Soil, and especially agricultural soil, is one of our most valuable non-renewable resources. We depend on it for food, for fibre that helps produce clothing, and even for fuel. Even so, we continue to lose farmland and soil to non-agricultural uses like urban sprawl. One of the biggest impacts of urban sprawl is soil sealing.
Soil sealing is defined by the Food and Agriculture Organization of the United Nations as the “permanent covering of the soil surface with impermeable artiﬁcial materials, leading to non-reversible loss of soil and most of its ecosystem services” (FAO, 2016). For instance, when a subdivision is built and asphalt is put down for roads, sidewalks, or parking lots, the area under the pavement is effectively sealed off from the world above. Not only is the total area of sealed soil increasing worldwide, but the sealing is often done on some of the most fertile farmland in a region (European Commission, 2019; Burghardt, 2006). For instance, the Greater Golden Horseshoe in Ontario is one of the fastest-growing regions in the country and it also contains some of the most fertile farmland in Ontario (Greenbelt Foundation, 2019). This means that we are rapidly losing some of our best farmland to the irreversible process of soil sealing.
When we lose farmland to urban sprawl, and thus to soil sealing, we are losing so much more than the ability to produce food and fibre. We also lose all of the vital ecosystem services that both soil and farmland provide us.
When it rains soil can absorb the rainfall, before slowly letting the rain seep into the groundwater. In comparison, hard surfaces like asphalt do not absorb water at all. This lack of absorption is why runoff from rainfall in developed areas is greater and must be managed with specific infrastructure to prevent damage. For instance, subdivisions have storm water retention ponds because the asphalt is not able to absorb the rainfall. Without ponds to contain the water that is not able to seep into the soil because of soil sealing, rain events would quickly cause floods in nearby streams and rivers that would be overloaded by the water (Clemson University, 2020; Burghardt, 2006). To illustrate this relationship, a study by Nirupama & Simonovic (2007) tracked the risks from floods in London, Ontario over a period of 35 years, and found that there was a strong relationship between the amount of urbanization and soil sealing in the area and the risks of floods. Additionally, when rainfall is absorbed into the ground in natural environments it helps to replenish the supply of groundwater, which many communities also depend on for drinking water. When soil sealing occurs the water is diverted and is unable to enter the groundwater supply, so the supply of groundwater is gradually being diminished (European Commission, 2012).
Another service that we lose when soils are sealed is the potential for carbon sequestration within the soil. Carbon sequestration is when carbon is locked in soil through the accumulation of organic matter like decaying plants, thus preventing it from entering the atmosphere (Ontl & Schulte, 2012). This process is more important now than ever because of the role that carbon plays in climate change. Where soil sealing occurs, it is no longer possible to sequester carbon because organic matter is not able to enter the soil, and we lose the possibility to sequester carbon in that area forever.
Once we lose farmland to soil sealing, we can never get it back.
We need to make sure that our farmland is protected, so that we don’t continue losing one of our most precious resources to this irreversible process. When we protect farmland, we are protecting our soils, our local food system and economy, and our environment for the benefit of our future generations.
Burghardt, W. 2006. Soil Sealing and Soil Properties Related to Sealing. Geological Society London Special Publications. 226(1):117-124.
Clemson University. 2020. Stormwater Pond Design, Construction and Sedimentation. Retrieved from https://www.clemson.edu/extension/water/stormwater-ponds/problem-solving/construct-repair-dredge/index.html
European Commission. 2019. Soil Sealing. Retrieved from https://ec.europa.eu/environment/soil/sealing_guidelines.htm
European Commission. 2012. Soil Sealing. Retrieved from https://ec.europa.eu/environment/archives/soil/pdf/sealing/Soil%20Sealing%20In-depth%20Report%20March%20version_final.pdf
Food and Agirculture Organization of the United Nations (FAO). 2016. Soil Sealing. Retrieved from http://www.fao.org/3/a-i6470e.pdf
Greenbelt Foundation. 2019. The Greenbelt. Retrieved from https://www.greenbelt.ca/learn
Nirupama, N., &Simonovic, S. P. 2007. Increase of Flood Risk Due to Urbanisation: A Canadian Example. Natural Hazards. 40(25).
Ontl, T. A. & Schulte, L. A. 2012. Soil Carbon Storage. Nature Education Knowledge 3(10):35