Structural Soil: an Infographic
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Structural Soil for Urban Trees
Currently most urban trees are planted directly into existing compacted urban soil or tree pits with limited root space. Trees that are planted in areas surrounded by paving tend to struggle for air space and usually decline well before they should. Where soil volume is limited by pavement, tree roots suffer and tend to take the path of least resistance searching for air, usually in and around pipes, foundations, or to the surface. Healthy trees need a large volume of non-compacted soil with adequate drainage and aeration and reasonable fertility.
While the need and desire for large trees in the urban landscape still is the desired intent, the trees do not survive long enough to fill the need. Not planning for root growth is ignoring the biological requirements of trees and is not economical or environmentally prudent. The failure to provide adequate soil for both drainage and root growth is critical to the life of the tree and without an engineered soil specific to this application, trees have a shortened life span and may die. Ensuring a good supply of air to the tree roots is essential for satisfactory tree growth, however in urban situations, the movement of air into the soil is often restricted. By providing additional root space below the pavement in what otherwise was compacted urban soil, Structural Soil can allow most newly planted trees to have a chance for healthy growth. This mix consists of 80% 75 mm angular clear aggregate and 20% approved sandy clay loam. The aggregates bear the load, providing the structural stability for the pavement above. The angularity of the rock create for the non-compacted soil, providing space for air, water and nutrients the roots, as well as provide for future root expansion. Engineered structural soil provides a resource for root growth beyond the traditional tree pit, allowing for much stronger root growth and ongoing tree health.
Denbow has been providing structural soil to many municipalities within the Greater Vancouver and Fraser Valley area. These departments use Denbow’s manufactured soil to allow city planners and engineering departments to add trees for the health and beauty of urban communities.
Contact Denbow’s soil experts today to find out more about how structural soil can work in your city or municipality.
What is Structural Soil Why is it good for Trees?
Why is structural soil effective?
Structural soil is a type of soil mixed with a specific type of gap-upgraded rock (typically 75mm clear). This is a very important part of the specification; the reason structural soil works well is that the gaps created by the rock are then filled with soil, enabling penetration by tree roots. This penetration allows the tree access to a larger space underground, and has the double function of preventing the tree roots from lifting up the hard surface, i.e. the sidewalk that is on top.
How does the structural soil get tested?
The testing of structural soil is essentially a three-part process. The soil must be tested separately. The rock must be tested and sourced with a specific sieve size attached to it to determine that it is in fact a clear rock and not a minus rock. With a minus rock all of which would be filled and therefore the soil would be effective.
There is also a stabilizing compound that is used with structural support to help the soil adhere to the rock. The specifications for this material would come directly from the stabilizer supplier; there are more than one of these.
Who can make structural soil?
Like any engineered soil, structured soil is a technical process which is best done by people who understand all the specific components and have all the components readily available with testing and prior knowledge. Making good structural soil to specification is an extremely important endeavour. Structural swell that is not up to specification is a very difficult situation to remediate once it’s already in the ground.
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The River Delta Soil
There’s a perception in the lower mainland of British Columbia that because we live in a river delta, all of our soil is ideal for growing. This puts to question the need to purchase manufactured soil.
We decided to “Ask Tom” his opinion on the matter.
Tom, we live in a river delta so why do people need to purchase engineered soil? Shouldn’t our native soil be ideal for growing?
Yes, we do live in a river delta and much of our soil is good for growing. However, in the land development process, the top layer of organic soil is usually removed. Some of this soil is actually really good. The problem is you can’t guarantee it’s composition. To ensure that your soil is the right composition for growing, it is best to purchase engineered soil.
Soil structure an important aspect of a grow medium. There is a difference between good soil and the right soil structure for a grow medium.
What is right soil structure?
Good Question. Let me tell you about our engineered soil and why, in my professional opinion, it provides the best soil structure for your grow medium.
First, We ensure that our engineered soil has the right amount of Sand, Silt and Clay. We also test that the ideal amount of water and air are able to move throughout the soil.
The sand used in our soil composition is a medium-fraction river sand. Since we have such a large river running through the southern portion of our province we have amazing access to high quality of sand. The Canadian Government of Fisheries does an amazing job of conserving the river while still allowing proper access to the sand.
So there are three main components, sand, silt and clay, to an engineered soil. At Denbow, we regularly test our soil to make sure it is in the right amount of these parts. Our soils adhere to strict laboratory-tested guidelines and meet BC Landscape Standards. When you just mix the excavated dirt on sight, you are unable to achieve this same standard.
When you have the right structure then the right amount of air and water are able to move naturally through the soil. Air is just as important as water as it allows the right drainage for plant growth. Residents of British Columbia know we get a significant amount of rainfall. This amount of water requires our soil structure in BC to have a specific amount of sand to allow the water to drain.
To conclude, there you have it. Although the mighty Fraser Runs runs through B.C.’s entire lower mainland, we still need engineered soil so we can be successful with our landscaping.
In our next post we’ll be discussing the importance of pH balance in addition to soil structure.
Stay tuned for more Ask Tom Articles.
An interview with Tom on soil
We decided to ask Tom the question, “What the single most important aspect of soil?”
We were surprised by his response.
Denbow: Ok Tom, what is the single most important aspect of soil?
Tom: Ah that’s a simple question!
Denbow: Really? Tell us more.
Tom: Of course. Now my first response was a little tongue-in-cheek. You see, there isn’t a single most important aspect of soil. Soil is so much more dynamic than most people understand. As I will hopefully help illuminate in this series, soil has multiple components, which can be engineered to best support what needs to grow. These components are sand, silt, and clay, together with organic matter. The proportions may vary, as does the PH level. So, in fact, your question is not simple at all. A better question might be “What is the best soil composition to support what needs to grow?”
The soil is the grow medium for what needs to grow. When sourcing soil for your project, it is important to ask the right questions. In what geographic region is the soil being used? What kinds of natural environments surround the soil? What are you planting in the soil? What is the slope of the land? There are literally hundreds of questions you could ask when considering what type of soil to use on your specific project.
I could also argue that the single most important aspect of soil is having standardized soil regulations that govern soil usage.
It’s ironic that when constructing a building there are stringent rules regarding the building code, yet when it comes external landscape components of the building, there are mixed standards and styles. However, when you consider the impact of the landscape and the potential the external part of the building has on the environment, as well as any surrounding water sources, you start to see that the exterior of a building is just as important the interior.
Soil manufacturers are not equal and soils are not equal. Many different components like yard waste, compost, and, in some situations even bio-waste, can potentially contain metallic elements in the soil. This is why we need to educate the public as well as lobby for more consistent and informative guidelines for the uses of soil.
So, in constructing a building you have some clear guidelines. Attempting to create similar standards for soil has been challenging to say the least. Over the last 20 years it’s been difficult because we haven’t passed along the right information.
I can explain this way, Tom says. Soil in British Columbia and the regulations of soils in British Columbia has been like a telephone game (you know, the game you played as a kid). One person starts the conversation, whispering in the next person’s ear. “Soil needs to have these three components,” they say. The next person passes it on: “There are three things we should discuss when it comes to soil”. The next person says, “There are three important ways to install soil”. This continues until the last person says, “Soil is great for growing.” Yes, this is true but it has very little—if any—of the information that is actually useful for soil regulations.
Conversations managed this way can get lost in translation. And that’s what’s happening with BC soil. We have several people trying to establish rules regarding the soil views in different cities and municipalities around the lower mainland of BC. Although these rules are somewhat informative, they also need to be consistent and useful, and therefore easily followed.
So, to sum it all up (and sorry for the tangent at the end!), there is no single aspect of soil that is most important. Soil is a dynamic grow medium that needs to be properly engineered to meet the requirements of each specific project. From there we need to have a governing body helping define and classify the different soils we use so the project manager, landscape architect or informed home owner can make the right soil decision.
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