Probably the most critical factor for a functional and healthy tree is providing sufficient quality soil for the tree roots. When planning trees in an urban environment, it is crucial that the intended soil volume is determined in an early design stage. This will maximize the return on your green investments and ensure that they quickly grow into large healthy specimens.
In a natural environment, root systems can extend up to 3 times the radius of the treeprojection. Probably the easiest way to calculate a minimum required soil volume is to take the projected awning area of the mature tree, multiplied by a depth of 0.6.
Please note, this formula does not apply to fastigiated and columnar habit trees. The soil volume depends on a large number of factors. Besides tree species, and design goals (size of the tree). Water management is the most important factor; irrigation system yes or no, groundwater accessibility, climate zone etc.