Every year, landscapers, arborists, and other green professionals rely on fertilizers as a quick fix instead of addressing the root of the problem. In urban environments, soils are often imported from surrounding areas, altering the site’s original soil properties. These imported soils are frequently selected for their ability to be compacted, great for building homes, but problematic for trees, as their roots become trapped in clay bowls with limited space to grow.
In this post, we’ll explore some of the most common methods for addressing compacted soils, along with a few tools that can make the job a little easier. We’ll focus on hand tools due to the high cost of air-powered alternatives.
Access the current situation:
Before making any recommendations, take time to thoroughly assess the soil and tree conditions on site. Every site is unique and should be treated accordingly. A one-size-fits-all approach is often ineffective—and can even lead to serious problems.
Here is a list of tools that can help you with your assessment:
Brix Refractometer(https://amzn.to/4jkyatU) : Measures the sugar concentration in plant sap, serving as a proxy for plant health. Higher Brix readings often indicate better nutrient uptake and overall vitality.
Soil Penetrometer(https://amzn.to/43sMef1) : Evaluates soil compaction by measuring resistance at various depths. High resistance suggests root growth may be restricted by compacted layers.
Screwdriver: A simple but effective tool: if you struggle to push a screwdriver into the soil, it’s likely compacted and poorly aerated.
Auger Bit (2”–4”)(https://amzn.to/4ksnzhw) : Ideal for drilling small holes for vertical mulching or sampling deeper soil horizons. Auger holes also serve as delivery points for soil amendments.
Create a Plan:
Now that you know what the soil is lacking, here are some organic amendments you can add to improve soil health and reintroduce vital organic matter.
Humic Acid:Improves soil structure and increases the cation exchange capacity (CEC), helping soils hold onto nutrients and water more effectively.
Fulvic Acid:A smaller molecule than humic acid, fulvic acid grabs on to micronutrients and improves their uptake into plant cells.
Calcium:Essential for cell wall structure, calcium also clumps together clay particles, helping open up compacted soils and improve drainage.
Biostimulants: A broad category of products that stimulate microbial life and enhance root growth without acting as direct fertilizers.
Seaweed Extract:Rich in micronutrients and natural growth hormones, seaweed extract promotes root development and stress resilience.
Unsulfured Cooking Molasses: Provides a quick food source for soil microbes, encouraging biological activity. It’s often used in compost tea or mixed with water for drenching.
Execute:
Now, if we could just throw everything on the ground and walk away, that would make things too easy. Below are a few techniques that help ensure your hard work stays in place and does what it’s meant to do, create healthier soil.
Vertical Mulching: Involves drilling vertical holes (using your auger bit) around the root zone and backfilling them with a mix of compost, biochar, sand, or organic matter. This loosens compacted soils and creates air/water channels.
Radial Trenching: Cutting radial lines from the trunk outward (like spokes on a wheel) and filling them with amendment-rich soil. This mimics natural root growth patterns and facilitates deeper root development.
Trunk Application: Certain biostimulants or liquid additives can be applied directly to the trunk (via bark sprays or injections) when root uptake is impaired. Always follow the label and only apply if trained to do so.
If you enjoyed this post, be sure to check out the e-learning course available on our website, taught by Board Certified Master Arborist Matt Latham. This course explores his holistic approach to tree management, lasts approximately one hour, and is approved for ISA CEUs.