The Importance of Quality Soil for Healthy Trees in Urban Environments

By: Richard J. Magill, Magill & Associates, Inc. & Simon Leake, Principal/Director of Operations SESL Australia

It is now widely accepted by green building professionals that quality soils and 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.

Why We Need Quality Soil in Urban Centres

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.

Because individual project and site conditions vary so dramatically, it is often necessary to adjust the soil ingredient mixture accordingly. Simon Leake, principal of Sidney Environmental Soils Laboratories (SESL) in Australia, favors a “Logical Approach to Soil Specification”. This approach promotes the idea that performance-based specifications are more effective than “recipe-based” soil specifications. Leake’s laboratory specifies the pH value, nutrient content, CEC (cation exchange capacity), OM (organic matter) content, permeability and whatever other properties they think necessary to compose a project-appropriate soil, rather than rely on a set recipe. These “fit for purpose” soils will optimize the water, nutrient and aeration properties to be consistent with the intended use. However, Mr. Leake cautions: “This is always a compromise between availability, cost and intended use.”

A commonly used field reference for planting soil is: two cubic feet of loam soil for every one square-foot of tree canopy (at tree maturity). Simply stated, a mature tree that provides 400 square-foot of canopy should be planted with 800 square feet of quality soil. Evidence of the benefits of using quality soil can be found in a study by the Bartlett Tree Lab. This study demonstrates that the quality loam soil grows trees that have 300 times more leaves and are 1.7 times taller than those grown in compacted soils.

Developing effective soil is a science, and it should be customized to meet individual site-demands, but it is important to point out that certain traits are universal for all quality soils, regardless of the application. These traits should include: a relatively high percolation rate to avoid prolonged surface ponding, a large capacity to sequester pollutants as water moves through it, and the nutrients necessary for vigorous plant growth.

What Role Does Soil Play in Maintaining Healthy Trees in Cities?

Soil quality plays a crucial role in maintaining healthy trees in urban environments. Trees rely on soil not only as a physical support for their roots but also as a source of essential nutrients, water, and oxygen. In urban areas, soil is often compacted due to construction, foot traffic, and other urban infrastructure activities. Compacted soil lacks the necessary pore spaces for water infiltration and root growth, which can lead to poor drainage and limited root expansion.

Healthy soil in general should have a balanced composition of minerals, organic ‘loam’ matter, oxygen, and microorganisms. This ensures the tree has proper water retention, aeration, and nutrient availability, all of which contribute to tree growth and vitality. Quality soil allows tree roots to spread, anchor securely, and access essential nutrients and water, thereby supporting overall tree health.

Without proper soil conditions, trees in urban areas may struggle to establish themselves, leading to stunted growth, reduced canopy size, and increased susceptibility to pests and diseases. Addressing soil quality is vital to ensure the long-term success of urban trees, as healthy soil directly impacts their ability to thrive amidst the challenges posed by urbanization.

Why Do Urban Trees need Different Soil Considerations than Rural Trees?

Urban trees require different soil considerations compared to rural trees due to the unique challenges posed by the urban environment. Here are several key reasons:

1. Soil Compaction: Urban areas often require a high level of soil compaction to support the weight of foot traffic, construction, roads, buildings, and heavy machinery. Compacted soil lacks the necessary pore spaces for air, water, and root movement, hindering root growth and nutrient absorption. leading to a poorer performing tree. Urban trees need uncompacted loamy soil that can promote healthy root development.
2. Limited Soil Volume: Urban trees are typically planted in small spaces like sidewalks, medians, and plazas. This limited soil volume restricts root expansion, making it essential to provide soil that can efficiently support root growth within confined areas.
3. Environmental Stress: Urban environments have higher pollution levels, heat island effects, and variable microclimates. Trees must cope with these stressors, requiring soil that can buffer against temperature extremes, filter pollutants, and provide optimal water and nutrient availability.
4. Infrastructure Interference: Underground utilities, pipes, and structures in urban areas can impede root growth and access to nutrients. Soil needs to be carefully selected and managed to accommodate these challenges while supporting tree health.
5. Stormwater Management: Urban trees play a vital role in stormwater runoff management, especially as Water Sensitive Urban Design (WSUD) becomes more integrated. High-quality soil combined with urban trees offer an effective method to retain water within the environment, purifying pollutants and particles from the air and water produced in urban areas.

Improving the Surrounding Substratum

The other important aspect of proper tree planting in an urban context is the condition of the underground areas (substratum) surrounding the immediate “tree pit” or “tree trench” location. Trees, and all other landscape plants for that matter, require a certain amount of lateral space underground to accommodate the biological needs of the plants for optimum root volume. Also necessary in urban streetscape situations is structural integrity and support.

The need for adequate space and soil for healthy root growth and the need for various hardscaped areas, such as sidewalks, streets, and malls, are typically at odds because paved areas demand a certain compaction below them to function properly and compacted soil severely restricts root growth. The use of quality soils is critically important for healthy trees, but without sufficient un-compacted, lateral space the root zone will be severely restricted and poor tree growth will result. The use of structural soil cells (discussed below) provides space for root growth in quality soil and the strong structural support necessary for successful urban infrastructure projects.

Related: 5 Reasons Why Soil is so Important for Trees

There have been a number of projects over the years that have utilized the “suspended pavement” approach to address the problems of poor soil and impaired substratum by constructing concrete “post and beam” structures in excavated areas and backfilling with quality soil, thus providing for structural support and adequate root growth. Unfortunately, these projects have proven to be somewhat cumbersome and quite costly.

What are Structural Soils?

In an attempt to resolve unfavorable growing conditions that arise from existing soil conditions in urban environments, 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.

Related: What is Structural Soil?

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 to Improve Quality Soil in Cities

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 Citygreen®, 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.

Conclusion

Quite often trees are planted without quality loam soil and are “squeezed” into places that lack the proper space to develop healthy root structure. In the past, designers and engineers failed to address this problem and simply hoped for the best, not knowing the actual biological and physical requirements of the plants. This of course resulted in multitudes of failed tree plantings across the world and unfortunately continues in some areas.

See Related: How to Plant Trees in Urban Areas

Simon Leake of SESL points out that there have been a number of advances in soil technology in recent years, including:

• “Gap-graded” soils that cope with compaction issues from increasing population and resulting uses. “Gap-graded” means that in the particle size grading there is a “gap” where no or few particles of a certain size exist, whereby insuring that there is sufficient pore space in the soil to resist compaction from pedestrian or other traffic. These soils are specified by the United States Golf Association for heavily trafficked turf areas, such as greens and tees, and are increasingly employed by sports field designers and urban parks, because they have better water and nutrient-holding capabilities, and improved compaction resistance.
• Less use of “structural soils”. Even though these “soils” continue to have their applications, they only contain 20% of useful soil for root zone volume. Structural soil cell systems like those manufactured by Citygreen, are much more effective for optimal tree growth in urban situations. Soil cells allow for the use of quality, non-compacted loam soils, and provide the root-zone space necessary for vigorous tree growth.
• Improvement of urban stormwater runoff management by using sandy filtration-type soils to improve water quality for irrigation and other uses.
• Promotion of stormwater harvesting through the use of gap-graded soils that filter the runoff prior to discharge or storage. Filtered stormwater can be directed from street drains and culverts to tree pits whereby significantly reducing irrigation and maintenance.

Today, green urban design and building professionals better understand the need for quality soil and improved substratum for healthy tree growth in difficult urban environments. But even armed with this knowledge, trees are often planted improperly due to design constraints, budgetary issues, and substandard installation practices. It is important that design guidelines that require the use of site-specific quality soil and innovative structural support systems be considered and adopted into the local building codes to insure that the use of appropriate materials becomes standard operating procedure for urban landscaping projects. Increased diligence by all of the key decision-makers in the approval, design, and installation of urban infrastructure will promote the use of quality soils and innovative support structures to insure the future long-term success of our urban forests.

About the Authors

Richard J. Magill, MLA, ASLA, LEED Green Associate
Magill & Associates, Inc. Landscape Architecture and Planning
Aspen, Colorado 81611

Simon Leake, B Sc (Ag) ASSSI
SESL Australia Principal/Director of Operations
Thomleigh, NSW 1715