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Soil turns paved paradise into urban jungle


An engineered soil which helps trees thrive in a concrete environment could help ease urban pollution problems (Image: iStockphoto)

The 'paved paradise' made famous by folk singer Joni Mitchell may soon be sprouting greenery thanks to an engineered soil created by US researchers to combat urban pollution. The engineered soil is made with natural and locally available materials. It can filter stormwater as well as provide a better soil bed for trees, which offer shade, scrub the air of emissions, reduce ambient temperatures, and intercept rainfall. "Paved surfaces account for up to 20-40% of a city's surface," says Dr Greg McPherson, director of the US Forest Service's Center for Urban Forest Research, at the University of California, Davis. "How do we green up these impervious surfaces?" McPherson and colleague Dr Qingfu Xiao, a research water scientist, are working together on improving water quality as part of a larger project to minimise run-off from pavement. The work involves Cornell University and Virginia Tech. The answer, they say, lies with trees. A 12-15 metre maple or oak tree can soak up about 190 litres of rain in its canopy alone, preventing the water from becoming run-off. Plus trees provide shade and take in carbon dioxide and other air pollution. Unfortunately, conventional soils and methods for tree planting reduce their effectiveness. When building a car park or footpath, about 60-90 centimetres of the topsoil is scraped off and the remainder is compacted and filled in with layers of rock and then asphalt. Although the compacted soil keeps out moisture and provides good support for the pavement, it works against a healthy Trees, which are typically planted in soil islands throughout the car park, cannot stretch their roots into the dirt, or likely reach their full canopy potential. The hard soil also promotes contaminated run-off and other pollution. McPherson and Xiao's solution involves a soil mixture that is 70% stone and 30% clay loam. The stone is a very porous, lightweight lava rock found in the nearby Sierra Nevada range. In laboratory tests, Xiao found the so-called Davis soil filtered out at least 50% of nitrogen and phosphorous from run-off water and removed about 75% of heavy metals. They also showed that if the soil was used to plant a tree in a hole that was 1.2-metres deep by 1.2-metres square, it could collect and filter 568 litres of water on top of the 190 litres a tree crown can naturally capture and store. The Davis soil can be used for more than planting car-park trees. It can also be deposited under footpaths, and used as a small soil reservoir alongside paved areas. The soil reservoir can catch rainwater run-off and filter it before it seeps into the subsoil and replenishes the groundwater. "The beauty of that is that you are much more closely mimicking the natural system," says Dr Susan Day, research assistant professor of urban forestry at Virginia Tech and lead researcher on the project. "The tree is a functioning part of the stormwater system management instead of just an add-on." The vision, says McPherson is for landscape architects and engineers to eventually think of the soil and the trees as mini reservoir systems designed to filter a quantifiable amount of water and air. "We will have more success in arguing to have adequate space for trees, for the soil volume that they need to grow to fulfill their potential and improve the quality of life in our cities," he says.

Tags: environment, science-and-technology, engineering