A great new way of dishing the dirt

Most visitors to the BRE Innovation Park are probably there to see the homes. After all, the first house to achieve the top level of the Code for Sustainable Homes is on the site. But, although visitors may not know it, plenty of eco-planning has gone into the ground beneath their feet as well. All the earth around the show homes was carefully landscaped to help BRE reach that coveted top rating. Sustainable drainage systems and biodiversity were an integral part of the grounds’ green credentials, but the foundation of all these efforts was the very earth itself: topsoil.

Topsoil is the upper layer of earth which nourishes plants and holds their roots and plays an important role in draining water. But despite its importance it is discussed relatively little. The original topsoil at the BRE Innovation Park was nutritionally poor for plant life and was also full of remnants of concrete foundations and building rubble. Under pressure to sort the site out quickly in time for the opening of the Offsite exhibition in 2007, landscape designer Macfarlane Wilder bought in new topsoil made by British Sugar from soil washed off their sugar beet. It’s a recycled product and therefore gains credit under the Code for Sustainable Homes.

Demand for good topsoil has been increasing because part 2a of the 1990 Environmental Protection Act, which came into force in 2000, says that topsoil should be tested to meet the requirements of local authorities and warranty provider the NHBC.

There are several options for dealing with the existing topsoil on a site if it is found to fall short of requirements.

Replace topsoil

One option, particularly in cases of heavy contamination or when a fast completion is necessary, is to buy in completely new topsoil. Naturally occurring topsoil would be suitable, but may be difficult to obtain, as it only becomes available when a large greenfield site is developed. It may also have to be transported a long way to reach a development so it’s not always the most environmentally friendly choice.

An increasingly popular alternative is manufactured topsoil. This is usually made up of a mineral component, like sand, plus organic ingredients, such as green compost from garden waste. Other elements can include cleaned subsoil, river dredgings, used mushroom compost, cleaned sewage particles, or bark chips. “It often ticks the sustainability box because one or more components are recycled. It should also have consistent quality and can be produced all year round,” says Tim O’Hare, partner at landscape consultant Tim O’Hare Associates.

Some companies sell demolition waste that has been sieved and blended with organic components, like compost. “It is of variable quality and has been overtaken by manufactured topsoil,” says O’Hare. “Some can be very alkaline or saline.”

Enrich existing topsoil

Often topsoil on site can be supplemented with other ingredients, like compost or gypsum, to enrich it or open it up. “The cost savings are phenomenal because it doesn’t have to go to landfill and you don’t have to bring in topsoil,” says O’Hare. “Bringing in compost and blending it on site is 25- 50% the cost of disposing of the spoil and importing new topsoil. That’s something the housing industry isn’t really aware of.”

Not all soils can be turned into good topsoil – sticky clays are often unsuitable – so it’s important to get an analysis of the existing soil before deciding whether to enrich it or choose to replace it. Spoil that is to be re-used as topsoil needs to be kept separate from rubble and there will need to be adequate storage for it.

Even soil on some contaminated sites has the potential to be cleaned and reused.

Bioremediation

There are various ways of using the power of nature to break down pollutants in processes known as bioremediation. Some trees and plants can absorb hydrocarbon, nitrate or phosphate contamination, for example, and dropped leaves add organic matter to the soil which helps microbes to break down some pollutants. Petrol and paints evaporate and break down when exposed to UV light.

Mobile and factory-based soil washing plants offer services to separate out contaminants, although they may not be able to remove all pollutants. If it is beyond rescue, heavily contaminated soil can be dug out and sent to a suitably certified landfill site. But this should be a last resort. In general, as long as the pollutants are not likely to move or get into water, they can be sealed in the ground and left. “You have to decide if the cost of clean-up means it is cheaper to bury the soil in landfill. We tend to go with bioremediation of large sites and let time deal with it,” says Peter Wilder, director of Macfarlane Wilder. “There’s almost always a way of improving what you have on site.”