How much soil does an urban tree need to grow?

There is no universal soil volume for all trees. What is needed depends on the eventual size, the species, the quality of the ground, the climate and the water available.

A single figure can be useful for comparing solutions, but it does not guarantee growth. Ten cubic metres of compacted or permanently saturated soil are not equivalent to the same volume with porosity, drainage and continuity.

A bigger crown loses more water and needs a root system able to supply it. When the volume is small, the tree tends to grow less, becomes more dependent on watering and copes less well with hot spells.

The size at planting does not change this future need. A mature tree arrives with a large rootball, but the roots need to leave that volume and occupy the urban soil.

The calculation should count only soil that the roots can actually explore. Compacted sub-bases, foundations, chambers and zones without oxygen do not work as usable volume.

Texture and structure change the capacity to store water. A soil of good quality can support more root activity than a larger but degraded volume.

A strip of soil connecting several trees can be more effective than isolated tree pits. The roots gain alternatives for finding water and getting round obstacles.

Under paving, systems that reconcile load-bearing and porosity can be used. The construction solution should be sized and coordinated with drainage and underground services.

Urban roots often develop in the upper layers, where there is more oxygen. Increasing depth alone is no substitute for width and continuity.

A very deep profile with compacted sides can still confine the tree.

More volume offers a bigger reserve, but it does not do away with establishment watering. In a hot, dry climate, the system needs to apply water where the roots are and allow gradual expansion.

Too much is also a problem. If the volume collects water without draining, the roots lose oxygen.

Instead of looking for a generic number, the design should state the intended size, the volume possible, the quality of the soil and the watering method.

When the space cannot support a large tree, it is better to change the species, the position or the street cross-section than to promise a crown that the site cannot sustain.

Trunk growth, crown density, mortality and water use help to show whether the solution works. Inventory data should feed into the projects that follow.

The discussion about soil volume should stay tied to observed performance, and not just to the figure on the drawing.

Guide values can help to compare alternatives, but they should state their assumptions and the quality of the soil. A number taken from another climate or another construction system does not automatically become a universal requirement.

The design should record the planned volume, the continuity and the method. After that, the growth and condition of the trees make it possible to assess whether that solution works on the site.

1. European Arboricultural Standards. European Tree Planting Standard.
2. Plano Municipal de Arborização do Porto.
3. Technical literature on soil volume, root continuity and the growth of urban trees.