There are many considerations that must be accounted for when planting street trees in our cities. One important point being soil compaction. It is critical to determine site environment and soil conditions in order to mitigate the potential constraints of soil compaction, in order to create healthy tree planting conditions. 

In previous articles I have detailed the soil requirements of healthy trees and the amount of soil urban trees should have. Now, we discuss why uncompacted rooting volume is so critical to the health of street trees. 

Once a site has been assessed, it is important to reconcile the conceptual design intent with site conditions to determine the limitations of any given site. Many potential urban soil limitations can be addressed with specie selection – such as spatial constraints, soil PH, cold hardiness, wet & dry soil conditions, sun & shade patterns, and even salt contamination. One soil condition that cannot be mitigated by plant selection however, is soil compaction. Compacted soil that tree roots cannot penetrate is as useless as no soil at all. 

Soil compaction is a reduction in large pore space which reduces soil oxygen levels and decreasing soil drainage. As a result, rooting depth is reduced. According to Colorado State University, soil compaction is the most common factor leading to the decline process of trees. And at very least, compacted soil results in stressed plants that are far more susceptible to secondary problems like disease and pest insects. 

Soil compaction is the most prevalent of all soil constraints on urban tree growth. Every place where humans and machines exist, and the infrastructures that support them are built, soil compaction will be present. There are very few soil areas without some form or extent of soil compaction. In the past, compacted soil was an accepted fact of life for urban trees. 

Avoiding Soil Compaction

Soil support cells, and before that 'structural soil' were developed to address the dilemma of creating uncompacted soil for trees, while providing durable stability for roads and paved surfaces to be built. 

Early methods focused on a rock and soil mixture, known as structural soil, to provide pavement support while also attempting to permit some root growth beneath the pavement. Since then, structural soil cells have moved this principle forward by replacing the rock (which had accounted for 80% of total volume) with engineered modules (that only account for 6% of volume). 

Structural soil cells are modular units that assemble to form a skeletal matrix, situated below pavement level, to support the pavement load while providing large volumes of uncompacted soil for root growth within the matrix structure. 

GreenBlue's StrataCell provides over 94% void space for soil and root growth, while also providing structural stability for pedestrian and traffic loads. There are different designs of soil cells to address the need for strength while maximizing available space for roots and common services and utilities. 

Industry professionals are increasingly insisting on the use of soil cells for trees planted on streetscapes and other paved areas. It has been recognized that while soil cell technology builds upon earlier structural soil concept, it is clearly superior in performance. Not only is vastly more soil made available to the tree, installation is straight forward and avoids the need of extensive calculations and testing required for the use of structural soil. 

GreenBlue soil cells, such as StrataVault, safeguard the all-important root zone, keeping the load off the soil, and keeping the soil open and free draining – giving trees the long term future they require in order to offer the many benefits that mature brings contribute for generations to come. 

Please consider sharing this article and downloading our FREE guide to the Soil Requirements of Urban Trees. You can also view the original article here.