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 ErosionRaindrops 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 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
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.