It is now widely accepted by green building professionals that healthy trees play a significant role in successful green infrastructure systems in urban environments. Trees provide a wide range of benefits to society at-large and to the urban dweller in particular.
Healthy trees reduce the urban heat island effect, help manage stormwater, reduce air pollution, and provide shade that lengthens the life of building materials. Trees also serve an important purpose in the design of urban spaces and ultimately in the creation of healthy, purposeful, and aesthetically-pleasing places for human interaction.
Unfortunately, far too many urban projects across the globe are subject to entirely inadequate tree planting practices, resulting in failed or diminished results. These commonly accepted practices often lack the proper improvement of inherently poor soil, and fail to adequately address the impaired surrounding substratum, as these are the site conditions typically encountered in urban landscaping projects. Inadequate mitigation of these two important aspects in the root zone typically results in trees with less than optimal health, i.e.: severely restricted growth and habit, shortened life spans and costly re-plantings.
Soil quality, also called soil health, is simply defined as: how well the soil performs and “does what we want it to do”. Specifically, soil quality is the capacity of a particular type of soil to function, within managed or natural ecosystems, to sustain plant and animal life, maintain and enhance water and air quality, and to support human health and habitation. Understanding soil quality involves the proper assessment and management of soil so it functions optimally now, and is not degraded for future use.
There are a number of factors that characterize typical urban soils and whereby reduce their productivity, such as: little or no organic matter, human artifacts that disrupt water movement, elevated salt content, interrupted nutrient cycling and modified micro-organism activity, higher pH values resulting from the introduction of cement, plaster and road salts, and compacted soil layers.
Trees need significant volumes of low-compaction soils, with suitable pore space, drainage, and organic matter to provide for long-term growth. A good rule of thumb for preserving older trees and assuring the success of new plantings in urban environments is to use large amounts of loam soils; comprised of varying percentages of sand, organic compost, and clay silt. The ratio of the ingredients listed above may vary dramatically depending on the requirements of specific plants, identified stormwater management objectives, intended land use, and the natural or man-made constraints of a particular site.
An attempt to resolve unfavorable growing conditions that arise from existing soil conditions, and address the necessity for structural support in typical urban streetscape situations, Cornell University developed Structural Soils in mid-1990. Structural soil is a mixture of stone aggregate and soil, with a small amount of polymer gel to hold the mix together. This mix can be compacted to 95% dry density and support paving and yet allow for some tree root growth. The mix typically consists of: 20% loam soil, 80% angular gravel with no fines, and .03% polymer gel. However, structural soils have limited success for good tree growth. The main reason for this is the lack of actual soil content in the structural mix. Trees will not grow any larger than the volume of loam soil in the mix. To create one cubic foot of usable soil under a streetscape or sidewalk, approximately five cubic feet of structural soil must be installed. This requires significant space (not usually available in urban areas) and substantial budgets (also not widely available in today’s world economy) to achieve the necessary soil volumes for healthy plant growth.
Structural Soil Cells
A relatively new and innovative approach to the problems associated with urban tree planting is the use of structural soil cells. These modular, pre-engineered structural cell systems, particularly those manufactured by Greenleaf are designed to accommodate irregular urban conditions and provide 95% of the space within the cells for tree-rooting soil. The structural cell provides for a “tree pit system” that allows for improved underground spaces for quality soils, optimum tree root volume, and superior structural support for urban hardscapes.
Structural soil cells are constructed of recycled thermoplastic material that is relatively lightweight, extremely durable, and non-toxic. The use of these materials reduces environmental impacts because they simply require less embodied energy to produce, transport and install. Recycled plastic soil cells are also more cost-effective than other systems due to lower production and shipping costs.
The structural cells also serve an important function in stormwater management because they allow water to move more freely through the root zones, and ultimately into the aquifer, as opposed to running on the surface and into overburdened urban stormwater management systems. These cellular systems are proving to be much more efficient and cost-effective than previous approaches.