By Kaylen Foley, M.Sc.
For many landowners and foresters, sugar maple (Acer saccharum) is more than just a tree, it’s a cultural symbol, a keystone species, and an anchor of forest health in Ontario. But decades of acid rain, poor soil recovery, and nutrient loss have left many sugar maple stands stressed and struggling. Trees on thin, base-poor soils are especially vulnerable, with signs of dieback, weak growth, and reduced vigor becoming increasingly common.
What if a local waste product could help reverse that decline?
In my master’s research at Trent University and with the help of the Friends of Muskoka Watershed, I studied whether non-industrial wood ash, a byproduct of home heating, could improve soil health and support sugar maple recovery in central Ontario. This type of ash is created when homeowners burn firewood in stoves and fireplaces. While often landfilled, it contains valuable nutrients like calcium (Ca), magnesium (Mg), and potassium (K) that could be recycled back into forest soils.
The Experiment
I set up a field study in a sugar maple dominated stand near Bracebridge, Ontario. Over two growing seasons, I tested the effects of five different ash doses (0, 2, 4, 6, and 12 metric tonnes per hectare) on sugar maple trees. I measured changes in soil chemistry, root biomass, leaf nutrients, and stem growth using dendrometer bands for precision.
What We Found
The results were promising but nuanced:
Soil improvement: Ash raised soil pH and boosted base cations like Ca and Mg, especially in the forest floor. This effect was strongest in the first year after application.
No metal toxicity: While some metals like zinc and manganese increased slightly in the forest floor, leaf concentrations stayed within safe ranges. No signs of toxicity were observed.
Root response: Fine root biomass declined at higher ash doses. This could be due to a “pH shock” from the ash’s high alkalinity or nitrogen limitation (ash contains very little nitrogen).
Tree growth: No immediate increase in stem growth was detected over the two years, but this isn’t unusual. Growth responses to soil amendments often take more time, especially in mature forests.
What Landowners Should Know
If you’re burning wood at home and want to give back to your forest, applying ash to the soil may help but it should be done carefully.
Let it weather first: Fresh wood ash is highly alkaline (pH 9–13). Letting it sit outside and get rained on for several weeks before applying helps reduce the risk of pH shock to roots.
Use it only where needed: Sugar maples growing on shallow or acidic soils are more likely to benefit. Avoid applying ash on already rich or calcareous sites.
How much is safe? Our study showed that small applications—around 2–4 Mg per hectare can improve soil without causing harm. For context:
4 Mg/ha equals about 400 grams per square metre, or roughly 2 cups of ash spread evenly over a 1 m² area. That’s about one 20-litre pail (~5 gallons) of ash per 50 square metres (roughly the size of a backyard garden).
Spread it thinly across the soil surface, ideally in fall or early spring, and avoid dumping large piles at the base of trees.
A Circular Future for Forest Recovery
Ontario soils have lost significant calcium and other nutrients due to acid rain and logging. Replacing those nutrients is costly—lime is effective but expensive and carbon-intensive to produce. Wood ash, in contrast, is a low-cost, locally available material with clear potential to restore soil health—especially for calcium-loving trees like sugar maple.
This research shows that non-industrial wood ash can safely improve soil chemistry under sugar maple stands, at least in the short term. While growth benefits may take longer to appear, applying small, pre-weathered doses of ash offers a promising way for landowners to support their trees and reduce waste—turning a fireplace by-product into a tool for forest recovery.
Kaylen Foley is a recent M.Sc. graduate from Trent University whose research explores the use of wood ash as a tool to support sugar maple recovery while examining its effects on soil health and carbon cycling. Her work bridges forest ecology, dendrochronology, and sustainable management to better understand how soil amendments can enhance resilience in Ontario’s hardwood forests.