Ode To The Air Knife
I wanted to share a bit about my favorite tool for tree preservation – the Supersonic Air Knife®.
What is it?
The tool consists of a high-strength aluminum tube fitted with a precision nozzle and air valve, powered by a commercial air compressor. Its core performance comes from the nozzle, which accelerates compressed air to roughly twice the speed of sound.
This ultra-high-velocity air stream penetrates the soil to a depth of about one foot, momentarily creating a cavity that loosens and displaces the surrounding ground. Because buried utilities such as pipes, cables, and tree roots are non-porous and structurally resistant to airflow, the soil is pushed away from them rather than through them, allowing excavation around these features without causing damage.
How does it help trees?
The relationship between soil air and root function is, unfortunately, overlooked by many arborists.
While it is widely understood that plants use carbon dioxide, sunlight, and water during photosynthesis to produce carbohydrates and oxygen, it is less commonly appreciated that oxygen is equally essential for tree survival. Through respiration, trees break down stored carbohydrates and organic compounds to release energy that fuels all metabolic processes. This respiration occurs not only in the leaves, but also in actively growing root tips, cambium, developing shoots, and fruit tissues.
Insufficient soil aeration is one of the most significant limiting factors in landscape plant health. Roots depend on pore spaces within the soil to initiate and sustain new growth. In heavily compacted soils, these pore spaces are reduced or eliminated, physically restricting root expansion. Contrary to common misconception, roots do not force their way through solid soil—they grow only into existing voids.
Beyond physical limitation, roots also rely on pore spaces for gas exchange. These voids supply oxygen while allowing carbon dioxide to diffuse out of the root zone. When soils are compacted and pore space is lost, this exchange is disrupted. Oxygen becomes unavailable, carbon dioxide accumulates, and water movement is impaired. Under severe compaction, soils can become anaerobic, creating conditions that can ultimately lead to tree decline or death.
How is it used?
The Supersonic Air Knife is ideal for relieving soil compaction in the tree’s root zone, especially in lawns. We use a process of vertical mulching in which we drill a grid of small holes two a depth of several feet. We then incorporate soil amendments such as biochar and compost that maintain the pore space created by these vertical channels.
Root collar excavation is another important service made possible by the air knife. In my experience, nearly all urban trees suffer from malformed root systems caused by improper planting practices, soil compaction, or construction impacts. Exposing the root collar allows us to identify and remove girdling roots before they restrict vascular flow and destabilize the tree. It also provides an opportunity to correct common planting issues such as buried root flares, synthetic burlap, wire baskets, or excess soil around the trunk. Restoring the root collar is often one of the most important steps in improving long-term tree health and stability.
Trees and utilities often come into conflict both above and below ground, but air excavation tools can help preserve the critical root zone before construction projects. Unlike backhoes, which can tear and shred roots, air tools make it possible to selectively expose and cleanly prune roots when necessary. They are also highly effective for trenching near trees, allowing utilities or irrigation to be installed with minimal disturbance to soil structure and root health.
Air excavation tools also transform how we can plant and transplant trees. The tool can be used to carefully bare-root trees, making transplanting possible without the excessive weight and stress of a traditional root ball. Even in dense clay soils, air tools allow us to loosen compacted ground and excavate planting areas simultaneously, creating a healthier environment for root establishment from the start.