Underground in the city

Most people living in large towns and cities probably give little thought to soil. Why should they? At a first glance, much of the ground in towns and cities is sealed with concrete, asphalt and bricks, and most city-dwellers have little reason to have contact with soil. To most, soil in cities is simply dirt. But soil is actually in abundance in cities: it lays beneath the many small gardens, flower beds, road and railway verges, parks, sports grounds, school playing fields, and allotments of the city, where it plays many under appreciated roles. In fact, almost a quarter of the land in English cities is covered by gardens, and in the United States, lawns cover three times as much area as does corn. Cities are surprisingly green and beneath this green is soil, and it is dawning on city planners that soils of the city actually play a crucial role.

The most obvious role for urban soil, and the place where people have the most intimate contact with soil, is in the many gardens, parks, and allotments that scatter the city. Here, soil supports the plants that adorn our gardens and city parks, and garden lawns, and it provides nutrients and water for the vegetables that city dwellers grow. In fact, millions of people worldwide grow their own food in one way or another in cities, often by necessity, and during the Second World War allotments in British and American towns and cities were a mainstay for food production for civilians threatened by shortages of food. It might be a surprise, but soils of city allotments are often more fertile, and support higher yields, than those of farmland, largely because allotment owners often add substantial amounts of compost to their soil, made up household wastes, which boosts organic matter content of soil.

Soil also helps cities in less obvious ways, such as by soaking up heat, keeping in check the urban heat island, absorbing excess water after intense rainfall, thereby reducing pressure on sewerage systems, and it forms the base of a perfect sports pitch for football, tennis, cricket and golf; for all these sports, the meticulous management of turf and its underlying soil is key to sporting success.

Despite its many roles, soil of the city is under threat. The most obvious threat is the sealing of soil with impermeable materials such as asphalt, bricks, and concrete. Worldwide, cities are growing fast, and as they grow they consume massive areas of fertile farmland and its underlying soil. The scale of urban expansion is enormous. Across Europe, for example, upward of 500 square kilometers of land are sealed by asphalt, bricks, and concrete each year. To put this into perspective, this is an area roughly half the size of the city of Berlin. And in Germany alone, around 27 hectares of land are sealed every day, which is roundly the size of 30 football pitches. The consequences of this sealing for soil are dramatic: it prevents plants from being able to grow and abruptly ends the many functions that soils perform, such as the recycling and storage of water or nutrients, and the exchange of gases between the land and atmosphere. Soil sealing effectively suffocates the soil.

Another big problem for city soils is contamination with pollutants from a long history of industrial activity. These pollutants, which can include a cocktail heavy metals, organic pollutants, and even asbestos, can pose a significant health risk, especially in the vast areas of derelict land where industry once flourished. These areas, often termed brownfield land, can be considerable; in England, there is some 65,000 hectares of brownfield land in our towns and cities, which is roughly the size of 100,000 football pitches, and in Germany this figure is almost double. Cleaning up this land for housing and new businesses is a major priority for city planners, but it is not an insignificant task. But much can be done. The 2012 London Olympics, which took place on what was derelict industrial land contaminated with heavy metals, solvents, organic toxins, and rubble, is an excellent example what can be done. Restoring this land required an enormous soil cleaning operation, involving ‘soil hospitals’ and several soil washing plants to test, treat and recycle contaminated soil. And now, a once derelict industrial wasteland is a vibrant and sustainably landscaped area of London supported by clean and healthy soil.

Worldwide, cities are expanding rapidly. And with this come the sealing of considerable areas of fertile soil and increasing pressure on unsealed soil as the urban populations grows. City planners are beginning to realise that this is, and will, have severe consequences for the urban environment given the many important functions that healthy soil plays in urban life. I have mentioned a few, such as in helping excess water drain away after storms, in storing carbon and nutrients, and in providing a foundation to grow vegetables and flowers, and play sports. But more needs to be done: urban expansion needs to be more soil friendly. This might be through restricting sealing to poor quality soils, or unsealing and restoring health to sealed soils that are no longer in use. Or it might be through bringing back life to soils of derelict land and encouraging city-dwellers to nurture their soils to grow food, as done so effectively during the Second World War. Education is also important, to inform gardeners how best to build the fertility of soil, and how to reduce the health risks of contaminants that potentially lurk in urban soils.

Next time you walk through a city, don’t just look at the buildings, pavements and roads that cover much of the land. Look also to the ground and the abundance of soil that supports the many gardens, flowerbeds, parks, sports grounds, and allotments of the city. This green space, and many aspect of urban life, depends on healthy soil.

Featured image credit: Southeastern Penang Island Bird Eye view by Marufish. CC BY-SA 2.0 via Flickr.