By Bonnie Kirn Donahue
Wood ashes are a surprisingly useful byproduct of winter heating. In addition to household uses such as making soap or adding traction to driveways, wood ashes can be used to amend your garden and lawn soils.
Wood ash, like limestone, is high in calcium, and can raise soil pH to provide the most optimal plant growth.
The pH of soil is important because it helps make certain nutrients more available for plants to use in the soil. For example, when soil pH falls outside of optimal ranges, critical nutrients for plant growth, like nitrogen, phosphorus, potassium, sulfur, calcium and magnesium, may be less available for plants to take up.
The pH is measured on a scale of 0 (acidic) to 14 (alkaline). An optimal soil pH for most vegetable and berry crops is 6.5 to 6.8. Generally, a 6.0 to 7.5 pH should support most crops. Different species have their own pH preferences, however, so be sure to look up the preferred pH of your plants prior to amending the soil.
Blueberries like acidic soil with a pH of 4.5 to 5.2, so they probably won’t appreciate wood ash. Azaleas also prefer acidic soils with a pH between 4.5 and 6.0. Vegetables such as asparagus, kale and beans need basic soils with a pH between 6.0 and 8.0, while many landscape shrubs and trees, such as dogwood, do best in alkaline soil (5.5 to 6.0).
Before adding amendments, first get a soil test to measure the existing pH of your soils. The test will tell you the pH and offer amendment options.
The University of Vermont Extension Agricultural and Environmental Testing Lab offers soil testing. For information, go to go.uvm.edu/ae-testing.
Following your soil test results, you can apply a thin layer of wood ash (remove any large charcoal pieces) on top of your soil. Use an application rate of 15-20 pounds of wood ash (one five-gallon bucket) per 1,000 square feet. One cord of wood will produce approximately 20 pounds of wood ash. For more information, check out: go.uvm.edu/wood-ash.
Moisten the soil prior to applying the wood ash to help it adhere to the soil and become less airborne. Ash can be irritating, so protect your skin and lungs when spreading it. Lightly water the area to help the ash adhere to the soil.
Make sure that the wood ash you are using contains only untreated, naturally grown wood. Using wood ash that is contaminated by oil, chemicals or plastic can transfer the contamination to your soils. Wood ash from pellet stoves and bonfires also should be avoided.
Wood ash can be added in very small quantities to compost piles, keeping an eye on the pH. Optimal compost pH for microorganism activity is between 5.5 and 8.0. The key is knowing the pH of your compost pile before you add wood ash and spreading a thin layer of ash across the pile (versus dumping it in).
This year, after getting your soil tested, try using leftover wood ashes in your garden and on your lawn. This low-cost soil amendment just might be something you add to your garden to-do list from now on.
Bonnie Kirn Donahue is a UVM Extension master gardener and landscape architect from central Vermont.
