I typically start introducing new plants right after New Year’s, and I will next week. But since so many of us are heating our homes with wood stoves, I thought it might be more timely to discuss what you can do with the buckets of wood ash you shovel out of the stove or fireplace.
Did you know that wood ash is a useful soil additive? Well, it is. It can supply a variety of major, minor and trace nutrients that we typically add by way of fertilizers to our soil. Those nutrients include potassium, phosphorus, magnesium, calcium, iron, manganese, boron, copper, sulfur and even a little zinc. It’s difficult to pinpoint exact nutrient quantities, but wood ash generally contains about 1 percent phosphorus, 10 percent or less of potassium, up to 25 percent calcium carbonate and trace amounts of all of the rest, which our plants require only in minute amounts anyway.
I’m betting calcium carbonate grabbed your eye when you read over this list. The lime we use to modify or sweeten our soil contains calcium carbonate, which gives away the biggest secret of wood ash’s usefulness in the garden. Top agricultural universities Purdue and Ohio State specifically state that wood ash is from 40 to 50 percent as effective in sweetening an acidic soil as is lime. Hardwood ash is closer to the 50 percent end; softwood nearer the 40 percent end.
Know your pH
So, how much can we safely use? First, let’s do a quick review. The pH scale runs from 1 to 14. Smack in the middle is neutral. The soil becomes more acidic the lower you veer, with 1 being pure acid. Climbing up from 7, the soil becomes more basic or “sweet”; 14 is lye. Evergreens and a few others such as blueberries and oak generally prefer an acidic pH in the range of 5.0 to 6.5, lawns range from around a 6.3 to 7.0.
Most of the vegetables, trees, shrubs and flowers in our area of upstate New York enjoy a range from 6.5 to 6.8. Remember, each plant has its own preference, and pH plays an incredible role in how soil particles release and make available various nutrients to the roots of the plant. So, to provide the best possible conditions for your particular plant, match its pH needs to the soil.
A soil test is the recommended first place to start. You’re probably saying, right, Nancy, and if you haven’t noticed, the ground is frozen! But listen, as gardeners, we have a pretty good feel for how our gardens performed last season. We know the trees overhead, the leaves and needles they shed, when our last lime application was ... sometimes we just need to think “outside the book” and rely on our “eyes” for knowledge.
My large gardens are below overhanging oak and pine branches. The leaves and needles fall, and come spring, they are incorporated into the soil to build up the organic matter (I do not remove them but rather work them back into the soil). Knowing this, I also know that the pH will be affected in an acidic way; pine needles, oak leaves and organic matter all help to lower pH.
As a gardener, I also know that soil pH doesn’t change in a year in huge increments but is altered slowly. Plants could not survive drastic changes by nature or by man, which is why we need to test our soil only every few years.
I work organic matter like leaves, needles and debris back into the soil come spring, and it naturally and slowly lowers the pH. I offset this alteration by spreading a 5-gallon pail of wood ash per 1,000 square feet.
This rate gives me roughly the same calcium carbonate rate as a recommended lime application to raise the pH by one-half increments. The pH never really gets out of kilter, the ash offsets the organic matter, the organic matter offsets the ash and the soil is the true winner. Do I test my pH as recommended? Absolutely. I don’t rely on my eyes to see what’s below ground, but I do allow myself to use common sense.
So, in conclusion, wood ash, particularly hardwood ash, can be used as a soil additive for both nutrients and as a conditioner for pH. Here are a few other tips:
• Using a 5-gallon pail (or 15 to 20 pounds) per 1,000 square feet will raise the pH roughly one-half increment (about ¼- to ½-inch deep). Remember, wood ash contains 40 to 50 percent of the calcium that lime contains, so the application rate will appear to be heavier.
• Apply only cooled ash. Hot ash is a potential fire hazard.
• Keep the ash dry until you spread it to avoid any leaching of nutrients.
• Do not use ash from treated wood.
• Lilacs and daffodils will appreciate a dose; both love a near-neutral pH. Acid-loving plants such as blueberries and pachysandra, however, will not.
• Ash can be used safely in vegetable gardens; however, it’s recommended that you wait at least two weeks to plant after application.
Because of its fine texture, wood ash acts fast and will dissipate quickly into the soil with a good rain.
Continue to test your soil’s pH.