What is Erosion?

Erosion is the wearing away of top soil or rock by rain, wind, sea/rivers or by the action of toxic substances. The primary effect of erosion is the reduction of topsoil thickness, which results in structural degradation, soil compaction, nutrient depletion, loss of soil organic matter, poor seedling emergence and reduced crop yields.

Erosion is one of the greatest problems of our time from the view point of agricultural as well as food supplies. It is a process where by nature removes soil more quickly than it is replaced by soil forming process. Ordinarily, soil erosion may be beneficial to man but when it is accelerated, loses its importance. Rivers are choked with large quantities of solid matter and becomes prone to flood and of course, become far less navigable.

Erosion is classified under to main categories: Geologic and accelerated erosion. Geologic erosion isnormal process of weathering that generally occurs at low rates in all soils as part of the natural soil-forming processes. It occurs over a long geologic time horizons and it is not influenced by human activity. The wearing away of rocks and formation of soil profilesare processes affected by the slow but continues geologic erosion.

Accelerated erosion is as old as agriculture itself, it dates back to the old civilizations in Mesopotamia, Greece, Rome, and other regions in the Middle East. The collapse of great ancient civilizations in Mesopotamia along Tigris-Euphrates. Rivers shows the consequences when lands are irreversibly degraded. Accelerated erosion is caused by human activity in addition to the natural rate of erosion. Cleared land in drought-stricken areas can produce dry soil which may blow away important soil nutrients. Felling trees removes the roots which bind the soil particles together and so exposes the soil to erosion by rainwater.

Indeed, low rates of erosion are essential to the formation of soil. In contrast, soil erosion becomes a major concern when the rate of erosion exceeds a certain threshold level and becomes a rapid, known as accelerated erosion. This type of erosion is brought about by anthropogenic causes such as, slash-and-burn agriculture, intensive ploughing, intensive and uncontrolled grazing and biomass burning.

Control and management of soil erosion are very important because when the fertile topsoil is eroded away the remaining soil is less productive with the same level of input. While the problem of soil erosion cannot be completely solved, excessive erosion must be reduced to manageable or tolerable level to minimize adverse effects on productivity. However, magnitude and the impacts of soil erosion on productivity depends on soil profile and horizonation, terrain, soil management and climate characteristics. The specific rates of maximum tolerable limits of erosion vary with different types of soil. For instance, moderate soil erosion may not adversely affect productivity in well-developed and deep soils, but the same amount of erosion may have huge effects on shallow and sloping soils. Thus, critical limits of erosion must be determined for each soil type, ecoregion, land use, and the farming system.

Types of Erosion

• Water erosion
• Wind erosion
• Glacial erosion

Water Erosion

Water erosion is the wearing away of the soil surface by rainwater, runoff, snowmelt, and irrigation. Rain water in the form of runoff is the main agen of water erosion. It refers to the movement of soil organic and inorganic particles along the soil surface with flowing water and deposition of the eroded materials at lowerlandscape positionand in aquatic ecosystems. On a global scale, water erosion is the most severe type of erosion, it occurs in the form of splash, rill, gully, tunnel, streambank, and coastal erossion. Runoff occurs when precipitation rates exceed the water infiltration rates. Both raindrop impact and water runoff can cause soil detchment and transport. Unlik wind erosion, watererosion is a dominant form of erosion in humid and sub-humid regions characterized by frequent rainstoms. it is also a problem in arid and semi-arid regions where the limited precipitation mostly occurs in the form of intense storms when the soil is bare and deviod of vegetal cover. One of the spectacular types of water erosion is the concentred gully erosion which can cause severe soil erosion even in a single event of high rainfall intensity. Excessive gully erosion can wash out crops, expose plant roots, and lower ground watertable while adversely affecting plant growth and landscape stability. Gullying is a mjor source of sediment and nutrient loss. It causes drastic alterations in landscape aesthetics and removes vast amounts of sediment. Sedimentation at the lower end of the fields in depressional sites can bury crops, damage field borders, and also pollutes water bodies. Gullies dissect the field and exaceerbate the non-point source pollution. For example, sediment, chemicals nearby water sources.

Picture showing a valley that was created by water erosion. Photo credit: Love South Africa (Facebook)

Wind Erosion

Wind erosion is a widespread phenomenon, especially in arid and semi-arid regions. It is a dominant geomorphic force that has reshaped the earth. Most of the material carried by wind consists of silt-sized particle. Deposition of this material, termed as loess has developed into very fertile and deep soils. The thickness of most loess deposits ranges between 20-30m, however, it can be as thick as 335m. For example, loess Plateau in China (extensive deposits of loess exist in Northern China, Midwestern USA and central Europe, Las Pampas. etc.). Excessive erosion due to soil mismanagement has however, caused the barren state of many arid lands. Anthropogenic activities set the stage for severe wind erosion by directly influencing soil surface conditions through deforestation and excessive tillage. Wind erosion is prominent but not unique to arid regions. High winds, low precipitation, high evapotranspiration, reduced vegetation cover, and limited soil development are the causes of wind erosion in arid and semi-arid regions. Unlike water, wind has the ability to move soil particles up and down-slope and can pollute both air and water. while arid lands are more prone to wind erosion than humid regions, any cultivated soil that is seasonally disturbed can be subjected to eolian processes in windy environment.

Wind erosion doesnt only alter the properties and processes of the eroding soil but also adversely affects the neighbouring soils and landscapes where the deposition may occur. Landscapes prone to wind erosion often exhibit an impressive network of wind ripples (<2m high). Formation of sand dunes can be as high as 200m in desert regions of the world. For example Saudi Arabia. The smaller sand dunes often migrate and form larger sand dunes.

An example of wind erosion in a desert, the cone-shaped rocks are formed due to excessive wind erosion

Glacial Erosion

The action of glacier and ice are very great both in the highlands and lowland areas of the world. In the highlands, glacier erosion is very dominant while at the lowland area, it does much of deposition works. Glacier does much of its erosive action through the following processes: Plucking, abrasion and sapping

• Plucking– the process of wearing away of blocks or rocks which have become frozen into the base of and sides of the glacier.
• Abrasion- this refers to the wearing away of beneath a glacier by scouring action of the rocks embedded in the glacier
• Sapping- this is process of breaking up rocks of alternate freezing and thawing of water at the bottom of cracks between a mass of ice and the sides and the floor of a valley.

Picture of Hanging valley formed by glacial erosion. Photo credit: Beauty of Nature (Facebook)

Causes of Erosion

1. Bush burning- grasses help to stop the rate at which water run down the soil, when these grasses are removed due to fire, the soil is exposed to excessive water which may in turn lead to flooding, hence the act of bush burning exposes the soil to erosion
2. Overgrazing- this occurs through excessive grazing or removal of grasses by animals thereby exposing the soil to erosion.
3. Excessive rainfall- high intensity rainfall creates serious risk as heavy drops on bare soil causes the soil surface to seal. The direct force of falling raindrops causes geyser splashing in which the soil particles are lifted and then dropped into new positions, a process termed splash erosion.
4. Deforestation- this entails the removal of natural vegetation, on a slope covered by grass soil, even a deep layer of overland flow causes little soil erosion because the energy of the moving water is dissipated in friction with the grass stems, which are tough and elastic. On a heavy forested slope, countless check dams made by leaves, twigs, roots and fallen tree trunks take up the force of the overland flow. Without such vegetation cover, the eroding force is applied directly to the bare soil surface, easily dislodging the grains and sweeping them down slope thereby leading to soil erosion
5. Excavation of soil for building and construction purposes equally trigger soil erosion.
6. Steep slopes- the longer and steeper the slope, the more erodible the soil and the greater the transport capacity of runoff under intense rain. Where there are steep runoff from torrential rains produces a more intense activity termed rill-erosion, in which innumerable closely spaced channels are scored into the soil and regolith if these rill are not destroyed by soil tillage, they may soon begin to integrate into still large channels called gullies.

Effects of Erosion

1. Soil erosion contributes to the chronic malnutrition and rural poverty in the third world countries where farmers are too poor to establish erosion counteractive measures
2. Erosion leads to reduction of or loss of farmlands
3. The removal of the subsoil by soil erosion reduces soil fertility
4. Erosion brings about environmental imbalance
5. It leads to the destruction of roads and other track ways
6. Severe soil erosion in an area could leads to loss of lives and properties
7. Soil erosion can lead to land and water pollution through deposition of sediments
8. Wind erosion causes dust pollution which alters the atmospheric radiation, reduces visibility and causes traffic accidents.

Control of Erosion

• Adoption of no-tillage farming should be considered, this is a practice where crops are grown without turning soil, and conservation tillage have also contributed in part to the reduction of soil erosion. These efforts have resulted in better soil management, but much should be done.
• Water and wind erosion preferentially remove the soil layers where most of agricultural chemicals like nutrients, pesticides are concentrated.