Soil Degradation

Soil Degradation, the decline in quality and quantity of a soil. This may be brought about by various processes: erosion, salinization, contamination, drainage, acidification, laterization and loss of soil structure, or a combination of these.

The most important process of degradation is accelerated erosion. This is the loss of soil by water, wind, and mass movements or, more locally, the action of vehicles and trampling feet of humans and animals. While only serious in some areas, its cumulative and long-term effects offer considerable cause for concern. The loss of the upper horizons containing organic matter and nutrients and the thinning of soil profiles reduces crop yields on degraded soils.

Salinization is the concentration of abnormally high levels of salts, for example, sodium, in soils due to evaporation. It frequently occurs in association with irrigation and leads to the death of plants and loss of structure of soils.

Frequent causes of contamination are farmyard waste and sewage sludge which may contain high levels of heavy metals. Soils have also been contaminated by radioactive isotopes from nuclear weapons testing and, on a restricted but locally serious scale, from the Chernobyl nuclear power station accident in 1986. Contamination may also result from other chemical wastes or byproducts of industrial processes.

Degradation of peatlands is brought about by drainage resulting in soil-loss due to oxidation and wind erosion.

Some soils are naturally acidic but may become more acidic due to acid rain or dry deposition of acid gases and particles. Acid rain has a pH of less than 5.6. The main sources of acidity in the atmosphere are the increasing quantities of sulphur dioxide and nitrogen oxides emitted by the combustion of fossil fuel.

The loss of organic matter due to erosion and oxidation degrades the soil and in particular its value as a crop-growing medium. The loss of organic matter also decreases the stability of soil aggregates which under the impact of rainfall may then break up. This process may lead to the formation of soil crusts which reduce infiltration of water into the soil and increase the likelihood of run-off and water erosion occurring. Soil crusts also inhibit the germination of seeds.

Loss of soil structure may occur due to loss of organic matter, compaction brought about by agricultural machinery and cultivation in wet weather, or dispersion of soil materials in the subsoil (“sodication”).