are formed along the slope, like hand ribs. The narrow lines eventually become larger in length and width. By the flow of rainwater, fertile soil is removed from the land. Thus the land loses its fertility, and it becomes problematic for the farmers to use implements.
Channel erosion: This erosion is the third step of sheet erosion. Rill erosion is the source of channel erosion. The small channels produced in rill erosion are increased in length and width due to longer duration. Therefore, the crop soils go under more erosion. After a certain period, these channels look like drains or small rivers. The more the rainfall is, the more channel erosion occurs. This type of soil erosion is found in the hilly regions of Bangladesh.
Riverbank erosion: A remarkable cause of soil erosion in Bangladesh is riverbank erosion. Every year hundreds of hectares of land go under the river in many regions. At the beginning, or the last of the rainy season, there is a heavy current in the rivers, and consequently, the agricultural lands on the bank of the river go underwater.
Wind erosion: The soil transportation caused by a strong current of air from one place to other is called wind erosion. The wind erosion is higher where there is plain land, comparatively lower vegetation, and lower rainfall. Sandy and sandy loam soils are loose and light. Therefore, these soils easily fly away if there is speedy airflow. Again, the soil in which organic matter content is very low, air erosion is more.
The fertile regions of the desert become unfertile due to sand deposition on that land by airflow. In the northern regions of Bangladesh like Dinajpur-Rajshahi region, a small amount of air erosion is noticed in the month of 2 Chaitra-Baishakh. Thus, due to airflow, % fertility of arable lands gets reduced.
Soil erosion by men:
Men need food to survive. From the beginning of agriculture, men have been using soil injudiciously to produce food. Tillage, irrigation, drainage, etc., are the main components of agricultural activities. Soil is continuously disturbed by these activities. Thus, lands are exposed to natural forces like rainfall, wind, etc., and get eroded. The more the soil is used, the more soil erosion occurs.
Uncovered lands are being attacked by rainfall, wind, and flood. Soil becomes loose due to zoom or step cultivation in the hilly areas. Hill-breaking occurs due to heavy rainfall and thus causes land erosion. This also becomes dangerous for human life and their properties. Besides, during grazing of livestock and movement on the field path, the soil particles fly away in the form of dust.
Demerits of soil erosion
The demerits of soil erosion are as follows:
- The nutrient-enriched surface soil is moved away. Thus soil fertility is lowered considerably.
- Soil erosion makes soil a deficit in plant nutrients. Consequently, it hampers crop growth.
- Continuous soil erosion fills up the river-canal, haor-bill, etc. For that reason, flood inundates the country. Thus it damages crops, birds, households, etc.
- A vast amount of eroded soil is deposited in the river. Thus it decreases the depth of the river and creates problems in navigation.
- There is a proverb that erosion of fertile soil means the erosion of civilization.
Causes of soil erosion
Soil erosion is caused by several reasons. From the above description of soil erosion, the causes of erosion can also be realized. The causes of soil erosion are cited below:
- Land slope
- Nature of soil
- Nature of crop
- Methods of land cultivation
- Intensive cultivation
- Human activities
Rainfall: Though rainfall is good for cultivation, it causes soil erosion. The intensity, number, and amount of rainfall affect soil erosion. If there is heavy rainfall, the rain droplets become larger and hit the soil with high pressure, and it loses the soil particles. When the soil loses its water absorption capacity, the excess water creates a flow and moves from higher to lower. During water movement, the loose and soft soil is transported with water. The higher the speed of water flow, the higher will be the soil erosion.
Land slope: In the sloppier soil, water moves with high speed towards the lower slope. For that reason, soil erosion is higher in the hilly area compared to plain land. Consequently, the soil becomes loose in zoom cultivation areas, and this soil moves towards the lower hilly slope with rainwater if there is rainfall. Within few years, the zoom cultivation area becomes unfertile.
Nature of soil: Soil erosion depends on soil structure, texture, and the presence of organic matter. The sandy-loam soil can easily absorb the rainwater due to its higher porosity. For that reason, erosion of this soil is lower. But the porosity of clayey and heavy soil is lower, thereby lowering its water absorption capacity. For that reason, a little rainfall causes soil erosion moving the soil surface towards the lower slope.
Methods of cultivation and nature of crops: If cultivation is practiced along the slope, avoiding across the slope in the hilly areas, the soil is eroded due to rainfall. The steps are created on the steep hills, and crops are cultivated on that steps. But if it is not cultivated normally, landslide or soil erosion may also be caused. Frequent cultivation of land also may cause soil erosion.
The crops covering the soil can save the soil from erosion, such as groundnut, black gram, grass pea, etc. But sugarcane, maize, rice, wheat, etc., do not cover the soil in their initial stage. Consequently, soil erosion occurs.
Airflow: In the regions where there is low vegetation, soil erosion occurs by air. This type of soil erosion occurs in Rajshahi and Dinajpur regions.
Human activities: Human beings themselves are the actual cause of soil erosion. To manage food for hunger, men started clearing forests. Thus soil surface becomes open and causes soil erosion. Men also cause soil erosion by destroying agricultural lands for the construction of residences, roads, etc.
Effective ways and means of controlling soil erosion
One of the important technologies of agricultural operations is controlling soil erosion. This technology is the summation of some methods of controlling soil erosion. These methods are-
Lowering the speed of water flow: It is important to decrease the speed of water flow to control soil erosion. The strong current of water flow can be decreased in several ways, such as setting up the ridge of the earth around the cultivable field. Then soil of the field gets time to absorb water, and thereby soil erosion can be prevented. If the small canals created by rill erosion are filled up and leveled, the water flow will decrease and control soil erosion.
Water flow can be decreased by allowing weeds in the larger drain, inserting sticks, and placing a net with the sticks at the end of the drain. Moreover, by dumping bits of dry grass, straw, and leaves at the foot of the wire netting, the current of water can be slowed down, and thus the erosion of soil can be prevented.
Facilitating well-drainage: If the water becomes stagnant in the land, rainwater adding with the stagnant water increases water flow and thereby soil from the land becomes loose and moves away. Therefore, soil erosion can be controlled by dividing the land into several pieces and removing excess water separately from each piece.
Increasing organic matter content in the soil: Granulation of soil becomes good if an adequate amount of organic matter is applied in the soil. The rainwater without causing soil erosion can easily enter the soil. The land in which organic matter is less, the soil of this land can easily be eroded.
Crop cultivation on the steps of the hill: Because of step/terrace cultivation, crops are cultivated on the lines created across the slope of a hill. Since the crops are cultivated across the slope, the speed of the rainwater flow becomes slower. And this type of cultivation does not erode the soil of the hill.
Cultivation in Contours: Cultivation of crops directly across the slopes of hills is known as the contours method. This method slows down the seed of rainwater and saves soil erosion.