In part 1 of this series, I summarized some of the lessons we learned from those people on the forefront of setting minimum soil volume policies. In today’s article, I will summarize how actual adopted soil volume policies compare to the minimum soil volumes trees need according to research studies.
In addition to the survey I wrote about in part 1, we also did a literature search on research about minimum soil volumes needed by trees. The studies we found determined minimum soil volumes in one or more of the following ways:
1) Field investigations: i.e., what was the minimum soil volume needed for trees to thrive
2) Calculations based on water requirements: i.e., how much soil does it take for a tree to have enough water to thrive
3) Calculations based on nutrient requirements – i.e., how much soil does it take for a tree to have enough nutrients to thrive
The median minimum soil volume from the 7 studies we looked at, extrapolated for a 30′ diameter tree canopy, was 1,500 cubic feet (42.4 cubic meters). The table below summarizes what the 7 studies found:
So how do those numbers compare to actual minimum tree soil volume policies? The table below compares how much soil volume trees need according to the research studies we reviewed versus actual adopted minimum tree soil volume policies.
You can see that the actual adopted volumes are generally lower than what research indicates trees need. The highest soil volume POLICY was equal to the researched lowest soil volume trees NEED. So, while trees in cities with minimum soil volume policies are a great start, the trees would like us to do more.
So, while trees in cities with minimum soil volume policies generally get much more than the volume in a standard tree pit (usually 4’ x 4’ x 2’ – providing a measly 32 cubic feet of soil), most current minimum soil volume policies are by no means overkill!
In sum, most trees, even those benefitting from minimum soil volume policies, aren’t getting as much soil as they need for a 30′ diameter canopy. We hope that the data we’ve gathered here can be used by arborists, planners, landscape architects, and others as evidence of the important role minimum soil volume policies should play in improving urban canopy cover.
Top image: Stradablog