Soil degradation and artificialization

Soils are fragile environments, sensitive to various degradations that can alter their fertility. The most fertile lands are the first to disappear in favor of human constructions. The erosion of intensely plowed soils is more than ten times the rate of their formation. Multiple forms of pollution affect most soils and organic matter levels are low in areas of arable crops and wine-growing lands.

From a quantitative point of view, soils are decreasing in surface and in depth. On the surface, due to the artificialization of the land , developed later in this report (see resilience pathway n ° 2 ). cause of soil erosion, that is to say the progressive loss of small particles, carried by the wind or the rains. The losses reach in some regions several tons of soil per hectare and per year, ie a rate more than ten times greater than the rate of soil formation by alteration of the bedrock .

Agricultural plot undergoing a phenomenon of accelerated erosion by runoff of rainwater. Erosion is aggravated by frequent plowing and the absence of plant cover. Crédits : © Thibaut Lorin.

Extreme weather events (floods, droughts, strong winds, etc.), whose frequency and intensity are increasing, accentuate the phenomenon of soil erosion. Conversely, this erosion worsens climate change by releasing carbon stored in the soil.

From a qualitative point of view, many types of pollution affect agricultural soils and affect their productivity ,: pesticide residues, plastic microparticles, hydrocarbons, heavy metals, etc. Pollution of industrial origin is often localized and can cause subject to monitoring and remediation: 6,800 sites have been identified in France. Other more diffuse pollution is widespread, and their potential impacts on biodiversity, soil fertility or human health are sources of concern.

In addition, the quality of agricultural soils depends on other parameters such as their structure and their organic matter content. Compaction of soils by the regular passage of agricultural machinery reduces their productivity and increases the risk of erosion and nutrient losses. The richness in organic matter promotes the biological activity of soils, the retention and gradual release of nutrients, the infiltration and storage of rainwater, and therefore the maintenance of soil fertility over time. A 2001 study estimated at around 40% the share of arable land lacking in organic matter in France. The regions concerned are mainly the plains of arable crops and wine-growing lands ( Figure 6 ).

Figure 6 : Organic carbon stocks in the top 30 centimeters of soils in mainland France in tonnes per hectare. A soil can be considered as deficient for values lower than 50 tons per hectare (about 2% of organic matter). The poorest lands are the areas of arable crops and wine regions. Source : Gis Sol (2011)

Background degradation : decrease in high quality arable land, decrease in soil fertility, impacts on human health

Soil degradation is reflected above all in a gradual deterioration of its fertility. However, degraded soils are more sensitive to extreme weather events and can increase the risks associated with floods, storms or droughts.

Crisis situations : worsening consequences of extreme weather events

Dust storm in the city of Stratford (Texas) in 1935 during the "Dust Bowl". Droughts that affected degraded soils in the southern Great Plains of America caused a serious agricultural and social crisis. Credits: NOAA George E. Marsh, public domain.