Since the ash is what’s left of burned plant material, it naturally contains many of the essential nutrients that were first provided by the soil. Burning wood loses nitrogen and sulphur but other elements such as calcium, potassium, and magnesium remain. According to Oregon State University’s website, “The carbonates and oxides in the ash are valuable liming agents that can raise pH and help neutralize acid soils.” Here’s a quick guide to what can and what shouldn’t be done with wood ash from your fireplace:
- Apply wood ash to unplanted soil and rake or dig it in. According to the Royal Horticultural Society, “[Applying wood ash in winter] will allow the compounds in the ash which could scorch plants to react with the moist soil and be rendered harmless before spring sowing or planting.” In Southern Ontario, where the soil in winter usually resembles permafrost and is hidden under a blanket of snow, waiting for your bed of dirt to become pliable in early spring is clearly a must. And then delaying planting for a month is a good precaution.
- Use only untreated wood. Ash from pressure-treated, painted and stained wood can harm plants.
- Pick hardwood over softwood ash. Hardwood, according to OSU, “produces about three times the ash and five times the nutrients per cord as softwoods.”
- Add some smaller branches to the fire. The ash from young wood has higher potassium levels (a good plant nutrient) than older, thicker wood.
- Save ash from your summer fire pit as well. Lawns needing lime and potassium can benefit from a light sprinkling of ash.
- Don’t leave wood ash in a pile on the ground. Harmful salts from the ash can leach into the soil, harming plant roots. Instead, store ash in a container until ready to use.
- Don’t think that wood ash is good for all plants. Because ash is alkaline, it’s detrimental to acid-loving plants such as rhododendrons and azaleas.
- Don’t mix ash with nitrogen fertilizers. The high pH in the wood ash cancels out the nitrogen. But National Geographic once reported a surprising additive that, when mixed with ash and used to fertilize tomato plants, produced plants yielding four times more fruit than non-fertilized plants. And this additive, unlike commercial fertilizer, is completely renewable, extremely easy to access and has a light footprint. Ok, two footprints.
For more details on using wood ash in the garden, try these helpful articles: