Urban trees face numerous challenges, particularly when it comes to soil structure. Compacted soil, a common consequence of urban construction, often lacks the necessary pore space to provide trees with adequate oxygen, water, and nutrients. Additionally, dense soil structures leave no room for roots to grow, severely limiting tree health and lifespan. Fortunately, engineers, researchers, and green industry professionals have developed innovative solutions to mitigate these issues and promote healthier urban trees. Two standout solutions are structural soil and soil cells.
Structural Soil: Pioneering Urban Tree Growth
The concept of structural soil was first developed by researchers at Cornell University in the 1980s. Their goal was to create an environment where urban trees could grow larger and live longer, contributing to greener urban spaces.
How Structural Soil Works
Structural soil, commonly referred to as “CU Structural Soil,” consists of a lattice of coarse-cut gravel (minimum ¾-inch size) mixed with a clayey loam and a hydrogel product. This mixture achieves dual objectives:
- The gravel provides the load-bearing capacity needed for sidewalks, parking lots, and other urban structures.
- The cavities between the stones allow for air and water infiltration, promoting root growth.
However, structural soil isn’t without its challenges. Its high composition of impermeable stone reduces water-holding capacity, and the stone’s chemical weathering can alter soil pH. To counteract these issues, clayey loam and hydrogel are included to enhance water and nutrient retention.
Benefits of Structural Soil
Research has demonstrated that CU Structural Soil allows for over 10 times the water infiltration rate of conventional compacted subsoil and 15 times greater porosity for root development. These characteristics enable tree roots to grow deeper, farther, and more robustly than in traditional urban plantings. Additionally, combining structural soil with porous asphalt can encourage roots to grow as deep as 3.5 feet, reducing the likelihood of above-ground damage to sidewalks and parking lots. Over time, this approach significantly reduces maintenance and repair costs.
Soil Cells: A Modern Innovation
Soil cells represent a newer solution to the challenges of urban tree planting, with development beginning in the early 2000s. These systems provide a support structure that holds soil in place while leaving room for root growth.
How Soil Cells Work
Soil cells function like a vaulted cathedral ceiling, with arched columns supporting the structure above while preserving the soil’s integrity below. They are designed to hold high-quality growing media or topsoil without the need for a rock aggregate, ensuring an uncompacted environment for roots.
Advantages of Soil Cells
Compared to structural soil, soil cells offer several significant advantages:
- Increased Soil Volume: Unlike structural soil, which is 80% stone by volume, soil cells are almost entirely composed of organic growing substrate, providing greater water and nutrient availability.
- Higher Water-Holding Capacity: This is crucial for supporting trees as their water needs increase with maturity.
- pH Stability: Without gravel, there’s no risk of chemical weathering altering the soil’s pH, making soil cells ideal for pH-sensitive trees.
- Reuse of Existing Soil: Excavated soil can be mixed with compost and amendments, reducing waste and cost.
- Stormwater Retention: The larger volume of soil in a soil cell system makes it more effective at retaining stormwater compared to structural soil.
Choosing the Right Solution
While soil cells generally outperform structural soil in terms of tree health and longevity, they are also more expensive to install. However, both options provide a better long-term investment compared to the repeated costs of planting and removing failed urban trees. Additionally, these systems can bridge compacted areas and connect tree roots to adjacent green spaces, further promoting growth and reducing costs.
Call to Action
At Homer Tree Care, we’re passionate about supporting urban tree health and sustainability. Whether you’re dealing with soil challenges or looking for expert advice on tree care, our certified arborists are here to help. Contact us today to learn more about how we can enhance your urban landscape and provide the best care for your trees!
