Estimating Soil Volume Needs of Trees in Urban Situations
In urban situations the amount of soil trees can access is critical to their long-term success. If existing soils are not suitable, or not available as with heavily urbanised or disturbed sites, we need to provide some that are.
So, how much soil do trees need?
THE CROWN PROJECTION METHOD
Various authors have investigated soil requirements for urban trees from different perspectives. e.g. Lindsay and Bassuk and James Urban estimate soil volumes required by using the area under a tree’s crown, generally arriving at a recommendation which, when converted to metric, equates to 0.61m3 of soil per m2 of crown projection (the Crown Projection Method).
Soil Volume Required (cubic metres) = 0.61 x Crown Projection (square metres)
This approach works well for trees of regular form. However, it is likely to overestimate required volumes for wide spreading species and underestimate volumes for upright species. It also takes no account of climatic factors or species factors such as drought tolerance.
THE FIELD SIZE INDEX (FSI) METHOD
If we adapt the concept of Size Index* as found in AS 2303:2018, designed for trees grown in nurseries, to a form that is better suited to trees in the landscape, Field Size Index** (FSI), where trunk diameter is measured at 1.4m rather than 300mm above-ground – Diameter at Standard Height (DSH), we can then use FSI to help us estimate soil volume requirements quickly, easily, and consistently.
If we assume that a tree of “regular form” has a spread that is approximately 90% of tree height we can use the soil volume requirement estimated using the Crown Projection Method, and the FSI to create a simple formula to calculate soil volume required in a way that allows for variations in tree form.
Soil Volume Required (cubic metres) = Field Size Index (mature trees) / 100
AN EXAMPLE
Consider a tree of “regular form” having an expected mature height of 10m and an estimated DSH of 400mm.
Crown Projection Method
Estimated Spread = 9m
Crown Projection (area under the crown) = 63.6m2
Soil Vol Required = 0.61 x 63.6 = 38.8m3
FSI Method
Using the FSI method for estimating required soils volumes the volume required becomes:
Soil Vol Required = (10 x 400) / 100 = 40m3
i.e. The soil volume estimates created using these two methods are effectively the same, for trees of regular form, and recommended volumes will range from around 5 cubic metres, for small trees, increasing with tree size, typically to around 40-50 cubic metres for medium sized trees. The FSI method of soil volume calculations is quick and easy and automatically compensates for tree form. However, like the Crown Projection Method, it also does not take climatic factors or species factors, such as drought tolerance, into account.
Note: To use the FSI method of estimating soil volume requirements, you will need to estimate both mature height and DSH. Estimations of height are readily available on grower sites and on the internet. Estimates of DSH will be more difficult to obtain. In the absence of better information, assume that the DSH will be approx.. 4% of tree height, for most species: a bit less for tall slender growing species, and a bit more for stockier/thick-stemmed species. See our article on Estimating Mature Tree Diameter for more information.
LEAKE & HAEGE SOIL VOLUME SIMULATOR
Simon Leake and Elke Haege offer a more scientific approach to calculating soil volume required which can be found in 2024, Soils for Landscape Development, 2nd Edition, CSIRO publishing .Appendix C: Soil Rooting Volumes: Table C4, found on page 180 lists typically recommended soil volumes of between 11 to 30 cubic metres for tall trees 9-20m high (one of the 4 tree design size categories).
You can access the “Soil Volume Simulator” at this link: https://www.elkeh.com.au/soils/. Using the simulator’s recommended soil volumes range from around 5 cubic metres to around 45 cubic metres, varying with tree size, site conditions and design life. It also takes account of eight factors such as climate, soil quality, availability of maintenance (irrigation, fertiliser), species suitability, and the tree's adaptability to stunting. We all appreciate that urban trees seldom reach their mature size. Shared root systems and connected soil networks are also built into the simulator.
FINAL NOTES
Use whatever method to calculate how much soil is needed you prefer, but make sure that accessible soil volume is considered, and an appropriate soil volume made available, if the long-term success of the new Urban Tree you are responsible for is important to you. Factoring in tree stunting and shortened lifespans when specifying trees in urban locations means a larger tree species can often be an acceptable design choice.
Note: All the above methods of calculating soil volume requirements are estimates only. Use them to calculate soil volumes you might need to provide, or to test actual available volumes against. Then make you decision to plant, or not to plant, based on how closely the soil volume possible approximates the theoretical volume calculated.
With thanks to Simon Leake and Elke Haege for their comments and input.
KEY
* Size Index = Height (m) x calliper (mm) where calliper is measured at 300mm above ground.
** Field Size Index = Estimated mature height (m) x Estimated Diameter at Breast Height (mm) – where Breast Height is 1.3m above ground
REFERENCES
AS 2303:2018 Tree stock for Landscape Use.
S. Leake and E. Haege, Soils for Landscape Development, 2nd Edition, CSIRO publishing, 2024
Up by Roots, James Urban, International Society of Arboriculture, 2008