Structural soil is used in the world of urban landscaping, particularly for urban tree-planting projects. With its unique composition, structural soil plays a crucial role in providing a supportive environment for tree roots to thrive in challenging urban environments. 

Structural soil is a specially engineered soil mixture designed for two distinct purposes;

  1. Provide a solid structural foundation for the surrounding urban infrastructure.
  2. Give trees access to nutrient soil in tricky urban environments.

Structural soil is not your typical soil blend— It’s made up of 80% gap-graded materials (crushed stones, etc) and 20% soil. Structural soil is generally compacted under a pavement system like a road or footpath to give structural support, creating minimal void spaces to ensure strength for the above and surrounding infrastructure.

the difference in soil volume in structural soil vs a soil vault

Difference in soil volume between structural soil & a soil vault

As highlighted in the graphic above the 'void space' shows the space available for soil that trees can actually use. When comparing Structural soil to soil vault systems(soil cells) such as Stratavault we see the dramatic difference in available soil. This is one of the key reasons soil vault systems grow high-quality and larger trees in a shorter time.

Is Compacted Soil Good for Tree Growth?

No, Tree roots will only grow where they have access to nutrients and water. Compacted soil is detrimental to tree root systems due to the lack of void space. Void space in soil allows for water and oxygen to trickle down and feed the tree roots. Void space is also needed for tree roots to grow and explore, in highly compacted soil conditions the tree root has to expend extra energy to push through the compacted soil in search for water and nutrients. We commonly see this impacting roads and sidewalks where the tree roots heave the pavement causing damage and safety concerns for the community.

Related: How to Stop Pavement Lift from Tree Roots

In a 10-year time lapse comparing trees planted simultaneously, we observe that the tree canopy of trees planted in Stratavault vastly outperformed the tree canopy of trees planted in structural soil.

What is CU-Structural Soil?

CU-Structural Soil, a patented soil mix developed by Cornell University’s Research Foundation, is engineered to foster tree root growth while bearing the load of pavements, roads, and urban infrastructure. This specialized blend offers a slightly expanded rooting area for urban street trees, effectively reducing tree mortality rates and preventing sidewalk failure due to compaction issues. For detailed insights into CU-Soil's research and application, refer to this link.

This soil mix allows for root expansion while prioritising necessary load-bearing capacity to support sidewalks and pavements, being compacted to 100% peak density. With CU-Soil, tree roots thrive, accessing vital nutrients and water without compromising the structural integrity of urban infrastructure.

Scientific studies confirm the efficacy of CU-Soil, showcasing somewhat enhanced tree health and longevity compared to conventional planting methods. Additionally, CU-Soil's contribution to LEED certification makes it a sustainable choice for projects.

Grow Better Trees with Stratavault

stratavault with raised foot plates

stratavault with raised foot plates

Stratavault is a structural soil vault system designed to support the above weight of urban infrastructure such as roads, pavement, parking lots, etc and provide void space that can be backfilled with high quality uncompacted soil which increases the growth rate of trees in urban environments.

20230330 101007 structural soil Citygreen

Download our specifier tech sheet to understand the engineering and environment benefits that using soil cells has in creating urban greenery.


Structural Soil Benefits

Structural soil was once the standard for planting trees in urban environments due to its ability to sustain healthy tree growth in challenging urban conditions when compared to fully compacted soil used in urban settings. By providing a stable foundation and some room for root expansion, it allows trees to establish themselves, withstand urban stresses like foot traffic, road traffic, and compacted soils, and ultimately grow in environments where traditionally highly compacted soil was used.

Here are some key advantages:

  1. Enhanced Root Growth: Structural soil provides a slightly improved environment for root development when compared to traditional planting. The larger coarse components in the soil create open spaces that allow roots to penetrate and expand, promoting healthy root growth. This leads to stronger, more stable trees that can better withstand environmental stresses.
  2. Improved Soil Drainage: Traditional compacted soils in urban areas often suffer from poor drainage, leading to waterlogged conditions that are detrimental to tree health, and the safety of the community. Structural soil, with its coarse composition, facilitates better water drainage, preventing excess water from accumulating around the roots. This helps prevent root rot and ensures a well-aerated root zone.
  3. Somewhat Increased Soil Volume: Urban spaces are typically limited in soil volume available for tree roots. Structural soil addresses this challenge by providing a higher volume of soil in a given area. The combination of coarse and fine components(rocks) allows for a greater amount of soil within the same space, providing more room for root expansion and nutrient uptake.
  4. Soil Compaction: Urban areas often experience soil compaction due to heavy foot traffic and construction activities. This compaction restricts root growth and limits the availability of oxygen and water. Structural soil, designed to mitigate total compaction, helps reduce the issue by maintaining its structural integrity even under pressure.
  5. Longevity and Sustainability: Structural soil is engineered to be durable and long-lasting, providing a stable growing medium for trees over an extended period. Its composition promotes long-term soil health and reduces the need for frequent soil replacement. This sustainable approach contributes to the overall longevity and sustainability of urban landscapes.

By utilizing structural soil for urban tree planting, city planners, landscape architects, and arborists can create better outcome from trees when compared to the traditional way trees of planting trees in urban settings. 

sidewalk tree being installed in structural soil

Types of Soil used in Urban Settings

When it comes to structural soil, there are various types available, each with its own unique characteristics and applications. Understanding the different options can help you make an informed decision based on the specific needs of your urban planting project. Let's explore some of the common types of structural soil:

  1. Engineered Structural Soil: Engineered structural soil is a specially designed blend of coarse aggregate materials, such as crushed stone or gravel, combined with soil components. It is engineered to provide both structural support and ample soil volume for tree root growth. This type of structural soil is often used in sidewalk planters, tree pits, and other confined urban spaces.
  2. Manufactured Soil Mixes: Manufactured soil mixes are custom-blended soil formulations that incorporate specific proportions of sand, compost, loam, and other organic materials. These mixes are designed to meet the requirements of urban tree planting, offering improved drainage, nutrient availability, and root penetration. Manufactured soil mixes are often used in larger-scale landscape projects where more flexibility in soil composition is desired.

Factors such as space constraints, project requirements, and desired outcomes will influence your choice. Consulting with landscape professionals or arborists can provide valuable guidance in determining the ideal structural soil solution for your specific needs.

How to Successfully Install Trees

Proper installation is crucial for creating a supportive environment that promotes healthy root growth and long-term tree vitality. Here are some practical tips to guide you through the installation process:

  1. Site Preparation: Before installing structural soil, it is essential to prepare the planting site properly. Start by removing any existing pavement, debris, or compacted soil that may hinder root growth. Excavate the area to the recommended depth, ensuring sufficient space for the structural soil and the tree's root ball.
  2. Soil Layering: When installing structural soil, it is common to layer it with different materials to maximize its effectiveness. Begin with a layer of uncompacted gravel or crushed stone at the bottom of the planting hole. This layer serves as a drainage system, preventing water from accumulating around the tree's roots. Above the gravel layer, add the structural soil mixture, ensuring it is evenly distributed and adequately compacted.
  3. Root Ball Integration: During the installation process, it is crucial to properly integrate the tree's root ball with the surrounding structural soil. Gently place the tree in the center of the excavated hole, making sure the root collar is at or slightly above the surrounding grade. Gradually backfill the hole with the structural soil mixture, firmly but not excessively compacting it around the roots. Be careful not to damage or disturb the roots during this process.
  4. Mulching and Watering: Once the structural soil and tree are properly installed, apply a layer of organic mulch around the base of the tree. Mulch helps retain moisture, suppresses weed growth, and regulates soil temperature. Be sure to leave a small space around the tree trunk to prevent moisture buildup. After mulching, thoroughly water the tree to settle the soil and provide initial hydration. Follow recommended watering practices to ensure the tree receives adequate moisture during its establishment period.
  5. Ongoing Care and Maintenance: After installation, ongoing care and maintenance are crucial for the long-term health of the tree. Regularly monitor soil moisture levels and adjust watering accordingly. Implement a proper pruning and fertilization regimen to support tree growth and vitality. Conduct periodic inspections to identify any signs of stress, disease, or pest infestation, and take prompt action as needed.

By following these installation guidelines and providing ongoing care, you can optimize the benefits of structural soil and create an environment where trees can thrive in urban settings. Remember, the establishment period is critical for the tree's survival, so attentiveness and proactive maintenance in the beginning stages of it's planting are key.

Interested in upskilling your team in how to design soil volumes for trees in urban areas?

Growth Comparison using Stratavault for soil and roots

Growth Comparison of the same tree using Stratavault soil cell system.

The Future of Soil for Urban Tree Planting

While Structural soil brings a multitude of benefits to urban tree planting projects it is no longer the best way to support healthy tree growth in hardscape urban environments. Soil Vault systems (soil cells) are a new technology rising in popularity around the world and case studies of plantings are starting to show the growth and environmental benefits using a soil vault system such as Stratavault have on improving the outcomes of trees in urban spaces. It's matrix design contribute to the long-term health and vitality of trees in challenging urban environments.

What are the significant advantages that soil vault systems provide:

  1. Enhanced Root Growth: One of the primary benefits of soil vaults is its ability to promote robust root growth. The open structure and ample void space within the soil matrix allow tree roots to penetrate deeply and spread extensively. This leads to stronger root systems, improved stability, and enhanced nutrient and water uptake.
  2. Reduced Soil Compaction: Urban environments often suffer from soil compaction due to heavy foot traffic, construction activities, and vehicle loads. soil vault systems addresse this issue by offering a low-compaction solution that supports healthy root growth. The 100% recycled engineered matrix design takes the structural load of the above infrastructure(not the soil or tree), providing a supportive environment for trees to establish and thrive.
  3. Improved Drainage: Proper drainage is essential for preventing waterlogged soil, which can be detrimental to tree health and can result in tree death. Soil Vaults systems are designed to include a layer of compacted gravel or crushed stone with supported drainage under the vault. When water enters the soil vault to nourish the tree, any excess water will flow out from the bottom of the vault and be directed into the supported drainage. This excess water can then be channeled into the city's infrastructure or nearby irrigation systems.
  4. Ample Soil Volume: Urban tree planting often faces space constraints, with limited soil volume available for root growth. Soil vault systems such as Stratavault solves this problem by providing a calculated soil volume within a structural matrix. The increased soil volume offers more room for root expansion and nutrient absorption, promoting healthier and more vigorous tree root growth at maturity.
  5. Longevity and Durability: Soil Vault Systems are designed to withstand the rigors of urban settings and has excellent longevity and durability. The engineered structural matrix ensures stability and strength, even under heavy traffic or pedestrian loads. This means that trees planted in soil vaults with adequate maintenance tree programs can thrive for an extended period, providing long-lasting beauty, shade, and environmental benefits.
  6. Stormwater Management: Another noteworthy benefit of soil vault systems are their contribution to stormwater management. The soil vault structure allows for the infiltration and storage of rainwater, reducing runoff and helping to mitigate urban flooding by ensuring that the water is used in the environment that it fell. This environmentally friendly feature promotes sustainable water management and supports the overall health of urban ecosystems while also reducing the burden on city infrastructure in a cost effective way.
  7. Flexibility and Adaptability: Soil Vault Systems are a versatile solution that can be customized to meet the specific requirements of different tree species and project site conditions. As soil vaults such as stratavault are modular it can be tailored to accommodate variations in project space, drainage needs, and root space requirements. This adaptability makes it suitable for a wide range of urban planting scenarios, from streetscapes and sidewalks to plazas and parks.

By harnessing the benefits of soil vaults, urban tree planting projects can overcome the challenges associated with limited space, soil compaction, and inadequate drainage.

Related Case Study: ROI of Planting Street Trees

Stratavault structural soil option for installing trees in urban environments to grow faster

Stratavault: Leading Soil Cell System Worldwide

Factors That Influence Structure Grade of Soil

Soil's structural grade is a critical factor that determines its ability to support healthy plant growth and overall ecosystem vitality. Whether it's in urban gardens or rural landscapes, the soil's structural grade influences water retention, drainage, aeration, and nutrient availability. A well-balanced soil structure fosters strong root development and contributes to the long-term sustainability of plants.

Several factors influence the structure grade of soil. These include:

  1. Organic Matter Content: The presence of organic matter, such as compost or decomposed plant material, greatly influences soil structure. Organic matter helps create aggregates or clumps within the soil, improving its overall structure and porosity.
  2. Particle Size Distribution: The distribution of different-sized soil particles affects the soil's ability to form aggregates. A balanced mixture of sand, silt, and clay particles can enhance the soil's structure by creating spaces for air and water movement.
  3. Soil Compaction: Soil compaction, caused by human activities or natural processes, can negatively impact structure grade. Compacted soil lacks pore space, restricting the movement of air, water, and root growth. It is essential to address soil compaction issues before planting trees to ensure optimal structure grade.
  4. Soil Moisture: The moisture content of soil plays a role in structure grade. Excessive moisture can lead to soil compaction, reducing pore space and affecting overall structure. On the other hand, adequately moist soil promotes the development of desirable soil aggregates.

Impact of Soil Structure Grade on Tree Health and Growth

Trees planted in soils with optimal structure grades are better equipped to establish strong root systems, access essential nutrients, and effectively manage water. Conversely, poor soil structure can hinder root development, leading to inadequate nutrient uptake and compromised water drainage.

The structure grade of soil significantly influences tree health and growth. Here are some key impacts:

  1. Root Penetration: Soil with a favorable structure grade allows tree roots to penetrate deeply and spread widely. This enables the roots to access essential nutrients, water, and oxygen, supporting healthy root development and overall tree vitality.
  2. Water Drainage: Soil with good structure grade facilitates proper water drainage. It prevents waterlogging, which can lead to root suffocation and the development of root diseases. Effective drainage ensures a healthy balance of soil moisture, promoting optimal tree growth.
  3. Nutrient Availability: Well-structured soil provides a favorable environment for nutrient availability and uptake. The presence of aggregates allows roots to explore a larger soil volume, accessing a greater pool of nutrients essential for tree growth and development.
  4. Soil Aeration: Adequate soil structure promotes air circulation within the root zone, ensuring sufficient oxygen supply to tree roots. Oxygen is vital for various physiological processes, and a well-aerated soil environment supports healthy root respiration.

By understanding the concept of structure grade in soil and its significance, tree planters can make informed decisions when selecting and preparing soil for tree planting projects. 

While understanding structural soil is key to promoting healthy tree growth in urban environments. By incorporating the right soil composition and ensuring an optimal structure grade, we can provide trees with the necessary conditions for ideal root development, efficient water drainage, nutrient availability, and adequate soil aeration. The use of soil vault systems, such as the Citygreen soil vault system, offers innovative solutions that improve the quality of urban tree outcomes and contribute to the long-term success of tree planting projects.

Remember, when working with structural soil, consider the specific needs of the tree species, the requirements of the project, and the site conditions. Adjusting the soil ingredient mixture accordingly and following a performance-based approach to soil specification can yield excellent results. Addressing factors such as organic matter content, particle size distribution, soil compaction, and moisture levels can greatly influence the structure grade and overall health of the trees.

By prioritizing the use of quality soils and implementing innovative support structures, create sustainable urban landscapes that benefit both the environment and the community. Investing in the health of our street trees and urban forests not only beautifies our surroundings but also contributes to cleaner air, improved stormwater management, and enhanced quality of life.

Book a Consult with our team to learn how you can use a soil vault in your project.