Consideration of soil formation and function is important in the urban environment where greenspace is to be established. This is because the basic building blocks of a soil, the natural soil materials and soil-forming materials, can be inadequate to be used for vegetation growth. Artificial soil formation involves more than mixing these ‘ingredients’ together – it requires an understanding of how soils are created naturally in order to encourage soil functional processes to act as quickly and effectively as possible.
In addition, purposeful soil creation using materials containing potentially toxic contaminants must take into account their fate whether in the soil or beyond it in the vegetation or water flowing through it. In an urban context, soil formation will usually be driven towards creating a functioning soil type capable of sustaining a particular type of vegetation, and providing it with water, nutrients and anchorage. There is also a need to consider other soil processes in the management of natural soil materials in urban greenspace schemes, in order to maintain soil sustainability and prevent soil degradation.
In the urban context, artificial soil formation will usually depend on four main sources of base materials:
- Mineral materials in the ‘made ground’ on a site, usually a mixture of natural but degraded soil materials, together with waste materials disposed of on the site during industrial use;
- Naturally unconsolidated, or ground, geological materials, e.g. sands and gravels, mining overburden;
- Specific waste materials from industrial processes, e.g. pulverised fuel ash, fines from washing plant;
- Remediated materials following application of in situ or ex situ soil remediation technologies.
All four types are likely to be infertile and depending on specific source or land-use history, may present chemical or physical properties hostile to plants. They may also pose a risk of environmental contamination depending on future use, and, indeed, the soil-forming technologies adopted. To produce a functioning soil using these types of material usually involves their amendment with organic materials to provide additional nutrient resources and promote biological activity, and to help restrict contaminant movement and/or assist in their breakdown.
Effective artificial soil formation depends on good planning and assembling important information. This includes:
- Physical and chemical properties of the proposed soil-forming materials and organic amendment materials, using involving laboratory analyses;
- Understanding of the edaphic qualities of the plant species proposed for planting;
- The risk of loss of potential contaminants, using a combination of leachate tests, bioassay testing and modelling;
- An understanding of the hydrological behaviour of the site where soil formation is to be executed, including assessments of likely risk of surface and groundwater contamination.
Best Practice Guidance
Forest Research has been pre-eminent in studying how to create sustainable, functioning soils, mainly from waste materials, for several decades. It can offer research and consultancy services for those interested in developing functioning soils from similar materials, for use at a national, regional, local or site based scale.