SOIL EROSSION : LOSS THE HEALTH AND PRODUCTIVITY OF SOIL
>> 30 March, 2009
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.
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.
Ø 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.
Ø 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
Ø Control of soil erosion:
The following methods can be adopted to control soil erosion-
A. Biological methods
B. Mechanical methods
1. Agronomic practices: The important agronomic practices that help in the control of soil erosion are-
i. Contour farming: It is produced in hilly regions or slops.
ii. Keeping the land fallow: If the land is left uncultivated and sheep, goats and other cattle are allowed to graze and sit over it for sometimes, the soil becomes fertile and avoids erosion.
iii. Vegetative cover: Keeping the soil with vegetative cover with different crops, soil erosion can be reduced.
iv. In hilly areas soil erosion can be reduced by following practices-
a. Tree crops are to be planted
b. Minimize slope of the land and make it less susceptible to erosion.
c. Try to increase the infiltration and percolation rate by proper management practices and by applying organic matter and thus reducing the run-off loss of water.
v. Mulching: By covering the soil surface with straw, leaves or grasses, soil erosion can be controlled.
2. Agrostological method: The following agrostological methods are followed to check soil erosion-
i. Cultivation of grasses in relation with agricultural crops.
ii. Retiring the land by thick covering with grasses.
iii. Afforestation and reforestation.
iv. Checking of over grazing: A system of restricted and rotational grazing may be helpful in checking soil erosion to some extent.
3. Dry farming practices: Dry farming is useful in those areas where rainfall is very low, uncertain and uneven in distribution.
B. Mechanical methods: The mechanical practices of soil conservation includes various engineering techniques and structures which are adapted to supplement the biological methods when the latter alone are not sufficiently effective.
Objectives of mechanical practices:
i. To reduce the velocity of run-off water to minimum level by dividing a long slope in several small parts.
ii. To allow maximum water to be absorbed and held in the soil.
iii. Protection against erosion by wind and water. Some mechanical methods for soil conservation are-
1) Basin leaching
2) Pan breaking
3) Sub-soiling
4) Contour terracing
5) Contour trenching
6) Terrace outlets
7) Gully control
8) Digging of ponds & reservoirs
9) Stream bank protection
Importance of soil erosion:
i. Erosion hampers sustainable agriculture
ii. River channels are filled and domestic water supplies are damaged.
iii. Soil erosion compels people to form the slopping to hilly land.
iv. Affects irrigation water reserve.
v. Causes drastic / huge reduction in food production resulting from the washing away of the fertile surface soil.
Universal soil loss equation: analysis of cropping management and erosion control practice factors led Wischmeier and Smith (1960) to propose the universal rainfall erosion equation. The equation is-
A = RKLSCP
Where,
A = Average annual soil loss from a given field (ton/acre)
R = Rainfall factors or rainfall erosivity factors
Topographic factors
S = Steepness of slope
C = Crop management factors or vegetative cover
P = Erosion control practices factors in sloppy lands (contour farming, stripe cropping, terraceing).
This equation can be used in planning farm programmes and conservation practices to minimize soil erosion.
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