Soil erosion - Causes and Effects

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Ecological Problems

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Global concentration of atmospheric pollution seriously threatens the ozone layer. It means the increasing concentration of greenhouse gases and increased ozone depletion. This threatens to lead to dramatic climatic changes or global warming. To reduce these threats, the scientists of the various countries of the world say that global emissions must be curtailed.

A number of gases contribute significantly to the stock of greenhouse gases. The burning fuels by automobiles and industries are main sources of greenhouse gases. Less damaging sources include deforestation, animal husbandry, wet rice cultivation, decomposition of waste, and coal mining.

Because incomes and consumption are higher in the wealthiest countries, per capita emissions are much higher. For example, the level of per capita emissions in the United States is more than twice bigger than that in Europe, 19 times higher than that in Africa, and 25 times higher than that in India.

A study jointly sponsored by the World Meteorological Organization and the United Nations Environment Program shows that global warming is growing. It says that if current emission trends continue, the mean global temperatures may rise 3 C by the end of the twenty-first century.

The potentially catastrophic consequences of climate changes have caused widespread cries for joint preventive policy to combat environmental pollution, concentration of greenhouse gases and ozone depletion.

Statesmen and scientists stress that responsibility for reducing emissions must be divided across the members of the international community. The share of responsibility may be remarkably different depending on industrial development, income, social structure and political orientation of a country.

There is great controversy over the extent to which each government must control the emissions produced by its local population, industry and agriculture.

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Erosion is a natural and continuous process. Soils are created through erosion of parent material and either local deposition elsewhere. Erosion, is defined as the detachment or uptake, and transport over a certain distance of material of the upper layer of the earth crust by an agent, like water, wind or ice. This mass movement of soil particles, is part of the process of soil degradation. Before erosion takes effect, the degradation process often has started with qualitative changes in the soil, like loss of nutrients, loss of organic materials, reduced soil life and loss of soil structure.

Soil erosion is one of the forms of soil degradation along with soil compaction, low organic matter, loss of soil structure, poor internal drainage, salinisation, and soil acidity problems. These other forms of soil degradation, serious in themselves, usually contribute to accelerated soil erosion.

Soil erosion may be a slow process that continues relatively unnoticed, or it may occur at an alarming rate causing serious loss of topsoil. The loss of soil from farmland may be reflected in reduced crop production potential, lower surface water quality and damaged drainage network.

Soil erosion is a naturally occurring process on all land. The agents of soil erosion are water and wind, each contributing a significant amount of soil loss each year in different countries.

Water erosion depends on four factors: rainfall, soil type, slope gradient, and soil use/vegetation cover. Loss of soil structure becomes often most visible in encrustation of the soil.

Both rainfall and runoff factors must be considered in assessing a water erosion problem. The impact of raindrop on the soil surface can break down soil aggregates and disperse the aggregate material. Lighter aggregate materials such as very fine sand, silt, clay and organic matter can be easily removed by the raindrop splash and runoff water; greater raindrop energy or runoff amounts might be required to move the larger sand and gravel particles.

Soil erosion potential is increased if the soil has no or very little vegetative cover of plants and/or crop residues. Plant and residue cover protects the soil from raindrop impact and splash, tends to slow down the movement of surface runoff and allows excess surface water to infiltrate.

Wind erosion less common, but again takes place after vegetation has been lost and when soil particles are loosened. Early signs of wind erosion include deposition of sand particles around plants and micro-ripples on the surface of exposed areas. The final extreme is the classic sand desert dune structures. The lack of windbreaks (trees, shrubs, residue, etc.) allows the wind to put soil particles into motion for greater distances thus increasing the abrasion and soil erosion. Knolls are usually exposed and suffer the most.

Sheet erosion is the most common form of erosion. Unprotected soil particles are loosened by trampling, through wind erosion and by the impact of rainfall. The soil particles are then transported by rainwater surface flow to the river and stream systems. Sheet erosion is characterized by a general lowering of the soil level, leaving raised pedestals where the root mass of the remaining vegetation protects it. Sheet erosion is soil movement from raindrop splash resulting in the breakdown of soil surface structure and surface runoff; it occurs rather uniformly over the slope and may go unnoticed until most of the productive topsoil has been lost.

Rill erosion results when surface runoff concentrates forming small yet well-defined channels. These channels are called rills when they are small enough to not interfere with field machinery operations. The same eroded channels are known as gullies when they become a nuisance factor in normal tillage.

Gully erosion is the most obvious and dramatic demonstration of erosion, although in most areas actually less significant in terms of total land degradation. Gully erosion rarely occurs without sheet erosion. It can also be triggered by erosion along livestock tracks, footpaths and road edges. The process can start with "rills" and end up with gullies that are tens of meters deep.

There are farms that are loosing large quantities of topsoil and subsoil each year due to fully erosion. Surface runoff, causing gull formation or the enlarging of existing gullies, is usually the result of improper outlet design for local surface and subsurface drainage systems. The soil instability of fully banks, usually associated with seepage of ground water, leads to sloughing and slumping of bank slopes. Such failures usually occur during spring months when the soil water conditions are most conductive to the problem. Poor construction, or inadequate maintenance, of surface drainage systems, uncontrolled livestock access, and cropping too close to both stream banks has led to bank erosion problems.

The effects of soil erosion are complex. Some of the impacts may appear to be reversible by suitable soil conservation programmes and improving cultivation practices, whereas there are other types of degradation which are irreversible. The latter type includes land lost by gulling, or cases of severe sheet erosion where the soil cover has been removed to a great extent. In cases where degradation is reversible it is generally difficult to assess the actual extent and impact of land degradation as farmers do convert their land to less demanding uses or increase the level of inputs. Methodologies for prediction of soil erosion have been developed since the early thirties.

Many farmers have already made significant progress in dealing with soil erosion problems on their farms. However, because of continued advances in soil management and crop production technology that have maintained or increased yields in spite of soil erosion, others have not been aware of the increasing problem on farmland. Awareness usually occurs only when property is damaged and productive areas of soil are lost.

Certain conservation measures can reduce soil erosion by both water and wind. Tillage and cropping practices, as well as land management practices, directly affect the overall soil erosion problem and solutions on a farm. When crop rotation or changing tillage practices are not enough to control erosion on a field a combination of approaches or more extreme measures might be necessary. For example, contour plowing, strip cropping, or terracing may be considered.

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