Soil Erosion : Introduction, Definition, Causes, Effects, Degradation, Preventions

Soil Erosion

Soil Erosion is the process of removal and transportation of the top layer of soil from its original position to another place, under the effect of strong winds and fast running rainwater.Soil Erosion normally occurs in bare areas i.e. areas without plant cover.

Soil is the portion of the Earth’s surface that consists disintegrated rocks and decaying material. It provides support for many plants and animals. Thickness of soil on the Earth’s surface ranges from a few millimeters to 3-4 meters. Terrestrial and aquatic plants depend upon the soil and water bed, respectively for their nutrients, water supply and anchorage.

Soil Erosion
Soil

1. Formation Of Soil

Soil is formed from the rocks undergoing the following two processes :

(a) Weathering

(b) Paedogenesis

WEATHERING

Breakdown of bigger rocks into smaller mineral particles is called weathering. Weathering occurs by following three means :

(i) Physical Weathering – Various climatic factors such as temperature, wind, rain water, ice, snow, glaciers and running water contribute to physical weathering. Water and high temperature cause corrosive humidity and bring about unequal expansion and contraction of rocks, facilitating their breakdown. Water causes weathering of rocks by three methods :

(a) Wetting And Drying

(b) Frost Action

(c) Abrasion

The freezing water expands in the rock crevices and breaks the rocks. Wind action also causes the weathering of rocks. Strong winds erode the rock surfaces by rubbing and striking its abrasive particles against the rock surface. River water grinds rock chips and stones into sand and into a finer form – the silt. Soluble components of rocks such as calcium, chloride, sulphides, etc are removed by water in solution; they percolate downward. The roots of the plants also have a role in weathering process. They penetrate into the crevices of the rocks and assist in the rock-breaking process.

(ii) Chemical Weathering – It involves a variety of chemical processes such as hydrolysis, hydration, oxidation and reduction. The breakdown of complex compounds by the carbonic acid present in water and acidic substances derived from the decomposition of organic matter in soil, are some examples of chemical weathering. The primary end products of the chemical weathering are silica, clay, inorganic salts and hydrated oxides.

(iii) Biological Weathering – Biological weathering is done by living organisms such as lichens and bryophytes (mosses). Lichens growing on rock surface extract minerals from the rocks. This creates small crevices at places where a thin layer of soil builds up. Mosses grow over these crevices and result in build up of more soil inside them. Deeper crevices from cracks. These cracks became wider and deeper when the roots of short lived herbs pass into them. With the passage of time, the roots of bigger plants (like peepal, banyan tree) pass into the cracks. Cracks gradually widen and cause slow fragmentation and eventually pulverisation of rocks.

PAEDOGENESIS (SOIL DEVELOPMENT)

This process concludes the decomposition by bacteria and fungi during which organic materials are broken down, leading to humification and mineralization. Detritivores such as nematodes, earthworms, arthropods such as scolopendra, mllipede, mites and ants consume organic matter and add excretory nitrogen to it. Thus, addition of organic matter (humus) from dead and decomposed plants and animals is the final stage in soil formation. A mature soil thus, has minerals, stored energy in the form of organic matter (such as starch, sugars, cellulose, lipids, proteins), oxides of nitrogen (such nitrogen dioxide, nirogen trioxide), NH+ ions, water and air.

2. Soil Profile

The term soil profile represents the vertical section of Earth crust, which is made up of a succession horizontal layers (horizons), each of which varies in thickness, colour, texture, structure, consistency, porosity, acidity and composition.

The upper or A-Horizon is the top soil. It contains most of the litter and humus. It also has a zone of leaching through which dissolved materials seep downward. The roots of small plants are embedded in top soil. The second or B-Horizon is composed of the mineral soil (sub soil). The third or C-Horizon contains the unconsolidated parent material. The last or D-Horizon comprises of the rock or unmodified parent material.

3. Composition Of Soil

Top soil mainly consists of following four types of the rock particles which differ in their size, look and texture :

(i) Gravels- These are large particles (small stones) which can be picked up by hand. Particle size of gravels is greater than 2 mm in diameter.

(ii) Sand Particles- They are coarse to touch and have particle size ranging from 0.05 mm to 2.00 mm. They can be seen easily by naked eyes.

(iii) Silt Particles- These soil particles have particle size between 0.005 to 0.05 mm.

(iv) Clay Particles- These are the smallest soil particles withe size less than 0.005 mm.

4. Types Of Soils

Based upon the relative amounts of particles and types of particles soils are classified into three main types :

(i) Sandy Soil– This soil contains a large amount of sand particles and very small portion of silt and clay. It is found in desert areas. It cannot hold much water.

(ii) Clayey Soil– This soil mainly contains clay particles and a small quantity of humus and silt. Clayey soil is compact and can hold water, but cannot trap air.

(iii) Loamy Soil– This soil contains clay, silt, sand and humus. Loamy soil has good water holding capacity and is porous to allow aeration of roots.

From agricultural point of view, soils of India are divided into following types :

A. RESIDUAL SOIL

This soil exists at the place of its formation. Residual soil may be black, red and laterite.

(a) Black Soil – It is derived from a basaltic rock. It is rich in iron, calcium, aluminium and magnesium. Black soil is porous and contains humus. Humus is a dark, finely divided, amorphous organic matter which consists of decomposed plant and animal material and is rich in C, N, P and S content. Black soil is best suited to grow cotton and sugar cane and is mainly found in Maharashtra, Andhra Pradesh, Gujarat and Madhya Pradesh. It is also found in Southern Western Uttar Pradesh, Eastern Rajasthan and Haryana.

(b) Red Soil – This soil is red due to the presence of iron oxide in it. It contains quartz and clay particles and forms the top soil. Red soil is poor in lime, magnesium, phosphorous, nitrogen and contains very little quantity of humus. In India, red soil is mainly found in Kerala, Tamil Nadu, Southern Karnataka, Andhra Pradesh, Orissa and Madhya Pradesh.

(c) Laterite Soil – This soil is also red in colour and is found in regions that receive heavy rainfall. It contains hydrated oxides of aluminium and iron. This soil is rich in nutrients and is good for growing tea, coconut and coffee. In India, laterite soil is found in Western Ghats, parts of Tamil Nadu, Andhra Pradesh, Orissa and Assam.

B. TRANSPORTED SOIL

This type of soil gets displaced and settled at the places away from its origin due to gravity, flowing water, wind or glacier. For example in deserts when sand is deposited in sand dunes it is also called Aeolian Soil. Transported soil is of following three types :

(i) Alluvial Soil – It is formed by the deposition of silt brought down from the mountains by the flowing river water. Alluvial soil is layered and consists of smooth round particles. This soil is rich in humus and contains gravel, sand and clay. This soil is suitable for growing wheat, rice and sugar cane. Alluvial soil is mainly found in the plains of Uttar Pradesh, Haryana, Bihar and Bengal. Alluvial Soil is also known as Khadar.

(ii) Desert Soil – This soil is coarse, sandy and porous. It is greyish brown in colour and rich in minerals. Desert Soil produces rich crops when it is found mainly in Rajasthan, Gujarat and Ladakh.

(c) Mountainous Soil – This soil varies in its contents from place to place. It consists of sand, stones, clay, shales and limestones. It also contains a good amount of humus. Mountainous soil is mainly found in the Himalayan Region.

Shales are soft rocks formed from compressed mud or clay that can be split into thin layers.

Soil consists of a sufficient amount of air, water and many living organisms. Soil air occurs within pore spaces of the soil and contain three gases – Oxygen, Carbon Dioxide and Nitrogen. It is rich in moisture. Soil water which is available to plants is the capillary water which exists as a thin film around soil particles. This water in fact occurs in smaller soil channels and is held by capillary forces (Eg surface tension and attraction forces of water molecules). Water which is retained as a thin and tightly-bound film around the individual soil particle is called hygroscopic water. It is not used by the plants. Rest of the water in the soil is called gravitational water as it is free to drain downwards, through the soil under the influence of gravity. This water causes leaching i.e. the washing away of minerals (nutrients). The level to which gravitational water drains is called water table.

Soil becomes the habitat for many living organisms such as bacteria (including nitrogen-fixing bacteria and blue green algae), fungi, protozoans, nematodes, land snails, millipedes, centipedes, ants, termites, amphibians, reptiles and mammals (rats, mice and rabbits).

Minerals – Earth has  limited non-renewable quantities of minerals distributed unevenly in the different parts of the globe. Coal, petroleum, copper, aluminium, zinc, lead, iron, silver, gold are the important natural resources to the humankind. Fossil fuels (coal and petroleum) are being widely used by the human beings in industry, transport, house-hold and agriculture. The ever increasing demand of the minerals may lead to their depletion within few decades Therefore, all the non-renewable minerals (metals and non-metals) should be used judiciously.

What Is Soil Erosion ?

The removal and transportation of the top layer of soil from its original position to another place, under the effect of strong winds and fast running rainwater is called soil erosion. The top layer of the soil is very fertile. It provides anchorage (firm support) to plants and is also a source of nutrients and water to the plants.

Soil Erosion normally occurs in bare areas i.e. areas without plant cover. It is so because of the bare topsoil is loose and thus can be easily carried away by strong winds or fast moving water of heavy rains or rivers.

What Are The Causes Of Soil Erosion ?

The soil that we see today in one place has been created over a large period of time. However, some of the factors that created the soil in the first place and brought the soil to that place may be responsible for the removal of the soil too.

1. Strong Winds – The soil which is uncovered and loose, is eroded when it is exposed to strong winds. The winds carry away the fine soil particles to other places.

2. Heavy Rains – When rain falls on the unprotected top soil, rain water washes it down into the streams and rivers etc.

3. Improper Farming And Suspended Cultivation -Farmers loosen the top soil of the agricultural fields either for the cultivation or for removing the weeds. Such soil can get eroded due to the winds or rains. Sometimes due to certain reasons, ploughed agricultural fields remain fallow (not cultivated) for a long time and is affected by soil erosion.

4. Human Actions – Human activities such as expansion of urban areas has led to the removal of vegetation from certain regions. The bare land is thus exposed to agencies (winds, rains) of soil erosion. Thus, large scale deforestation and overgrazing by our domestic animals, not only destroys biodiversity but also leads to soil erosion.

5. Dust Storms – Dust Storms shift huge amounts of loose soil from one place to another.

6. Frequent Floods – Frequent flooding of rivers is another cause of soil erosion. Fast moving water in the rivers removes the top soil of the fields near the river banks and carries it away.

What Are The Effects Of Soil Erosion ?

Soil Erosion results in the following human crisis:

1. Loss Of Fertility And Desertification – Soil Erosion results in the displacement of the top soil from one region to another, thus, reducing its fertility. When the top fertile soil is constantly removed from a region, only infertile sub-soil is left behind. In such a soil, only sparse vegetation can grow. This way, soil erosion gradually turns lush green areas into deserts.

2. Landslides In Hilly Areas – Barren hills or hills with sparse vegetation are constantly exposed to heavy rainfall that makes the top soils of the hills loose. Due to soil erosion, rock pieces of various sizes and loose soil from hills, suddenly slide down the steep slopes of mountains/hills. This phenomenon is called landslides. When these rock pieces and soil block the narrow river bed they result in floods. Sometimes, landslides block the roads and disrupt hill-life. Landslides occasionally kill the people living in the downhill and downstream with the displaced soil tend to disturb the soil quality.

3. Flash Floods – Vegetation in the hilly regions absorb a lot of rain water and keep the top soil intact. Barren hills or hills with sparse vegetation cannot absorb much rain water and thus cannot keep the soil intact. So, heavy rains result in rapid movement of water in the areas resulting in flash floods in lower areas causing enormous loss to life and property.

4. Famines (excessive shortage of food) – Continuous soil erosion from a region removes the fertile top soil leaving behind only infertile subsoil. Texture change in eroded soil, reduces its water holding capacity. Crops thus, cannot grow in such infertile, dry soil leading to shortage of food grains in the region. Ultimately such a situation leads to famine in an area.

5. Silting Of Water Reservoirs – Top soil when washed down by water, clog drains, water channels etc. due to deposition. Silt pollutes the water. Problem of silting in water reservoirs lowers the water level in them which ultimately leads to the shortage of production by the hydroelectric power stations.

Positive Effects Of Soil Erosion – Fertility may be enhanced when soils are washed down from hills into river valleys and deltas or are deposited on prairies by wind.

What Are Steps/Methods To Prevent Soil Erosion?

1. Intensive Cropping – If the fields remain covered throughout the year, their top soil will not be exposed to winds or rains. In such a condition no soil erosion will occur.

2. Sowing Grasses And Planting Xerophytes – Soil should not be left uncovered. Sowing grasses on barren soil or planting of xerophytes will bind the loose soil. The roots of the grasses and xerophytes hold the soil in place. Vegetative cover on the ground also helps in percolating water into deeper layers of the soil.

3. Terrace Farming (Terracing) – In terracing the slopes are divided into a number of flat fields to slow down the flow of water. In hilly regions, small crop fields are thus formed in the form of steps or terraces for cultivation of crops. Such terrace farms reduce the flow of rain water down the slopes of hills. Moreover, eroded soil from upper regions of hills gets deposited in lower terraces.

4. Contour Bunding – Small bunds (embankments or dikes) are raised on the edges of the fields to prevent loss of top soil through wind or water.

5. Conservation Tillage – Instead of conventional tillage, reduced or no tillage can be practiced. It prevents soil erosion.

6. Wind Breaks – Rows of trees and shrubs are planted at right angles to prevailing wind flow, to check the erosion of the soil by wind.

7. Proper Drainage Canals Around The Fields – This method involves the removal of excess rain water through small drainage canals formed around the fields.

8. Making Strong Embankments Along The River Banks – Formation of strong embankments of stones, sand bag etc. on the both sides of the rivers particularly in erosion prone areas, this will check soil erosion caved due to the fast moving river water.

Soil Degradation Due To Extensive Farming

Modern farming practices involve the use of large amounts of fertilizers and pesticides. Use of these substances over long periods of times can destroy the soil structure by killing the soil microorganisms that recycle the nutrients in the soil. It also kills the earthworms which are helpful in making the rich humus. Fertile soils can quickly become barren if sustainable practices are not followed. Removal of useful components (nutrients) from the soil and addition of other substances (fertilizers, pesticides etc) which adversely affect the fertility of soil and kills the diversity of organisms that live in it is called soil pollution.

Soil Compaction resulting from intensive cultivation withe ever larger and heavier farm machinery, definitely reduces yields. About half of the irrigated lands of the world are damaged to some extent by salinization (salt accumulation) or alkalinization (alkali accumulation).

Soil Pollution – Soil Pollution is caused by solid wastes and chemicals. The slag heaps from mines spoil the beauty of sites of mines. Pulp and paper mills, sugar mills, oil refineries, power plants, chemicals and fertilizer manufacturing units, iron and steel plants, plastic and rubber producing complexes are some major contributions to soil erosion. Most industrial furnaces and thermal power stations produce fly ash, which is a grey powdery residue of unburnt material and causes pollution. This fly ash hampers the growth of crop plants and also decreases crops of orchards. Domestic waste also add a large amount of solid. These include food scraps, vegetable remains, packing materials, cans, rags, papers, cinders, ash, broken gadgets (electronic wastes, old computers), wood, metals, bones of dead animals, plastics, polythene bags, ceremics, glass, aluminium, rubber (vehicle tyres, tubes), leather (old shoes, belts, purses, suit cases etc), construction rubbish, bricks, sand and other junk. This solid waste has two types of pollutants :

(i) Degradable

(ii) Non-Degradable

Degradable Pollutants include domestic wastes and sewage that decompose easily.

Non-Degradable Pollutants are the pollutants which are not degraded and persist in the environment Eg- plastics, inorganic metallic compounds, oxides, pesticides (DDT), radioactive substances.

Most of the solid wastes are used in sanitary landfill, some of these are recycled and some is burned by incinerator.

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