Urban development fundamentally changes the ways by which water flows through the local environment. Without the application of best management practices to control stormwater runoff and improve its quality, the health of natural ecosystems that urban streams, lakes, and wetlands support declines.
Healthy soil provides important stormwater management functions, including infiltration and storage, adsorption of nutrients, filtration of sediments, decomposition of pollutants, and moderation of stream flows and temperatures. In addition, healthy soil supports vigorous plant and tree growth that intercepts rain, returning much of it to the atmosphere. The health of the soil, vegetation and the rivers, lakes and wetlands they drain to are intrinsically related and these relationships should be better recognized in land development planning and urban construction processes in order to produce more functional landscaped areas.
The challenge of compacted topsoil
Today, conventional construction practices involve mass stripping and stockpiling of site topsoil in large mounds for periods of six months to a year or more until a portion, typically less than 30 per cent of what is stripped, is reapplied to landscaped areas. This process compacts the topsoil, radically changing its structure and water holding capacity, and depletes it of beneficial soil organisms that cannot survive the anoxic conditions experienced in the topsoil mounds. Often, the only living portion of topsoil stockpiles is limited to the top 30 centimetres. This produces stockpiles that are either poor or highly variable in quality. Standard practice is to apply 10 to 15 centimetres of the stockpiled site topsoil to landscaped areas, without taking any measures to reverse compaction of the underlying subsoil caused by construction equipment traffic and storage of building materials.
If best practices to preserve or restore healthy functioning soil in landscaped areas are not applied during construction, changes to soil structure, biology, and organic matter content and the effects of compaction can cause them to function more like impervious surfaces.
This makes the standard practice of directing roof drainage to these surfaces less effective than it could be at reducing urban runoff and contaminant loads to receiving waters. Furthermore, poorer quality planting environments are produced that require more irrigation, fertilizer, and effort to re-establish and maintain vegetation and urban tree canopy. Application of improved soil management practices during construction can reduce stormwater runoff and outdoor water use, produce more lush yards and landscaped areas that are easier and cheaper to maintain, and provide the growing environment needed by urban trees to reach maturity.
With the release of a new best practices guide on preserving and restoring healthy soil, the Toronto and Region Conservation Authority (TRCA) hopes to foster change in construction industry practices and municipal standards. The guide recommends that soil in all landscaped areas should meet minimum standards for quality and uncompacted depth, especially those that receive runoff from adjacent roofs or pavements. In terms of quality, the topsoil should contain at least five to 10 per cent organic matter, a critical component to the water holding capacity and biological health of soil. If topsoil is low in organic matter, it should be amended with compost to meet the standard. In terms of uncompacted soil depth, topsoil should be at least twenty centimetres deep, representing double the amount that is typically applied to yards and parks during construction, and the total uncompacted soil depth should be at least 30 centimetres.
Higher standards are recommended in areas where shrubs and trees will be planted as they need richer and deeper soil to thrive. To make adoption of these practices easier, guidance is also provided on how to develop a soil management plan for your site, including templates forms for planning and field inspection and tools for calculating the quantities of topsoil and compost needed.
Best practices and optional methods to meet the standards are described in stepwise detail. Examples include
- Leaving existing trees, vegetation, and soil undisturbed to the greatest extent possible;
- Stripping, stockpiling, and preserving existing topsoil on-site for reapplication in areas to be landscaped;
- Restoring post-construction soils in areas to be landscaped to meet minimum soil quality and depth standards.
Recommended approaches to restoring healthy soil functions involve reversing compaction through the use of subsoiling or tilling equipment, and incorporation of compost and mulch, to increase organic matter content. Incorporating compost helps reverse the effects of compaction, and adds organic matter. Every one percent of organic matter in a 30-centimetre deep topsoil can hold up to 16 litres of plant available water per square metre, which adds up to substantial volume of water storage capacity if all landscaped areas in a development met the recommended standards. Compost also has soil binding properties, acting like glue which aggregates and holds soil particles together, making it more resistant to erosion. When subsoiling and tilling is combined with compost amendment, studies have shown that the volume of runoff produced by a landscaped area constructed on compacted soil can be reduced in the order of 75 to 90 per cent.
The lawns, gardens, sports fields, parks and open space components of our communities all contribute to how absorbent our urban landscapes are to rain and snowfall. They also contribute to making our cities, towns and villages, beautiful and healthy places to live. Everyone’s yard should be a functioning part of the “treatment train” of stormwater management best practices helping to manage urban runoff. By improving on conventional construction practices and municipal standards to ensure all landscaped areas contain healthy functioning soils, the impacts of urbanization on the local water cycle and the health of our urban rivers, lakes and wetlands could be reduced.
Dean Young is a project manager with the Sustainable Technologies Evaluation Program of the Toronto and Region Conservation Authority.