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My grandparents used to spread the ash from the fireplace on the vegetable plot. This was in reasonably large quantities (I mean, not just a thin trail of ash to deter slugs), and was done to "add" to the soil. Visually, it was an eyesore, but they didn't worry about that.

I am sure I've read elsewhere that this practice is without value, and in fact I dump our fireplace ash out for the bin men. But every time I do it I wonder whether I should be using it in the garden. And it's free, and I have to cart it straight past the veg anyway, to get to the dustbins.

So, is it useful to add ashes as a kind of fertilizer?

Edit We are mostly burning firewood, sometimes with a small amount of Taybrite

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Wood ash from hardwoods is a useful garden soil amendment, in reasonable quantities. Don't use coal ash or other sources. Be aware of what was burned: I wouldn't want ash that has anything left over from burned plastics or stuff like that.

Ash has a very high pH, and can be useful for helping to raise the pH of acid soil. I use all the ash from my wood boiler both on my lawn and in my garden. (We have very acid native soil.)

It also has (varying depending on the woods that were burned) quite a few nutrients -- which makes sense, since it comes from a plant that consumed lots of nutrients while it was growing. In particular, it is a good source of Calcium and Potassium as well as several micronutrients.

Just don't overdo it, you can raise your pH too far.

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  • Interesting this pH thing. I was thinking of smoke which is definitely acidic (steam locomotive condensing equipment are notorious for acid corrosion from smoke; acid rain; etc). But yes I would expect things like Potassium - and those minerals will form alkalis...
    – winwaed
    Commented Jun 15, 2011 at 21:28
  • @win - You can make lye by pouring water through wood ash. (Not that I've had need to do such a thing.) Very strongly alkaline.
    – bstpierre
    Commented Jun 15, 2011 at 21:31
  • accepted because this is closest to the article cited in @Mancuniensis link
    – Tea Drinker
    Commented Jun 17, 2011 at 11:50
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I think there's a big "depends". There can be useful minerals in ash, and fly-ash (i.e. commercial coal burning) is sometimes used in agriculture and landscaping. Coal definitely has lots of non-combustable minerals, but it can also contain some nasties - so I'm not sure what the agriculture people do.

I would NOT use charcoal briquette ash (e.g. as used on grills). These definitely contain some nasty things in them - as do the unburnt briquettes, and the whole briquette manufacturing process is notorious for dioxins.

So in comparison, wood ash is quite benign but I can't think of any minerals that would be present and survive burning. I have seen wood ash being dug into a heavy clay soil to break it up. That was quite effective, but that was for texture rather than fertilizer.

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There is an excellent article on the composition and use of wood ash in the garden here

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My grandpa raised his garden for 40 years on an old slag dump (he called it "Manufaturer's Glass") the soil is still very productive, very loose for our area.

You might want to add it to your compost rather than directly to your garden.

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