It’s that time of year. With winter come nights with clear, cold skies, often accompanied by the smoke and aroma from wood-burning fireplaces. Many folks enjoy a roaring wood fire and the warmth, whether it be physical or psychological comfort, that it provides. This innate fascination with a wood fire is primal.
I can vividly remember a time when my grandparents heated their farmhouse with wood-burning stoves and cooked on them, too. There was a big woodshed in the barnyard and chores associated with gathering, splitting, stacking, curing and transporting the firewood. For many people back in the day, and particularly those living in the country, using firewood to meet daily energy needs was an absolute necessity and a formidable challenge.
Fast forward to the present. Fewer homes being built in the United States have fireplaces, let alone burn wood for heat. According to the 2021 American Community Survey by the U.S. Census Bureau, only 1.9% of U.S. households rely on wood for heat. Predictably, rural regions, where winters are harsh and wood is a sustainable resource, are more dependent on wood. Vermont and Maine are at the top of the list, at 12.3% and 7.8% respectively, while Virginia is at 1.6% and North Carolina is 1.2%.
Nevertheless, at times during the winter in my urban neighborhood, there is plenty of wood smoke in the air. But what happens to the wood ash? Just a guess, but I suspect that most gets dumped into the yard waste or the trash bin and hauled away.
Is there, you might ask, a more sustainable use for this byproduct at the source? Indeed there is: in your garden and landscape.
Wood ash is a valuable source of several elements necessary for plant growth. The secondary macronutrient calcium is far and away the most abundant, typically around 20%, according to the University of Wisconsin Extension. Next is the primary macronutrient potassium, around 5%. The primary macronutrient phosphorus and secondary macronutrients magnesium and sulfur each weigh in at about 2%. Last, boron, iron, manganese and zinc may occur in trace amounts.
But that doesn’t mean wood ash is a good fertilizer per se. It is more or less a 0-2-5 “low nutrient analysis” fertilizer. That means its ratio of the three primary macronutrients — “macronutrients” being the ones plants need the most of — is zero nitrogen (N), 2% phosphorus (P) and 5% potassium (K).
Its potential for raising soil pH is where wood ash shines as an amendment. Depending on the type of wood and method of burning, high levels of carbonates may result; they can raise pH and neutralize soil acidity. Compared to off-the-shelf acid neutralizing products, such as pelletized or agricultural lime, wood ash has about 25% to 50% of their acid neutralizing ability. As a starting point, 2 cups of wood ash can be substituted for 1 cup of agricultural lime. Another bonus is that wood ash, being more soluble and reactive, can yield a more rapid response.
But before you try to round up your neighbors’ ash, you need a professional soil test. That will tell you exactly what amendments your soil needs for the optimum pH for crops and for maximum soil fertility.
Wood ash is highly alkaline. Too much of it can hurt the growth of plants that prefer and thrive in acidic soil. Elevated soil pH — more alkaline — can restrict the availability of nutrients that these plants require.
If you do choose wood ash, keep in mind these points. They’re from the University of Wisconsin Extension and the University of Saskatchewan College of Agriculture and Bioresources:
- Apply it in fall or winter and on a calm day, to moist soil, to prevent drift of the fine ash.
- Not all wood ash is the same. Use only ash derived from native trees — not from treated or painted wood; coal; barbecue briquettes; or lump (wood burned to charcoal).
- Sift it first to remove large pieces of charcoal.
- Spread it evenly at no more than 15 to 20 pounds per 1,000 square feet per year. (A 5-gallon bucket will hold about 20 pounds of ash.)
And for safety:
- Wood ash is caustic. Wear eye protection, a mask and gloves.
- Never mix it with nitrogen fertilizers.
And one more thing (or two)
The winter workshops from the Chesapeake master gardeners and Virginia Cooperative Extension continue through March. Workshops include bonsai, espalier training, pruning, camellias, soil blocks, vermicompost and layering (see bit.ly/3JAjs33). For details and to register, call the Chesapeake Extension Office, 757‐382‐6348.
Did he say soil testing? Before you know it, spring planting season will be upon us. Get on it. For more information, see the Virginia Tech Soil Testing Lab, soiltest.vt.edu (bit.ly/3NXhXhn).
Send questions to wkspen@gmail.com