Aww…. the holiday season. Very few things compare with the presence of family and friends gathered around a toasty warm fire in the fireplace or wood stove. The warmth of the Yule Log on New Year’s Eve has become a distant memory, leaving behind the memories — and a pile of gray ashes. But what can be done with the wood ashes that have accumulated over the holiday season? Well, it just depends.
The burning of wood releases nitrogen and sulfur as gases. And the elements of calcium, potassium, magnesium, and trace elements such as boron, copper, molybdenum, and zinc are left behind, many of which are essential to plant life. In general, the principal nutrients in wood ashes are: potash (potassium) (3-8%), phosphate (1-2%), calcium (20-25%) and magnesium (2%). In terms of commercial fertilizer, a bag of wood ashes would be labeled 0-1-3 (0% nitrogen, 1% phosphate and 3% potash). Calcium is the most abundant element in wood ash and has the same effect on soil as lime — it reduces the acidity or raises the pH level of the soil. The chemical make-up varies with the type of wood burned. In general, hardwoods produce greater concentrations of chemicals than softwood.
Right off the bat, wood ash has three things going for it — it’s a fertilizer, it will raise the pH of the soil, and it’s cheap. Great stuff, right? Well, not so fast!
Wood ash works faster than lime. When wood ashes are applied to the soil, it raises the pH of soil, much like lime. Yet unlike lime, which can take six months or more to alter the soil pH, wood ash is water-soluble and works very quickly to change the pH.
Remember, the pH of the soil affects a plant’s ability to harvest nutrients from the soil. For example, certain nutrients like iron, copper, and aluminum become less available to plants in alkaline soil (higher-pH soil), while other nutrients, such as calcium and phosphorus, become less available in acid soils ( soils with a low pH). The recommended pH range for a vegetable garden is in the range of 6.2-6.8. Therefore, wood ashes should NOT be added to the garden if soil pH falls within the recommended range. Those wood ashes could raise the pH to a point that it becomes detrimental to many garden plants. The only way a gardener can make an informed decision on adding wood ashes to the garden is by doing a soil test. The Virginia Cooperative Extension Publication 452-129 provides information on how to collect a soil sample, and soil test kits are available at your local Virginia Cooperative Extension Office, located a 460 Stagecoach Road, just off Fifth St. Extended in Charlottesville. Additional information on the value of soil testing can be found in our October 2016 issue of The Garden Shed; see the article titled Interpreting Your Soil Test.
Once you have received the results of your soil test, if the recommendation is to add lime to raise the pH, the general rule is it takes 2 pounds of wood ash to equal the neutralizing power of 1 pound of ground limestone. For example if the lime recommendation is to add 8 pounds of lime per 100 square feet, 16 pounds of wood ash would be the equivalent.
Wood ashes should NOT be placed around certain plants; for example, blueberries that prefer more acidic soils, or potatoes, as scab disease becomes more prevalent in soils with a high pH. Potatoes are less likely to develop scab when the pH is between 5.0 and 5.2.
Do NOT use ashes that result from burning coal, pressure-treated wood, painted or stained wood, or cardboard. These materials contain potentially harmful chemicals.
Careful consideration should be made before using wood ashes in the garden. A measured application can be beneficial to increase soil pH. Over-applications of wood ash will increase the likelihood of soil-related problems. So before you start spreading those ashes, be sure to have your soil tested and follow the recommendations.
Thanks for stopping by The Garden Shed; we look forward to your visit next month.
“Best Management Practices for Wood Ash as Agricultural Soil Amendment,” University of Georgia Extension, Publication No. B 1142, http://extension.uga.edu/publications/detail.cfm?number=B1142
“Soil Preparation,” Virginia Cooperative Extension, Publication No. 426-313 https://pubs.ext.vt.edu/426/426-313/426-313.html
“Using Wood Ash in the Garden,” University of Illinois Extension, http://web.extension.illinois.edu/state/newsdetail.cfm?NewsID=12505
“Soil Sampling for the Home Gardner,” Virginia Cooperative Extension Publication Number 452-129, http://pubs.ext.vt.edu/452/452-129/452-129.html
“Potatoes, Peppers and Eggplant,” Virginia Cooperative Extension Publication No. 426-413 https://pubs.ext.vt.edu/426/426-413/426-413_pdf.pdf
“Wood Ashes for Gardening,” Penn State Extension, http://extension.psu.edu/lackawanna/news/2013/wood-ashes-for-gardening