Using wood ash in the garden

Gardeners often ask if wood ash is a good amendment for the garden and compost pile. If you are burning hardwoods or softwoods then yes, wood ash helps the garden depending on where you live, the type of soil you have and how you apply it. Used sparingly, it can even be added to your compost pile. Wood ash is a low-cost soil fertilizer and a good way to recycle something that would otherwise go in the landfill. 

Is wood ash a good fertilizer? 

Wood ash adds nutrients to your soil, but the amount varies according to the kind of wood burned. Generally, the largest ingredient in wood ash is calcium carbonate (about 20%). This is followed by potassium (less than 10%), phosphorus (1%) and trace amounts of micro-nutrients such as iron, manganese, boron, copper and zinc. Wood ash does not contain nitrogen.  

Used in moderation, wood ash helps to fertilize your soil. However, since wood ash has no nitrogen at all, it is not a complete fertilizer. Adding compost to your soil will help meet the other nutrient needs of your plants. Over-use of wood ash is detrimental to soil bacteria, so use it sparingly. 

Many gardeners use synthetic fertilizers on their gardens and lawn, but it is dangerous to mix wood ash with synthetic nitrogen. Because wood ash is very alkaline mixing will cause a chemical reaction that produces ammonia gas. Never mix ash with ammonium sulfate (21-0-0-24S), urea (46-0-0) or ammonium nitrate (34-0-0).  

What about pH?  

The calcium carbonate in wood ash is alkaline and can change pH in certain types of soil. In some areas of the continent, wood ash is a common treatment for raising soil pH levels from acidic to alkaline.  

However, soils of the prairie plain regions of Saskatchewan, Manitoba and Alberta are different from other regions of the continent and are naturally more alkaline. These soils also have a natural buffering capacity that resists changes to pH. Wood ash will not change the pH of prairie soils, like it would in soils in other areas. Fortunately, prairie gardeners really don’t have to worry about pH, because even with slightly alkaline soil, nutrients are still available to plants.  

However, soils in northern areas of the prairie provinces are different and likely acidic. You can take a soil test to measure pH or look around you. If plants like bog bilberry, blueberry, creeping snowberry, dwarf bog rosemary, lingonberry, Labrador tea, or small bog cranberry grow naturally in your area, then your soil is acidic, because these plants thrive in acidic soil. The addition of modest amounts of wood ash to northern acidic soils will make it more alkaline, which helps if you are trying to grow vegetables. Root crops like potatoes grow well in northern soils because they tolerate some acidity. However, most other vegetables grow best in soils that are more alkaline. Adding wood ash to northern acidic soils (which lack the buffering capacity found in prairie soils) will become more alkaline. Northern soils also tend to be shallow, so while you’re at it, add lots of compost to build up your soil. 

Wood ash safety 

• Wood ash is caustic. Wear a mask to avoid breathing it in. Protect your skin and eyes by wearing gloves, long sleeves and eye protection. 
• Never mix ash with nitrogen fertilizers such as ammonium sulfate (21-0-0-24S), urea (46-0-0) or ammonium nitrate (34-0-0). These fertilizers produce ammonia gas when they are mixed with wood ash. 
• There are things you should not burn if you are saving ashes. These contain toxic ingredients like heavy metals such as cadmium, chromium, or lead, which are toxic to humans, soil and plants, especially if you grow fruits and vegetables. 
  • Treated lumber 
  • Painted or stained wood 
  • Cardboard 
  • Fake fireplace logs 
  • Coal 
  • BBQ briquettes or residue 
  • Any wood that was ignited with fuel like gasoline 
• Store wood ash in a metal container with a lid. Ensure that ash is completely cold before filling the container. 

How to use wood ash 

The best wood ash comes from burning dry, aged wood from trees like poplar, ash, birch, pine or spruce. It helps to remove or sift out large chunks before you use it.  

Sources

Bang-Andreasen, T., Nielsen, J. T., Voriskova, J., Heise, J., Rønn, R., Kjøller, R., Hansen, H. C. B., & Jacobsen, C. S. (2017). Wood ash induced ph changes strongly affect soil bacterial numbers and community composition. Frontiers in Microbiology, 8, 1400. https://doi.org/10.3389/fmicb.2017.01400 

Brewer, L. (2018). Are ashes good for my soil? [Featured question]. Oregon State University Extension Communications. https://extension.oregonstate.edu/ask-expert/featured/are-ashes-good-my-soil

Griffon, T. S. (n.d.). Bulletin #2279, Using wood ash on your farm . University of Maine, Cooperative Extension Publications. https://extension.umaine.edu/publications/2279e/ 

Lerner, R. (2000). Wood ash in the garden.  Purdue Consumer Horticulture. https://www.purdue.edu/hla/sites/yardandgarden/wood-ash-in-the-garden/ 

Perry, L. (n.d.). Wood ashes in gardens. University of Vermont. https://pss.uvm.edu/ppp/articles/woodash.html 

Saunders, O. (2018). Guide to using wood ash as an agricultural soil amendment. University of New Hampshire Extension. https://extension.unh.edu/resource/guide-using-wood-ash-agricultural-soil-amendment