Most urban trees suffer from compacted soils and will always be affecting their health and vigour. Compacted soils also heavily stress trees, lowering their ability to prevent decay and the onset of disease
What is soil and how do trees use it?
Soil is made up of the soil parent material (Sand, Loam, Clay, or combinations of the above) as well as minerals, nutrients, and organic material. Within the soil structure, there are, in healthy uncompacted soils, spaces around the soil molecules that contain air, water etc. These molecules of water are available for the trees to absorb through its fine roots. Spaces between the soil molecules are needed for the water to be held in the soil and to allow spaces for the roots to grow into. The water that is held in soil also absorbs a large number of macro and micro nutrients, that are also taken up by the tree to aid in day to day functions.
What are compacted soils and what effect do they have on trees?
A compacted soil is one that has been compressed, usually through vehicles, construction machinery or even pedestrian and cyclists moving over the soil. This compression presses the soil molecules together, making less space for water and oxygen to be held within the soil. Less water spaces within the soil mean there is less water available to be taken up by the tree, as well as less nutrients available to be absorbed as well. Less air spaces within the soil mean that it is much harder for the trees roots to grow into new spaces in the soil and mean it is more difficult for the tree to grown new absorbing roots.
Other problems associated with compacted soils are fewer microorganisms within the soil, less organic material leaching into the soil and greater rates of surface water run off.
How can compaction be avoided and what can be done once it has happened?
In an ideal world the best way to avoid issues from compaction is to avoid driving vehicles or construction machinery over the rooting area of trees. This is especially important when dealing with construction projects surrounding established trees. In addition, areas such as parks that see high numbers of pedestrians and cyclists should consider re-routing path ways or laying mulch around the base of prominent trees to encourage users to avoid those areas.
Where compaction has already happened, we can undertake several mitigation measures dependent on the circumstances:
Compressed air decompaction – This is where a specially designed lance is driven 30cm into the ground and high pressure compressed air is injected into the soil, creating fractures and gaps. Typically this de-compacts a radius of 1m surrounding the injection site and to a depth of up to 1m. this is usually recommended on heavily compacted areas and can also have the benefit of having a targeted fertilizer added to the soil at the same time to spread this around the area.
Vertical mulching – this is a method of excavating and breaking up the top 20-30cm of soil using our air spade. Typically, we only undertake 50% of the rooting area at a time to reduce any risk of destabilising the tree. This method of decompaction is generally recommended when lighter surface compaction has taken place, such as in parks or heavily used areas.
It is recommended that both mitigation methods above are finished by adding a layer of agricultural compost, well-rotted woodchips or bark mulch to the area to increase soil moisture levels, discourage further compaction and increase levels of organic material within the soil. Other substances can be incorporated at this stage including biochar, fertiliser and other organic matter.
For further information on our soil decompaction services please contact us.