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I wonder of the impact ash (probably combined with still existing coal) might have to compost. I know that coal is used to clean water and air. But does it improve compost or does it reduce activities inside of compost heaps?

I assume that ash is cold and nothing inside is burning!

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The problem with store-bought BBQ fuel is that they often add binders, accelerants and other things to the basic fuel in order to make it easier to manufacture/shape/pack/light/burn. Whilst the vast majority of store-bought BBQ fuels won't impart anything particularly bad into the food being cooked, the by-products (ash and partially-burnt coals) were never meant to come into contact with food.

Since the ashes of BBQ fuels were never meant to be reinjected into the food stream, their content is not regulated anywhere (that I know of). There are no safety protections in place.

Ash, in particular, is no longer 'organic' after being burnt. The heat of the fire has reduced it to mineral status. Whether there are things like heavy metals in there is anyone's guess. Putting that stuff into a compost pile won't 'help make it safe' because the bacteria/fungus in a compost pile feed primarily off organic molecules, not inorganic minerals. Nothing can turn cadmium into carbon.

Are there BBQ fuels being sold in stores that are 100% natural with no additives? Sure, but in my area you have to look hard to find them, they usually aren't cheap, and they often don't "work as well" as the ones that have been heavily bound and doped with accelerants.

If you make wise purchasing decisions in the store, you can throw as many coals into the compost pile as you like and they will act as moisture and nutrient stores, as well as habitat for microbes and fungi — all positive with no downsides. Small amounts of ash (less than a cup a week) will also be fine, and provide trace minerals that various organisms can use (without messing with the pH too much).

If you're producing more than a cup of ash a week, the typical 4-person compost pile won't be able to handle the load, and you should scatter the rest of the ashes evenly out onto fields/pastures, or under trees and bushes (assuming you don't want to use it to make lye for soap or any of a pile of other things). Treat the ash as a mineral cocktail that most plants can handle in very small doses, and you'll be fine.

Note: Since ash raises the pH of the soil onto which it is applied, it follows that plants that prefer acidic soils won't appreciate it. Avoid dusting the ground around such plants with ashes.

If you dump a pail of ashes into a single spot on a weekly basis — compost pile, base of tree, doesn't matter — I can pretty-much guarantee that you will kill everything there. I know of no plants or organisms that can survive that sort of abuse.

  • 1
    The charcoal for BBQ sold in our area is declared to be from organic growth and not otherwise treated. The question is: Should I trust it? – Salt Aug 17 '18 at 20:47
  • @Salt Yep, that's the problem. With no regulations in place, the marketing department can pretty-much put anything they want (or omit anything they want) on the packaging of a product. You, as a consumer, simply don't (can't) have confidence in what you read on the packaging. Online research about the product, the processes used to make it, and the company (or companies) involved in making it, may provide some degree of assurances, though. There are safe BBQ fuels out there — you just need to exert some effort and find them. – Tim Aug 18 '18 at 2:05
  • As far as specific things you can do to test charcoal, I always use the 'sniff test' first. If it smells like petrol, diesel, oil, grease, or wax then it definitely contains accelerants. The other test is 'ease of lighting'. Pure charcoal is not actually that easy to light. If you can light the fuel with just a match, it probably contains accelerants. Pure charcoal (in semi-round lumps) is nothing but carbon, and should require kindling or a blowtorch to get going. – Tim Aug 18 '18 at 2:21
  • On the other hand by the time the barbecue is out, there will be trace quantities at most of accelerants. I can buy lumpwood charcoal, rather than briquettes. The latter are reconsituted wood (from unknown sources), pressed together apparently without additives, but the former are made from forestry waste and still look like random bits of branch. The little BBQ ash I produce gets stirred into the compost bin – Chris H Sep 14 '18 at 15:52
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I'm not sure of the science behind it, but I know that my local compost collector is fine with accepting fireplace ashes, but has a ban on coal and charcoal ashes. They use heat-treating on the compost as well, so if that can't handle coal and charcoal ashes, maybe be wary of using it for your at-home pile. Incidentally, if you do want more information about it, I'm sure that someone at Bennett Compost (my local compost company) would be able to talk to you about it!

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Do you mean charcoal ash from a BBQ or coal ash ? Wood ash will add K which is very beneficial and some alkalinity , likely good as decaying organics in compost are acidic. Coal ash will have much more mineral content; silica ( sand), iron oxides, ( the pink or reddish color) aluminum oxides, some K and alkalinity ( eg. calcium oxide). Coal ash composition will depend on the source of the coal. If you want count the minuscule traces , like sea water , it contains about every element ( If you have the analytical equipment to find it).

  • By "K" are you referring to potassium (by its chemical symbol)? Or something else? – LShaver Aug 17 '18 at 16:58
  • Initially I was just thinking about charcoal used from BBQ, but the question may expand to ash from burned wood in a fire place. In my area the chimney sweeps recommend to put ash into metal trash cans instead of putting it into the garden. But this seems to be related to the fact that many people burn painted or treated wood, which is forbidden due to its content of heavy metals etc. Ash from fossil coal I would not use for compost, even not knowing why. Doesn't seem organic to me. – Salt Aug 17 '18 at 20:48
  • Yes, K = potassium, i am a lazy slow typist. – blacksmith37 Aug 18 '18 at 19:28

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