Study a clump of ordinary garden earth or soil, fresh from the garden – or better still a woodland area. If you look closely you will see that far from being a solid mass, it is honeycombed with holes and crevices, fault lines and voids. We call these gaps between the mineral particles and organic content “pores”- and there are both macro pores (larger) and micro pores (tiny). Both are very important for soil health, drainage, water and nutrient transport and of course air.

Healthy Soil
Healthy Soil

Within that healthy soil, a lot is going on. In fact, it is a veritable hive of microbial activity. Some estimates say that in about a tablespoon of soil as many as 2 billion microbes, in the form of beneficial bacteria, are going about their daily business. That’s a lot of activity and whilst this is going on, small amounts of oxygen are consumed, and small amounts of methane and carbon dioxide are generated. All good so far, and this is why trees would always prefer to remain in woodland and tree nursery areas if given the choice.  Optimal void ratio for a root friendly soil is 20-40% – this is provided principally by the macro pores.

For the hapless urban tree planted in a continuous paved surround, often with poor quality soil, the reality is rather different with devastating consequences. Why?

Failed trees
A site with numerous failed trees.

Decades of street design with increasing priority for vehicles have created less-than-friendly tree conditions underground and making it almost impossible to establish street trees successfully, let alone enable them to realise their ecosystem potential. It takes 60 small trees to achieve the benefit of one single large established canopy treeso simply replacing trees every few years is not the answer.

It takes 60 small trees to achieve the benefit of one large mature tree – so simply replacing trees every few years is not the answer.

To understand this challenge, take a healthy soil clump (or “ped”as they are sometimes termed) and crush it flat in a hydraulic press. Effectively, what you have done is destroy the actual physical structure of the soil. It isn’t dead yet, but it soon will be – because you have obliterated the macro pore structure which is necessary for the soil life to breathe, for nutrients to be transported, and for the water and air balance to be maintained. Not good. Next time you drive past a road construction project and admire all the heavy compacting machinery so necessary for civil engineers to sleep at night, spare a thought for future trees trying to live in that environment.

This is of course, one of the reasons why statistically, 50% of street trees die before they get to ten years old. They exhaust the resources that the limited tree pit soil originally provided and simply die off, unable to access quality aerated soil media. From thereon, nutrient deficiency, drought and anaerobic compacted soil, condemn the tree to a slow and barbaric death. Even if the tree struggles on for a while, it is left severely weakened unable to resist disease and thrive in the harsh urban environment it was originally planted to benefit.

It would be ludicrous to state that tree growth is not benefited from aeration, as aeration is key to these microbial processes, and therefore increase root growth for supporting healthy tree canopies. Thus, utilising a system like the RootRain Aeration/Irrigation system with pedestrian-friendly surface inlets and slit perforated piping designed specifically for this, helps us understand that including these components brings back the forest floor conditions that allow for the maximum health of trees. These systems also allow for nutrients to be added directly to the soil zone if required at a time of drought for instance, and serve as access ports allowing the soil underneath a hardscape to be inspected.

Statistically, 50% of street trees die before they get to ten years old. 

The message is simple “ if you look after the soil structure, the soil will look after the tree. Therefore, for future generations to enjoy mature trees in our increasingly heavily engineered cities, we have no option but to provide uncompacted, root friendly, aerated soil media in appropriate volume and location for the trees to access freely. Compacted so called tree sand type materials, even when topped off with a plastic crate, have no macro pores, so do not provide well aerated soil, and they require maintenance due to low nutrient levels determined by their predominantly high mineral content.

Healthy soil in StrataCells with Aeration pipes shown along the length of the tree pit

To conclude, best practice urban tree pit design should always include a load bearing structure to protect the soil from engineered loadings, with an air space above the soil. This airspace should be vented to pavement level with at least two maintainable inlets. If using additional perforated or mesh air distribution pipes, the perforations should be large enough to allow reasonable gaseous exchange, so a good pipe surface void area is necessary for this.

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