Temps de lecture :4 minutes
The degradation process can take different forms: hydraulic erosion, wind erosion, changes in the soil’s composition, physical degradation.
Soil provides living things with food, fibres and fuel. It supports wildlife and rural and urban activities. From the end of the 1940s to the beginning of the 1990s, over 90% of the degradation of productive land was due to overgrazing, deforestation and inappropriate agricultural practices. These changes in the soil affect over 2 billion people, most of the 852 million people suffering from hunger in particular.
Soil can therefore not be ignored in ecosystem mechanisms.
The state of affairs
According to the ISRIC World Soil Information data, 46.4% of soil is experiencing an important decrease in productivity and partially destroyed biological functions. A third of it is in Asia and a fifth is in Africa. 15.1% of soil can no longer be used for farming as its biological functions have been seriously destroyed and it would take large investments to restore them. About 9.3 million ha (0.5%) of soil is irreparably damaged and no longer has any biological function.
Over 50% of soils that have been degraded by deforestation are situated in Asia and 15% are in South America. Deforestation is the main cause of soil degradation in South America (41%), in Asia (40%) and also in Europe (38%), particularly in Central and Oriental European countries.
36% of soils degraded by overgrazing are in Africa. Overgrazing is the main cause of soil degradation in Africa (50%), in the South Pacific and in Australia (80%).
37% of soils degraded by inappropriate agricultural practices are in Asia. They are the most common cause of soil degradation in North and Central America (58%) and the second most common cause of soil degradation in Africa (25%).
50% of the 133 million ha degraded by the overexploitation of vegetation cover for domestic purposes is in Africa.
Almost all the soil degraded by industrial pollution is in Europe.
The soil system
Soil is a composite environment which is the result of abiotic factors (independent of human actions), that is to say of alterations to the bedrock (which provides soil’s mineral elements), atmospheric content (oxygen fixation, nitrogen cycle, water cycle) and biotic factors (linked to the actions of living things) such as the content of vegetation cover and the decomposition of living things. The analysis of a section of soil shows a superimposition of layers made up of different colours, different chemical compositions and different sizes of material. Each superimposition of layers creates a pedological profile.
The World Resources Institute published an article which highlighted the importance of soil ecosystem management especially in the search for solutions to fight desertification and certain forms of soil degradation. This involves taking the soil system as an element which is part of a whole. This is what these authors suggested for the risks linked to soil degradation to be managed better. [See debate]
Types of degradation
Human activity is the main cause of soil degradation. Agriculture plays a large part in soil degradation, especially clearing, irrigation, the spreading of chemical fertilisers and pesticides, overgrazing and even the passage of heavy farming equipment.
The clearing and deforestation of large plots of land to increase the agricultural surface area change humus composition and soil formation. This is because of varied primitive vegetation being replaced by secondary vegetation (monoculture being the extreme).
Tillage destroys superior layers of soil as well as the layer of humus and can even cause a plough sole (lower layer of compact land) to form because of ploughs regularly passing through soil at the same depth. Farming equipment also contributes to soil compaction especially when it weighs more than 5 tons.
Irrigation and soil drainage can cause soil acidification and salination whilst the use of chemical fertilisers and pesticides contributes to reducing soil capillarity (runoff) as well as its consistency. Irrigation in the Aral basin caused the salination and flooding of soils (this can be attributed to canals not being covered and bad drainage). It also famously caused the Aral Sea to dry out.
By threatening the productive capacity and/or reproduction of vegetation, overgrazing strips soils thus making them more vulnerable to hydraulic erosion (this is the case for 56% of soil degradation) and wind erosion (28% of cases).
In urban areas, pollutants such as heavy metals which are dumped can affect soils, just as old industrial sites sometimes leave behind waste land with severely polluted soils.
According to Lester Brown, a third of all cultivated land is losing its arable layers faster than it is gaining them and this lessens its productivity. Brown noted that one of the factors that can explain the collapse of certain civilisations, like the Mayas, is the erosion of land which leads to less food being produced.
Soil transformation leads to a change in water quality, especially when soil particles are washed into waters thus increasing their turbidity and their phosphorous and nitrogen content. [See debate]
Using pesticides and chemical fertilisers destroys soil fauna which is necessary for aerating soil. The well-known consequence of this phenomenon is soil runoff which causes floods and mudslides.
Wind erosion which has been worsened by overgrazing and tillage in some cases, causes dust storms like in the USSR in 1960 or in Africa where 2 to 3 billion tons of soil particles leave the continent every year, thus gradually wearing away the soil’s fertility.
By changing the composition and structure of soils, agriculture makes it more difficult for CO2 to be stocked in soil. Indeed, the conversion of meadows, forests and peat bogs into crops significantly reduces the amount of pedological carbon storage in soil.
As well as the loss of biodiversity (climax vegetation and environmental habitats) which comes with soil degradation and regression, there are also effects on climate change, especially through changes in the albedo on a local level and greenhouse gas emissions such as methane and nitrous oxide (soil asphyxiation).
– Brown, L. (2001) Eco-Economy: Building an Economy for the Earth. W.W. Norton & Co., New York
– Scherr, S. J. (1999) Soil Degradation. A Threat to Developing-Country Food Security by 2020? International Food Policy Research Institute, Washington, DC
– Cummings, D. (1999) What is soil degradation? Department of Sustainability and Environment, Victoria, Australia