Great work from Dr. Rebecca Abney and UGA’s Warnell School. City trees fight compacted soil, poor drainage and drought every day. Her research shows how biochar mixed into the planting soil can help them hold water and grow stronger. That’s exactly the kind of work we want Georgia Wood Char’s biochar supporting.
Street trees have a hard life below the sidewalk.
Trees along city roads and sidewalks are often planted in underground tree vaults or tree pits. Those spaces can be tough on roots: drought, overwatering, heat, nutrient deficiency, poor drainage and aeration, and contamination (TREE Fund). A University of Georgia study asked a practical question: can biochar help, and does how you apply it matter?
Note: this is independent research by the University of Georgia, funded by TREE Fund. It did not test Georgia Wood Char products, and Georgia Wood Char has no affiliation with the researchers.
The study
Dr. Rebecca Abney of the University of Georgia, with co-PI Julia Holly Campbell, received a 2021 TREE Fund Tree and Soil Research Fund grant titled “Utilizing biochar to improve community tree vault soils and tree health and to reduce watershed contamination” (TREE Fund). Dr. Abney is with UGA’s Warnell School of Forestry & Natural Resources (UGA Research Insights).
The team planted two common street trees, willow oak (Quercus phellos) and red maple (Acer rubrum), in simulated tree vaults. They compared biochar applied in relation to soil volume at:
- 1% mixed into the soil
- 5% mixed into the soil
- 1% as a top dressing
- 1% at the bottom of the pot
They tracked soil quality, water quality and tree growth.
What they found
According to the TREE Fund results summary, biochar increased tree growth when mixed to 5% of soil volume and when applied at 1% as a top dressing. The 5% rate had the greatest increase in soil water holding capacity and tree growth. The 1% top dressing also increased growth and water holding capacity, and the researchers concluded that different application methods should be used for different objectives (TREE Fund).
The results summary does not publish specific growth or water holding figures, so we don’t quote any here. The work was presented at the Soil Science Society of America Annual Meeting (November 2022) and the Georgia Water Resources Conference (May 2023), and in a TREE Fund webinar in December 2024.
What comes next
Dr. Abney also leads an NSF CAREER project on dissolved and pyrogenic organic matter in fire-impacted soils. Part of that work will establish a tree vault study with biochar in two soil types, sandy and clay, typical of Georgia, to describe how applied biochar carbon moves through soil over longer time scales (UGA Research Insights, NSF CAREER).
How to apply it: notes for arborists, cities and landscapers
New plantings and vault rebuilds: mix it in
When you’re building or refilling a tree vault, the study points to mixing biochar through the soil at 5% of soil volume as the strongest option for growth and water holding.
Existing trees: top-dress
Where you can’t dig, a 1% top dressing still increased growth and water holding capacity in the study.
Match the method to the goal
The researchers’ own conclusion is that method should follow objective. Results will vary with your soil, species and site, so trial it and measure. TREE Fund lists contact details for the researcher on its project page if you want the full findings.
Georgia Wood Char Grades as a practical option
If you want to try these application methods, our biochar Grades are made from Georgia wood with licensed, patented Biochar Now kiln technology; slow pyrolysis with no quenching keeps the natural cell structure.
- Medium (3 mm to 26 mesh): direct placement into soil or top-dressing.
- Small (26 to 50 mesh): direct placement into soil or top-dressing.
- Powder (50 mesh and under): direct placement into soil, water spray and irrigation systems.
- Dust (20 micron): slurry and mixed ground application.
Garden centers and landscape suppliers: ask about supply for your customers. Results available on request. Email sales@gawoodchar.com or contact us.
Precision built product, not a byproduct.
Sources
- TREE Fund, Utilizing biochar to improve community tree vault soils and tree health and to reduce watershed contamination (2021 grant, Rebecca Brianne Abney, PhD).
- UGA Research Insights, CAREER: Composition, transport, and fate of dissolved and pyrogenic organic matter in fire-impacted soils (NSF).
Frequently asked questions
What did the UGA tree vault biochar study find?
In simulated tree vaults planted with willow oak and red maple, biochar mixed in at 5% of soil volume gave the greatest increase in soil water holding capacity and tree growth. A 1% top dressing also increased growth and water holding capacity.
Who did the research?
Dr. Rebecca Abney of the University of Georgia, with co-PI Julia Holly Campbell, under a 2021 TREE Fund Tree and Soil Research Fund grant. It is independent research and did not test Georgia Wood Char products.
Which tree species were studied?
Two common street trees: willow oak (Quercus phellos) and red maple (Acer rubrum).
What application rates were compared?
Biochar applied in relation to soil volume at 1% and 5% mixed into the soil, 1% as a top dressing, and 1% at the bottom of the pot.
Should I mix biochar in or top-dress it?
The researchers concluded that different application methods should be used for different objectives. Mixing in at 5% gave the biggest gains in the study; a 1% top dressing is an option where the soil is already in place, such as existing tree pits.
Which Georgia Wood Char Grade fits tree vault soil?
Medium and Small Grades are suited to direct placement into soil and top dressing. Powder and Dust suit slurry, water spray and mixed applications. Contact sales@gawoodchar.com to match a Grade to your soil mix. Results available on request.