Introduction
Soil erosion is simply a systematic
removal of soil, including plant nutrients, from the land surface by the
various agents of denudation. Briefly, soil erosion can be regarded as merely a
geomorphological process, whereby the surface layer of weathering rock is
loosened and carried away by wind or running water and a lower horizon in the
soil is exposed. It
occurs in several parts of Nigeria under different geological, climatic and
soil conditions. But the degree of occurrence varies considerably from one part
of the country to the other. Thus, while it is true to observe that soil
erosion is one of the most striking features on the land surface of
Southeastern Nigeria, especially in Anambra and Imo States, only rare
occurrences of the phenomenon are recorded in some other States of the
Federation.
Soil erosion as a Natural and Global phenomenon
From this geological
perspective, erosion is just a means by which earth’s surface is shaped. As a
natural phenomenon, this process occurs in all areas, regardless of human
activities. A classical example of wind erosion in the absence of human
interference is shown in recent photographs taken
on Mars. Another example is the images of Titan
(Saturn’s giant moon) sent back by Huygens during its landing on January 14,
2005. The images show how river channels and deltas in Titans are carved by
methane rain
Factors that can expose Soil to erosion (Causes of Soil erosion)
We know
that except for some of the forest reserves in the country, there is hardly any
such soil under undisturbed cover. Man needs the soil for his cultivation and
has to clear the ground (bush or forest) for farming. He also burns the grass
and trees for project development due to urbanization (e.g. housing, erection
of buildings, construction of roads etc.) and man’s need to graze his animals.
Each of these activities leads to exposing the soil to the elements and,
invariably, to "accelerated" soil erosion and deterioration,
depending on the existence of other favourable conditions. These other factors
include climate, topographic disposition and lithology, especially the nature
of surface materials.
However, the factors of
soil erosion in Nigeria resolve into two components: physical (geologic or
"natural") and anthropogenic (human or "accelerated").
Studies have revealed that the human component in soil erosion is often
exaggerated while the effects of the physical component are usually
underestimated. It has been notice that soil erosion in the country is a result
of the so-called "bad farming techniques".
1. Physical factors
- Climate: for the humid tropics, rainfall constitutes the dominant sub-factor. High amount and intensity rainfall, with larger size and higher velocity of raindrops gives rise directly to splash erosion. Rainfall leads to infiltration (amount of water entering the soil) and also leads to runoff (water sliding on the soil into streams, river and other water bodies) which is the central agent in the sheet erosion (unconcentrated runoff) and gully erosion (results from concentrated runoff) provided that lithology (the general physical characteristics of rocks in a particular area) is favourable. Exposure of the B-horizon with lower infiltration capacity results in greater runoff for a given rainfall intensity – thereby causing even more erosion.
- Nature of surface materials: influences the rate of infiltration and rate of surface runoff. Wet, saturated soils will not be able to absorb as much rain water, hence, reduce infiltration and increase surface runoff, thus high erosivity for a given rainfall. The composition, moisture, and compaction of soil are all major factors in determining the erosivity of rainfall. Sediments containing more clay tend to be more resistant to erosion than those with sand or silt, because the clay helps bind soil particles together. Soil containing high levels of organic materials are often more resistant to erosion.
- Surface configuration
(relief/slope):
aids runoff, sheet erosion and gullying. The general. Sheet erosion is common
over fairly uniform and gentle slopes, while Gully erosion is expected to be
more characteristic of steeper slopes. It is known, however, that gullying also
takes place on very gentle slopes and is even more common on such gentle slopes
than on very steep ones. Runoff requires such gentle slopes to be concentrated,
and concentrated runoff is a prerequisite for gullying.
2. Anthropogenic factors
The human components in soil
erosion are connected mostly with agricultural practices and other land use
activities.
- Agricultural practices: Vegetation acts as an interface
between the atmosphere and the soil. It increases the permeability of
the soil to rainwater, thus decreasing runoff. It shelters the soil from winds.
In the humid tropics generally involve the destruction of vegetation by
clearing the land for cultivation and by forest fires, lack of crop rotation.
These activities cause great change in the relative proportions of infiltration
and runoff, with the dangers of erosion increasing with increased destruction
of vegetation and, thereby, reduced infiltration and increased runoff.
- Land use activities: surface mining, road building, urbanisation, industrialisation and general infrastructural development appear most important. these other land use activities help deprive the soil surface of its vegetation and also contribute directly to sliding, slumping, sheet and gully erosion.
No one could reasonably deny the
possible contributions human activities make to the development of soil
erosion, nor, could it be contended that different methods of land use and the
level of technology of land users necessarily affect the soil differently. Consequently,
the most important of the factors to explain soil erosion in Nigeria are not
human; that the main contribution of man to the inception and development of
the various forms of erosion in the country is essentially one of complicating
and accelerating an inherent problem rather than provoking it, given that the
environment is one whose physical characteristics are totally disposed to the
evolution of the worst types of erosion. However, man's role is a more recent
and easily decipherable phenomenon than that of geologic erosion, which Is why
his role readily attracts attention (Prof.
G.E.K. Ofomata).
Types of Soil Erosion
Water Erosion
Raindrops can be a major problem for farmers when they strike bare soil. With an impact of up to 30 mph, rain washes out seed and splashes soil into the air. If the fields are on a slope the soil is splashed downhill which causes deterioration of soil structure.- Sheet erosion: is defined as the uniform/downslope
removal of top soil in thin layers from sloping land. This, of course, is
nearly impossible; in reality the loose soil merely runs off with the rain. It
is a gradual process and it is not immediately obvious that soil is being lost.
- Rill erosion: is the most common form of erosion. It can occur on steep land or on land that slopes more gently. Because there are always irregularities in a field, water finds hollows in which to settle and low-lying channels through which to run. In other word, it occurs when soil is removed by water from little streamlets that run through land with poor surface draining. Although its effects can be easily removed by tillage, it is the most often overlooked. Rills can often be found in between crop rows.
- Gully erosion: usually occur near the bottom
of slopes and are caused by the removal of soil and soft rock as a result of
concentrated runoff that forms a deep channel or gully. They are larger than
rills and cannot be fixed by tillage. Water running downhill cuts a channel
deep into the soil and where there is a sudden fall, a gully head forms at the
lower end of the channel and gradually works its way back uphill.
Wind erosion
Wind erosion
occurs when the land surface is left bare in regions that are arid enough, as a
result of low rainfall, to allow the soil to dry out, and flat enough to allow
the wind to carry the soil away over several consecutive days. Wind erosion is
much more severe in arid areas, and during times of drought. Land may become
susceptible to wind erosion through grazing animals, which remove the
protective plant cover, and whose hooves break up the soil, especially round
watering points. Arable land that has been left bare is also a major problem.
Wind erosion is also a major source of land degradation, evaporation,
desertification, harmful airborne dust, and crop damage—especially after being
increased far above natural rates by human activities such
as deforestation, urbanization, and agriculture
Gravitational erosion
Mass movement: is
the downward and outward movement of rock and sediments on a sloped surface,
mainly due to the force of gravity. Mass movement process is always
occurring continuous on all slopes and is often the first stage in the
breakdown and transport of weathered materials in mountainous areas. It
moves material from higher elevations to lower elevations where other eroding agents
such as streams and glaciers can then pick up the material and move
it to even lower elevations.
Slumping: happens
on steep hillsides, occurring along distinct fracture zones, often within
materials like clay that, once released, may move quite rapidly
downhill. In some cases, the slump is caused by water beneath the slope
weakening it. In many cases it is simply the result of poor engineering
along highways where it is a regular occurrence.
Types of erosion in Nigeria
The gully types are the more obvious forms of
erosion in the country, mainly because of the remarkable impression they leave
on the surface of the earth. They are also a visible manifestation of the
physical loss of the land due to erosion. Good examples of gullies are widespread
in Nigeria, especially in the Agulu-Nanka (Plate 1), Obioma, Nsuka, Alo, Nnobi,
Nnewi, Olu, Ozuitem, Abiriba, Ohafia, Uruala, Amucha and Uyo areas of
Southeastern Nigeria. Other examples, but on a much smaller scale, exist on the
Jos Plateau, especially in Heipang (Plate 2), around Zaria, in Ankpa and at
Auchi.
Much more pernicious
and highly detrimental to agriculture is sheet erosion which often goes on
unnoticed due to its gradual, constant and uniform action, but which finally
results in a complete removal of arable parts of the soil. Through this action
of sheet erosion, the topsoil is gradually swept clear of its finer elements
and plant nutrients, and only coarse, infertile materials are left behind. Wind erosion occurs
more generally and more frequently in the extreme northern parts of the country,
but is limited in both time and space in other parts. Some of the materials
removed by the wind reach the southern parts of the country by way of
dust-laden winds (the North Easterly Trades) especially during the period of
the harmattan.
Prof.
G.E.K. Ofomata in his work reveals that sheet erosion is the most widespread
type of erosion in the country, and that every part of Nigeria is affected by
one form of erosion or the other. Also, erosion in four States - Borno, Kaduna,
Kano and Sokoto is a result of the combined effect of wind and water action,
while the other parts of the country are affected by erosion due mainly to the
action of running water.
Effects / Consequences of Soil Erosion
- Land degradation: Water and wind
erosion are now the two primary causes of land degradation; combined, they
are responsible for 84% of degraded acreage. The loss of soil fertility due to
erosion is further problematic because the response is often to apply chemical
fertilizers, which lead to further water and soil pollution, rather than to
allow the land to regenerate. In Africa, if current trends of soil
degradation continue, the continent might be able to feed just 25% of its
population by 2025, according to UNU's Ghana-based Institute for Natural
Resources in Africa.
- Sedimentation of aquatic ecosystems: Soil erosion
(especially from agricultural activity) is considered to be the leading global
cause of diffuse water pollution,
due to the effects of the excess sediments flowing into the world's waterways.
The sediments themselves act as pollutants, as well as being carriers for other
pollutants, such as attached pesticide molecules or heavy metals. It also
reduces their food supply, and causes major respiratory issues for them as
sediment enters their gills and reduced the biodiversity of aquatic plant.
- Airborne dust pollution: Soil particles
picked up during wind erosion are a major source of air pollution, in the
form of airborne particulates—"dust". These airborne soil
particles are often contaminated with toxic chemicals such as pesticides or
petroleum fuels, posing ecological and public health hazards when they later
land, or are inhaled/ingested.
- Desertification: Soil erosion also
leads to desertification. Desertification refers to increase of desert areas. Soil erosion
results in the loss of soil fertility and makes the land barren.
Prevention and Control of Soil Erosion
- Increase of Vegetation: when the land is covered with vegetation, the roots of the plants and trees interlock and interlace to bind the soil particles. This helps in two ways:
- does not allow the soil particles to be
carried away by wind or water
- does not allow free flow of water over
the soil which prevent erosion of soil by flowing water
- the falling leaves of the plants get
converted to humus by decomposing action of the soil microbes. This
enriches the soil.
- Crop
Rotation: the practice of growing
different crops at different times on the same land is called crop rotation.
This keeps the topsoil covered with vegetation. Rotation of cereal crops with
legumes also keeps the soil enriched with nitrogen (from the legumes).
- Reforestation: slopes are more subject to soil erosion by
running water. Growing trees on lands which have lost their vegetation is
called reforestation. Trees like Albizia, Cassia, Butia, etc. are suitable for
this.
- Strip
Croping: it involves growing of
crops in strips. The most common method followed is the contour farming where
the strips of crop are at right angles to the slope. Wind-strip cropping is
when the strips of crop are place at right angles to the direction of wind.
- Restoring Soil Fertility: fertile
soil supports vegetation. Loss of fertility results in loss of vegetation and
this exposes the land to erosion. Fertility of soil can be increased by
addition of natural and synthetic fertilizers.
- Control of Grazing: covering the land with small plants and grasses helps the
topsoil to remain in place as the roots of these plants bind with the soil
particles. Cattle graze on these plants and expose the topsoil Thus, grazing
should be allowed only on the land meant for the purpose and other areas should
be protected from grazing.
- Terracing: fields
are cut at right angles to the slope. This slows down the flowing water and
allows it to irrigate the crops, as well.
- Dam Building: with the dams the speed and
amount of water flowing can be controlled. This will control the soil erosion
of the river banks.
- Wind Breakers: trees
are planted across the wind direction to protect against the high velocity
winds. These rows of trees are called shelter belts or wind breakers.
Highly informative, should be brought to the attention of policy makers and land use planners in Nigeria.
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Soil erosion is the displacement of the upper layer of soil, the agents of soil erosion are the same as the agents of all types of erosion: water, wind, ice, or gravity. Running water is the leading cause of soil erosion, because water is abundant and has a lot of power. Wind is also a leading cause of soil erosion because wind can pick up soil and blow it far away.
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