Cause Of Soil Erosion

Soil Erosion:

  Soil erosion is the wearing away, detachment and removal of soil from one place and its deposition at another place through the action of natural agents such as water, blowing winds, strong waves, snow and gravity etc.

Kinds of soil erosion: Soil erosion can be divided broadly into two categories-

  1. Normal / Natural / Geologic erosion
  2. Accelerated soil erosion

1. Normal soil erosion:

  When the top soils are gradually removed under normal, physical, biotic and hydrological equilibrium in nature, it is known as normal erosion. It is a slow process in which complete equilibrium is maintained between soil removing and soil forming process. The normal erosion tends to produce wavy or undulating land surfaces with alternating ridges and depressions.

2. Accelerated soil erosion: 

  When the normal soil erosion does not keep harmony with the soil formation due to the disturbed by man’s exploits and by natural calamities that cause the loss of its (soil) resisting power and eroding agencies become more active and cause accelerated erosion. Actually accelerated erosion is soil erosion.

Agents of soil erosion: Two agents are responsible for soil erosion-

  1. Biotic agent 
  2. Climatic agent / Abiotic agents

 
1. Biotic agent: 

  Excessive grazing, deforestation, undesirable forest biota and mechanical practices by man are important factors which cause soil erosion.

2. Climatic agents: These are water and wind.

A. Wind:

Removal of soil by wind is called wind erosion. Stormy winds carry the soil particles to distant places and sometimes form sand-dunes. Wind causes the following 3 types of soil movements-

  i. Saltation: Major part of wind carried soil (1-1.5 mm dia) is moved in a series of bounces called saltation.

  ii. Suspension: Small soil particles (<>

  iii. Surface creep: Large soil particles (5-10mm dia) creep on surface soil due to wind velocity and move from one place to another.

  Wind erosion is not a serious problem in Bangladesh. In places where land is usually sandy and not thoroughly covered with vegetation, strong wind speed causes soil erosion.

B. Water Erosion: 

  Water erosion is a serious problem in Bangladesh. Water is an important factor in soil erosion where soil is directly affected by heavy rainfall, rapidly running water and by wave action.
Water erosion may be classified into following categories-

i. Splash or raindrop erosion: 

  The falling raindrop at an approximate speed of 30 ft/sec is capable of creating a force of almost 14 times of its own weight. With this falling force, the raindrops beat up the bare soil surface and causes erosion. Splash erosion is the forerunner of other types of water erosion.

ii. Sheet erosion: 

  It is the removal of a thin uniform layer of soil from large area is called sheet erosion. It is affected by run-off effect of rain water.

iii. Rill erosion:

  In this type of soil erosion heavy rainfall and rapidly running water produce finger-shaped groove or rills over the entire field. It is an intermediary stage between sheet erosion and gully erosion.


iv. Gully erosion:
 

It is more prominent type of erosion in which heavy rainfall, rapidly running water and transporting water may result in deeper cavities or grooves called gully. 
  Gullies may be ‘V’ or ‘U’ shaped. Gullies cut the fields into small fragment and make them uncultivable.

 

v. Land slide or slip erosion: 

  This type soil erosion is caused by heavy rainfall and it occurs in sloppy lands such as mountains and hills. In this type when the running water percolates through the crevices of rocks, great masses of soils and loose rocks lying on the steep slip downward.

vi. Stream bank erosion:
  On the banks of swollen rivers it is most active. During the rainy season, when fast running water streams take turn in some other directions, they cut the soil and make caves in the bank. As a result, soils become detached and deposited in other places.

vii. Sea shore erosion:

  This is caused by the striking action of strong waves which combine eroding effects of both wind and water.

Mainly erosion in Bangladesh is water erosion. 

Factors affecting soil erosion:

  There are several factors which affects the erosion at a particular area. These are given below-

1. Topography of the land (Land form): 

  Land form with special difference to nature degree and length of slope influence the soil erosion.
   

  0% Level Land
  5%
  10%
  15%  

a) Slope steepness (S)
b) Slope length (L)

The velocity of water movement increases exponentially as the slope steepness of land increases and thereby causes soil erosion.

2. Climate:

  Climate, specially distribution, nature and amount of precipitation (rainfall) and wind velocity influence soil erosion. Rainfall is the most important factor which directly influences the soil erosion through the following ways-

a) Intensity of rain (cm/hr)
b) Drop size of rain (mm)
c) Terminal velocity of raindrop (m/sec)
d) Duration of rainfall.

 Rainfall erosivity (R):

  The ability of rainfall to cause erosion in a particular soil under a given climatic condition is called rainfall erosivity and expressed by ‘R’.

  Ek=210.3+89 log10I
  Ek=Kinetic of energy (metric ton/ha/cm or rain) 
  I=Rainfall intensity (cm/hr)

3. Physical and chemical characters of soil:

  Soils with light textured with low organic matter content are more susceptible to erosion than soil with heavy textured, well structured, high organic matter content and clay content.


 Soil erodibility (K):

  Under a given slope, rainfall, vegetative cover, susceptibility to erosion depends on mainly soil type. This characteristic of soil to erosion is known as soil erodibility and expressed by ‘K’.

4. Vegetational cover, its nature and extent of coverage:

  Bare soil is more susceptible to erosion than the soils covered with vegetation. Crop canopy (reducing wind velocity) and root system (anchoring soil) also reduces erosion and thereby tree crops are recommended in hilly area.

5. Land use practices and soil management.
6. Natural phenomena such as earthquake, landslides and upheavals.

As functional equation soil erosion can be expressed as follows-

  Erosion = f (cl,v,t,s,h)
  F = Function of the dependent
  cl = climate
  v = Vegetation
  s = soil
  h = Human factors

Written By:-

BISMARK BANGALI

BSC in Agrotechnology

Khulna University, Bangladesh.