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Adding wood ash into your garden

We predominantly heat our home with a wood fire which does a bang up job. However dealing with all the wood ash that comes out of it has been a bit of a quandary for us. As a result we’ve had an impressive collection of boxes filling up with ash outside our back door, waiting for one of us to think properly about how we can integrate it into our property rather than putting it in the bin.

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Because, while a little bit of wood ash on your garden can be beneficial – a lot of it generally leads to tears as it increases the alkalinity of the soil drastically, affecting the plant’s nutrition. Nutrients are most readily available to plants when the soil reads 7 on the pH chart and wood ash can throw this out of balance, pushing this number up towards 10.

“Since wood ash is derived from plant material, it contains most of the 13 essential nutrients the soil must supply for plant growth. When wood burns, nitrogen and sulfur are lost as gases, and calcium, potassium, magnesium and trace element compounds remain. The carbonates and oxides remaining after wood burning are valuable liming agents, raising pH, thereby helping to neutralize acid soils” (soil scientist, Dan Sullivan).

Of course, the fertilizer value of wood ash depends on the type of wood you burn. As a general rule, hardwoods (i.e. eucalyptus) yield more ash per kilogram of wood burned and contain a higher percentage of nutrients compare to ash from softwoods such as pine.

ph_nutrient_chartHowever some plants, including asparagus, are more tolerant of slightly alkaline conditions compared to acid-loving plants, such as potatoes, rhododendrons and blueberries. Wood ash should never be used on acid-loving plants.

So while it’s well known that you can add small amounts of wood ash into your compost pile and spread some onto your vegie beds (it can help deter slugs from attacking your seedlings), I’ve always wondered what to do with BULK ash. At our place we’re trialing a little experiment in our garden which revolves around our ever useful and multifunctional swale pathways. Rather than putting a little bit of ash here and there, we’re seeing whether we can get away with bulk seasonal integration of wood ash going directly into our sawdust swale paths.

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Step 1: Simply spread the wood ash out roughly along the length of the swale path

Our swale path’s main job is to catch and store water, but they’re also slow in situ compost piles, meaning that one day, when the sawdust/woodchips have decomposed nicely they will end up on either a vegetable garden or in our orchard. With this in mind, it’s important that they have good mineral profile, we don’t want to mess with it too much, making it significantly alkaline or acidic. Currently we’re comfortable with the ash input, as the ratio of sawdust (mostly from hardwood timber) is high compared to the quantity of ash going into it. Saying that, we would still consider adding something like blood and bone (strong nitrogen content) to ensure it’s neutralised and perhaps even some pine needles (high acidic ingredient).

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Step 2: Vigorously massage the ash into the top layer of the sawdust. With time, rain and gravity the ash will settle deeply into the path, you also could water it in if it’s particularly dry in your area to accelerate this integration process.

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Step 3: Level it out with your rake and feet so it’s easy to walk along again.

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We also added a small amount into our deep litter chicken run which is another form of slow composting in action in our garden. We only put a small amount of ash in as chicken poo’s pH ranges from 6.5 – 8, 8 being fairly high on the alkaline chart so we don’t want to push it over the edge. But, in moderation we’re comfortable to add ash every now and then as a lot of plant waste and other other organic matter is being mixed in at the same time.

In summary:

  • Do a basic pH soil test on your garden so you know whether it’s leaning towards being alkaline or acidic,
  • If you’re using wood ash on your vegie beds or orchards (and don’t have acidic soils), use it very sparingly,
  • Add small amounts into your compost pile/bin every now and then, and
  • As a way of processing ash in bulk, consider adding it into in situ slow composting systems like the swale path or deep litter chicken run. Where required add in other ingredients which can help neutralise (blood and bone) or acidify the contents (pine needles).

The reason we’re so hell bent on keeping ash out of the conventional waste stream is because we’re big fans of nutrient cycling. We’ll do anything we can to keep nutrients onsite as opposed to sending them ‘out there’ where more often than not they transform into pollution. And also, it’s kind of fun to see how much you can really ‘close the loop’ in an urban environment – which is quite a lot!

*Your blogger is Hannah Moloney, co-director of Good Life Permaculture and lover of all things fun and garden-esk.

31 Responses to “Adding wood ash into your garden”

  1. Victoria

    Another super useful post, thanks! We don’t have wood heating (yet) but here in Argentina all the barbecuing (and there’s a lot of it) is done slow over open wood or charcoal fires and I’ve been long wondering what to do with the ash other than dump it under the hedge in random parts of the garden and cross my fingers that it won’t hurt anything. I’ve got sawdust swale paths in my veg garden so am going to give this a try. Cheers!

  2. Darren (Green Change)

    I put wood ash into a shallow wooden box/tray, and stick it in my chicken coop. The chooks seem to like dustbathing in it (maybe it helps keep mites etc away?). The ash also helps neutralise the acidity of the chicken manure, so when it goes eventually into the compost it’s more balanced.

    • Hannah Moloney

      Hi Darren,
      Yep, I agree the ash would be great to prevent mites from moving in. My research shows me that chook poo is actually more alkaline than acidic, depending on its age the pH ranges from 6.5 – 8. So technically too much wood ash isn’t so great. However if you’re putting in food scraps, plant materials etc it’ll probably all balance out all right :-).

  3. Carl

    Let us know how it goes Hannah. I’ve mixed in a couple of handfuls of ash to my 20L fertiliser bucket, as the ash reportedly contains magnesium and calcium (as oxides) as well as trace elements. If so this should cut down the amount of expensive kelpmeal I need to buy. We’ll see how it goes!
    Awesome nutrient chart too, by the way ♥

  4. Kelly-Dee Knight

    Thanks Hannah,
    Your article was really helpful, saved me a lot of trailing through the inter-web.
    I did a general Google search & it was a lovely surprise that the article I choose to read was by an old aquantiance, it’s great to see you still popping up in my life after all these years (I’m originally an old friend of your sisters from Brissy years ago, now living / gardening in Victoria).

  5. David

    David here from the Barossa valley in South Australia. Tested the vineyard soil. Ph 7.5, tested wood ash from the fire, 7.5. Tested sulphur, extremely acidic. I can therefore spread wood ash on the vineyard and garden, like a fine scattering, as an extra nutrient source. Is it a myth that wood ash is alkaline, or does it reflect the soils that the wood is grown on?

    • Hannah Moloney

      I;m not 100% sure David, but I can speculate that it could reflect the soils grown in. It’s been a while since I did some of this research and can’t remember any facts around this.

  6. Daniela Paun

    Thank you for this interesting article. Did anyone try to use ashes in order to keep away weeds? From between the rocks of a terass, for example? Thank you from Romania

  7. josh

    hi there I added wood ash in a large quantity to one batch of compost and actually found the worm content grow off the charts with that particular batch. I dont know if it was the ash or I just got the mix right on that occasion. I also found that particular batch of compost to be very fine and granulated. The best I have made so far. I also unknowingly added a small amount of woodash to a blueberry bush and found that it fruited off the charts aswell. I had added it because I heard that it was very good for flowering and fruiting, and have been adding it too our fruit trees and compost. We too have wood heating at home so we get alot of ash aswell. Hadnt heard of using it in the chook pen which we will trial soon. Great blog BTW!

    • Hannah Moloney

      Thanks for sharing all that Josh!! Great to hear other ways it can be used. We’re about to plant 6 blue berry trees this season so that’s handy to know :-). Cheers

  8. Katkinkate

    One traditional use for wood ash was as a source for lye to make soap. You run water through it and collect the resulting seepage then add fat and heat. There’s probably a way to calculate the strength of the lye and how much oil/fat to use, but I don’t know it. I’m sure it’s somewhere on the internet if you’re interested.

  9. Silversurfer

    I regularly use a mixture of wood ash, borax, and cayenne pepper, as an insecticide dust on my vegis. When it rains the ash then feeds the soil.

    I also use a similar mix without the borax to persuade bandicoots to relocate their burrowing.

    Strange but true

  10. Dotti

    some interesting reading on here from the blog ….thank you Hannah ….and from comments and replies……..my question is this…..we moved a year ago and there were some tall conifers blocking out the sun from the lawn and it is full of moss. Have had the trees chopped down as basically all dead and very brown in the middle….would applying wood ash to the lawn help to kill the moss or help it grow….looking forward to any replies and getting some sunshine this year to help kill the moss off….thank you

  11. Stewart

    I have been reclycling wooden pallets for wood work projects and the off cuts have been going into my outside fire place. Unfortunately I didn’t initially know about the difference between heat and chemically treated pallets. I’ve got rid of all the chemically treated pallets now but I may have burned some of these already and my father in law, as a nice gesture, unknowingly has emptied the ash from the outside fireplace into our garden beds. If some of the ash is from chemically treated wood, can I still use the bed?

    • Hannah Moloney

      Hi Stewart, I would do a round of soil testing in those beds as you really don’t want to be growing food in contaminated soil. See what the soil tests say – you may have to remove some of the soil and start again…. Sorry!

  12. Polly

    I had a large old wheely bin with no lid and broken wheels. I drilled holes in the bottom then began filling it up with layers of Newspapers, kitchen scraps, woodfire ash. Watering each layer well. When it was full I threw in a good handful of compost worms. 2 years later it is dark rich soil full of worms. I successfully grew spuds in it so it can’t have been too alkaline.

    • Hannah Moloney

      Sounds good Polly – thanks for sharing. I think that ageing process you did with diverse inputs and compost worms would have done wonders in creating a balance product. It’s a great idea to just have a compost system tailored for processing wood ash for folks who have a lot of it! 🙂

  13. Anthony Davidson

    Hi Hannah, thanks for the article. Can you give a rough idea of an appropriate amount of wood ash to add to a 400L Gedye bin, & how often? Any benefit to mixing it in with a bulk load of horse poo? Thanks 🙂

    • Hannah Moloney

      Hi Anthony, I’d do a couple of handfuls for each layer or nitrogen or food scraps. So not too much. It doesn’t matter if you mix it in with other materials or not :-).

  14. Nola Arnold

    Great info. I have a lot of gum tree bark lying around – can i use it to make wood ash for my vegie garden or would the eucalyptus be bad for vegetables and berries etc.?

    I don’t seem to be able to find which timber or bark is safe for making wood ash.

    Hope you can help.

    • Hannah Moloney

      Hi Nola, You can try and just check the pH of it as you go to ensure you’re not putting it out of balance at all. If possible, it’s best to use what you have!


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