Structural Soil VS Soil Cells: What’s The Better Choice?

With trees come their roots – and it’s something that needs considered when planting trees in an urban setting.

How can we ensure the roots of your urban trees will grow well under a sidewalk and not impact the utilities and services of nearby infrastructure?

What’s the best way to ensure enough soil for tree planting, even when it’s located in the city?

For years, people have been relying on structural soil to help trees mature in an urban environment.

However, recent developments of soil cell products offer city trees unmatched growth potential in a shorter time frame.

To help you identify the ideal option for your city, here’s a quick comparison guide about soil cells and structural soil & sand.

What is Structural Soil?

Structural soil is a commonly used soil medium for tree planting that’s compacted under a pavement system to give structural support, creating only small void spaces for tree roots to grow. It’s made up of 80% gap-graded levels (crushed stones) and 20% soil.

People often think that structural soil and soil cells are in the same league. However, they differ in the number of benefits they provide urban tree growth.

So what does this mean?

Structural soil, consists of crushed stones or larger objects to help provide structural stability for the above urban load such as pavements and roads. So, structural soil leaves fewer minerals, void space and nutrients available to the tree as only 20% of the available ‘soil’ is actually useable by the tree.

As a result, urban trees planted in this type of soil often experience weak roots and restricted growth due to the limited availability of nutrients and inadequate space for root expansion.

Read our Full Article Here: What is Structural Soil?

What are Soil Cells?

Soil cells are purpose-designed urban landscape solutions that provide trees with optimal soil conditions for growth without compromising local underground infrastructure.

Stratavault, specifically designed for easy installation and load-bearing capacity, ensures stability under pavement, parking lots, and roads while offering ample space within its interconnected matrix design for uncompacted, high-quality soil to deliver healthy tree root expansion.

The matrix also allows the integration of underground services, making it a comprehensive solution for sustainable urban development.

Related: How to Install Soil Cells

An additional benefit of soil cells is their integration with blue infrastructure, including stormwater management ecosystems.

Stormwater can be directed into the treepit, providing on-site water supply to the tree. The tree and soil work together to naturally filter the water, removing unwanted contaminants such as road pollutants.

Any excess water can be directed to a holding vault for later use, creating a circular environment of water use (Case Study), or time-gated for release into government utilities at a later date to help with capacity during high storm level events.

This integrated approach not only promotes sustainable water management but also enhances the overall environmental resilience of the urban landscape & the targeted water also helps to accellerate tree canopy growth by feeding water directly to the tree roots.

Related: Sustainable ways to Manage Stormwater

What are the Advantages of Soil Cells Compared to Structural Soil?

The use of soil cells in urban environments offers an array of advantages, with one of the most prominent being the substantial increase in “void space” allocated for high-quality soil to support tree growth. This concept of “void space” refers to the open areas within the soil cell system.

The soil cell matrix is designed to take the above weight load of urban infrastructure such as roads, pavements, carparks, etc off the tree and soil which reduces the overall stress on the tree and soil.

As the soil vault matrix bears the weight, we can fill the void space with uncompacted high quality soil specifically tailored to meet the unique needs of the trees.

Here’s an explanation on why this increased void space is so advantageous:

1. Enhanced Soil Quality: Soil cells provide an opportunity to replace this subpar on-site soil with a high-quality growing medium. This soil is carefully selected or engineered to provide optimal conditions for root development, water retention, aeration and nutrient availability.
   
2. Ample Root Space: Trees need space to spread their roots to establish a stable foundation. The increased void space in soil cells allows for extensive root growth, enabling trees to access the oxygen, nutrients and water they require to thrive. Lack of soil compaction also helps accelerate tree growth as less energy is needed for tree roots to push through the soil as it explores for new sustenance. This will create healthier, resilient trees that are better equipped to withstand urban stresses.
   
3. Improved Longevity: Space is a huge complication when planning trees in urban spaces. The lack of space allocated to trees is one if the main factors in their high mortality rate in urban areas. Soil cells treepits are designed with the trees maturity in mind. This way, tree roots will have sufficient soil and space to grow for decades.
   
4. Stormwater Management: Soil cells also play a crucial role in stormwater management. The void space allows rainwater to percolate into the soil rather than becoming surface runoff. This not only replenishes the tree’s water supply but also reduces the risk of flooding and soil erosion in urban areas.

Citygreen participated in a study called, “Comparison of Soil Treatments Under Concrete Pavement”.

The research examined the capabilities of structural soil and soil cells in providing root growth space for trees in urban areas.

The study found that soil cells are a superior choice in city environments because of the following advantages:

• Uncompacted soil within soil cells grows the largest and healthiest urban trees in a shorter timeframe.
• Soil cells enable maximum root spread and depth.
• Soil cells support the weight of tree parts.
• Soil cells ensure the moisture content of the soil is maintained.

Video Comparison: Tree Growth of Structural Soil Cell vs Structural Soil Planting Methods

Below is a video from our Mill Woods parking lot case study which highlights the growth difference of trees planted at the same time.

What are the cost differences between Structural Soil and Soil Cells?

The cost difference between structural soil and soil cells can vary based on several factors, including project size, location, product choice, and installation requirements. Generally speaking, here are some key cost considerations for both options:

Structural Soil:

• Structural soil involves blending soil with aggregates like crushed stone, large rocks or gravel to create a load-bearing medium.
• The cost of structural soil includes soil materials, aggregates, transportation, and labor for mixing and installation.
• Usage soil available to the tree is vastly lower.
• Excavation and preparation of the planting area may be necessary, adding to the overall cost.
• Initially, Maintenance costs are lower, but you leave yourself open to tree roots exploring and damaging surrounding infrastructure which becomes costly over the lifetime of the tree.

Soil Cells:

• Soil cells are modular systems that provide high quality uncompacted soil volumes for tree roots within a matrix that can be designed to site specifications.
• The cost includes the price of soil cells, soil, installation labor.
• Excavation and site preparation more extensive compared to structural soil.
• Lifetime cost to the tree and surrounding infrastructure is lower as tree roots are contained and reduce pavement heave.

While structural soil may seem cheaper upfront, planting a tree in a pit that isn’t right for its surroundings can lead to higher maintenance costs over time. If we don’t carefully match the tree’s needs, soil conditions, and space, its roots might struggle to grow. This can mean slow growth, a smaller canopy, and more susceptibility to pests and diseases, all requiring expensive remedies.

And that’s not all – surrounding infrastructure like sidewalks, roads, and utilities can suffer when roots can’t spread properly. This can lead to cracks, upheaval, and damage, which ends up costing more to fix than what you saved with the initial lower cost of structural soil.

Investing in systems like soil vault systems such as stratavault that support healthy root growth will pay off in the long run. These designs offer enough space for roots to access water and nutrients, promoting strong tree development. The result? Less upkeep, reduced infrastructure issues, and a longer, healthier life for the tree which will bring it’s benefits to the community quicker.

Aspect	Structural Soil	Soil Cells
Tree Growth	Stunted growth, reduced canopy development, susceptibility to pests	Robust growth, healthy canopy, improved pest resistance
Tree Health	Increased stress due to lack of soil, Vulnerability to diseases, increased need for interventions	Enhanced overall health, strong root development, reduced need for interventions
Infrastructure Impact	Upheaval of Sidewalk, road, underground utility damage due to root pressure and growth	Reduced infrastructure damage, minimized maintenance costs to city infrastructure
Maintenance Costs	Frequent pruning, fertilization, pest management, Early Replacement of trees	Lower maintenance costs, fewer interventions
Longevity	Shorter lifespan due to compromised health from lack of nutrient soil	Longer tree lifespan, sustained ecosystem benefits
Initial Savings vs. Long-Term Costs	Lower upfront cost savings overshadowed by expanded maintenance expenses	Higher upfront investment justified by long-term cost-effectiveness
Supporting Systems	Lack of proper systems to facilitate root growth and support.	WSUD systems ensure healthy root development and sustained growth
Overall Benefit to Urban Environment and Sustainability	Negatively impact urban green spaces and ecosystem services	Positively contribute to urban environment, stormwater management

The cost difference will depend on project-specific factors. It’s recommended to consult with our landscape professionals to get accurate cost estimates tailored to your project’s needs. Additionally, consider the long-term benefits of each option, such as improved tree health, stormwater management, and urban resilience, when evaluating their cost-effectiveness.

For years, Citygreen has invested in research and innovative development initiatives to explore the challenges, the causes of failure and the reasons for the premature mortality of urban trees.

Through it, we created effective soil vault systems that serve as the most viable solution to help build greener cities.

Download for free an extensive report on Comparison of Soil Treatments Under Pavements here.