As landscape architects, we understand the importance of ensuring the health and longevity of the trees we plant. One of the biggest challenges we face when planting trees in urban environments is providing adequate support and stability for their roots, especially in areas where space is limited. To meet this challenge, two common solutions are structural soil and soil cells. In this blog, we will explore the pros and cons of each approach, as well as some additional benefits that other solutions, such as porous pavements, tree grates, and root directing systems, can offer.

Structural Soil

Pros:

• Structural soil is a mixture of crushed stone or gravel and soil, which provides increased stability and load-bearing capacity. This can be particularly useful in areas with heavy foot traffic or vehicle traffic, where trees may be subject to greater stress and compaction.

• Structural soil can help to improve soil aeration and water retention, promoting better root growth and overall tree health.

• Structural soil can be more cost-effective than other solutions, as it does not require specialised materials or installation methods.

Cons:

• The creation of structural soil can be time-consuming, as it involves excavating a large hole and adding the crushed stone or gravel layer.

• Structural soil can be heavy, making it difficult to transport and install, especially in tight spaces.

Soil Cells

Pros:

• Soil cells are modular plastic units that can be filled with soil, providing an efficient and effective way to support street trees.

• Soil cells are lightweight, making them easy to transport and install, even in tight spaces.

• Soil cells help to prevent soil compaction, promoting better root growth and overall tree health.

Cons:

• Soil cells can be more expensive than other solutions, especially if large numbers are needed.

• The plastic material used in soil cells can become brittle over time, leading to cracking and potential failure.

Which option is used often comes down to the requirements of the site, the tree species used and the budget available. A number of studies have suggested that soil cells allow for better tree growth, but structural soils are also an effective solution which is affordable and readily available to clients and contractors. A bit further in this article, we will explore the findings of one study. 

In addition to structural soil and soil cells, there are several other factors that should be considered that can support street tree growth and improve their health and life expectancy. Each of these items will be covered in a future blog.

1. Porous pavements, such as permeable pavers can provide an attractive and functional solution for street tree planting. These systems allow water to penetrate through to the root zone, while also providing support and stability to the tree.

2. Tree grates are metal frames that are placed over a tree's root zone and filled with soil, gravel, or other materials. They can provide support and stability to the tree while also allowing water and air to circulate freely in the soil.

3. Root directing systems, such as root collars or root pruning baskets, can be used to guide and control the growth of a tree's roots, helping to prevent damage to sidewalks, roads, and other infrastructure.

The best solution for street tree planting will depend on the specific requirements of the project and the species of tree being planted. Structural soil can provide increased stability and cost-effectiveness, while soil cells offer a lightweight and efficient solution. Porous pavements, tree grates, and root directing systems can offer additional benefits, such as improved water management and root guidance. Regardless of the solution chosen, it is important to consult with a professional arborist or landscape architect to ensure that the needs of both the tree and the surrounding environment are met.

What is structural soil made of?

If you are looking to use structural soil, it is worth considering what is the best structural soil mixture to use. Mixtures can vary greatly, and there are many studies which look at variations in stone sizes, organic soil percentages and more. Many of these variables can be influenced by environmental factors and should be considered on a case by case basis. The below is a guide based on a number of studies and serves as a generic solution only.

The size of stone or gravel used in structural soil can vary, but it is typically in the range of 2 to 4 centimeters in diameter (sometimes referred to as "pea gravel" or "crushed stone"). This size range allows for good compaction of the material and provides the necessary load-bearing strength to support urban infrastructure. The specific size of stone or gravel used can depend on various factors, such as the intended use of the structural soil, the composition of the soil component, and the available materials in the local area.

There have been studies that have investigated the effects of using different sizes of stone or gravel in structural soil. For example, some studies have suggested that using larger-sized aggregates, such as stones with a diameter of 5 to 10 centimeters, can provide greater soil volume and better drainage, which may lead to improved tree growth and sustainability (e.g., see the study by Landry et al., 2010). However, other studies have found that smaller-sized aggregates, such as those in the range of 1 to 2 centimeters in diameter, can result in better compaction and load-bearing capacity, which is important for supporting urban infrastructure (e.g., see the study by Grabosky et al., 1998).

Overall, the choice of stone size in structural soil will depend on the specific goals and needs of the project, as well as the availability of materials and other site-specific factors. It is important to carefully consider the effects of different stone sizes on soil volume, compaction, drainage, and other important factors in order to select the most appropriate mix for a given project.