Managing soil compaction around tree roots

What is soil compaction?

Soil compaction occurs when force compresses soil aggregates (large units of bonded silt and clay particles), reducing macro pore spaces (spaces larger than 75um) that allow air and water movement through the soil (Hirons and Thomas, 2017), (Ashman and Geeta, 2002).

What effect does this have on trees?

Compaction causes an increase in the soil’s bulk density (the weight of a soil by volume) and the soil’s mechanical strength. This reduces the root’s ability to penetrate the soil as they grow (Roberts et al,.2006). The closing of macro pore spaces significantly reduces the availability of water, nutrients and oxygen, which are vital for development (Pollardy, 2008). Reduced water availability will cause leaf stomata to close to reduce water loss through transpiration, which will limit photosynthesis and the production of new carbohydrates. This combined with a lack of soil oxygen will reduce the rate of respiration and the production of a high energy compound called adenosine triphosphate (ATP) which provides the plant with the energy required for physiological processes such as primary and secondary growth (including new roots), reproduction and defence (Pollardy, 2008), limiting the trees development and function.

How do we manage compaction?

The most effective way of managing compaction is to remove the cause. This can be achieved by limiting access, for livestock this could be as simple as a suitable fence that restricts the animals from entering the root zone. For green spaces this could be something as simple as restricting access or moving footpaths and bench’s, making the area less attractive to people. For areas where vehicle access is required systems such as cellular confinement systems installed within the root zone will help to distribute the load of passing vehicles.

Figure 1 Cellular confinement systems used over tree roots (Rose, 2020)

In areas where bulk density is high and trees are starting to decline, physical intervention with a method of soil amelioration might be necessary. Mulching with materials such as biochar will help to breakup compacted soil and allow for water filtration and aeration as it is incorporated into the soil. Whereas the use of an air injection system to break up the soil and reduce bulk density. By firing high pressure air into the soil and the smaller pore spaces, the air brakes up large aggregates and creates new macro pore spaces allowing for water filtration and air exchange (Hirons and Thomas, 2017). This however has the potential to damage soil associations such as mycorrhizal fungi by damaging the delicate mycelium in the soil as well as the fine root hairs responsible for water and nutrient uptake, the fine root hairs however will quickly regenerate and expand into the newly formed pore spaces.

Video 1: Airspade and how it works (Airspade, 2025)