Using Coal Ash in the Garden

Well all the snow is gone now from the fields but it’s still there on the mountains in Snowdonia. Makes you cold just looking at them! I’m not getting much done outside except for the odd trip to the compost bin. Even that’s not doing much in this weather.

Smokeless Fuel Ash & Wood Ash

One thing I am adding is the ash from our multi-fuel stove. Basically it’s a mix of wood ash and a small amount of smokeless fuel ash. Now I’ve done a little research on this subject and this is what I’ve found out.

The official line on adding coal or smokeless fuel ash to the garden and particularly the vegetable plot, is don’t do it. Apparently the ash can contain trace elements of arsenic and metals such as cadmium, iron, lead, zinc, aluminium and so forth.

Official Line versus Reality

Now this brings us to the ‘official line’ versus reality. If I was a scientist or the representative of the solid fuel association, I’d take the safe line that it’s not good for the garden. That way, nobody sues me and I can’t be blamed.

Now in reality, the soil already contains traces of those elements and should do so. A little zinc or iron is needed by plants and by us for that matter. However, too much of anything is a problem and can be dangerous.

So will a small amount of coal ashes cause a problem in the garden?

I’ve found a reference to a scientist who says not but refuses to be quoted (it’s that blame game again) and my feeling is that he’s right. We have had a couple of mainly coal fires and the ashes from those were used to grit the path. Very effective they were too. My Grandad used soot on his onion bed and soil, ashes to lighten and break up clay and it didn’t seem to do him or the family any harm.

Our normal wood and bit of coal ash, is going into the compost bin. Wood ash is definitely safe and good for everything from trees to tomatoes. That assumes the wood was ‘clean’ i.e. untreated or not painted. It can go directly around fruit trees or be stored in an air-tight tin until needed for the tomatoes.

Adding it to the compost is just more convenient and hopefully the nutrients contained in it will still be available when required next year.

Free Wood!

I mentioned about community in a my post on rural living. Well our neighbours from the back knocked on the door yesterday and asked if I wanted any wood. They’ve found a source of scrap wood.

They refused my repeated offers of payment or at least a contribution to their petrol costs for transporting it and we unloaded half a pick-up truck round to the cow shed. Some is a little damp, but it’s drying out. Incidentally, there’s a big difference between dry timber that has gotten damp and unseasoned timber that still contains high levels of sap and moisture. Even damp ‘dry’ timber will burn far better than unseasoned timber.

Most of wood is clean, there are a few bits of painted or stained wood. Some of it is actually good enough to use for something. That’s the allotment attitude in me coming out. See a skip? Dive into it as there’s bound to be something useful. Even old net curtains make good horticultural fleece in an emergency.

Missing the Allotment

Had an email from Crewe the other day from a chap asking about the allotment association there. Must admit it made me realise I’m missing the allotment. Not so much the actual plot as the people on the site.

Mind you, this is a depressing time of year. You get up and it’s dark and you’ve hardly finished lunch and it’s going dark again. Not long to the winter solstice now, the shortest day, on the 21st December and then the days will slowly lengthen again.

Posted in Allotment Garden Diary
30 comments on “Using Coal Ash in the Garden
  1. Steve in Salford says:

    I agree on the trace elements but some of them are not good for the environment. Lead and arsenic are water soluble, not only that they can bio-magnify, getting into the food chain. Natural elements are one thing but long polymer chain or man made substances are not good for the environment they take years to break down if at all.

    I did some work on some contaminated land which the local community wanted to use as a growing space. Now the high priced consultants said it was unsafe to use and could not work out where the contamination came from.

    We took one look at the site and said what is up there, it was of course railway lines, loads of sidings and some nasty chemical factories in the local area, not forgetting bleaching, dyeing and finishing works and lots more besides.

    The contamination had leached down from the railway, our solution place a physical barrier between the ground and your growing space. Using poly-tunnels on the hard standing. Remember the contamination is in the trees as well, leaves falling on the planters could cross contaminate any food grown.

    It’s all about minimising risk, there are other plans for this site to use it as a community asset, wildlife area and growing space. At the same time re-mediating the land.

    Incidentally I asked the question of Defra, RHS and others as to what the research showed about contaminants organic or otherwise getting into food chain, the answer there is no definitive research to turn to. Hmmm…when much of our land is post industrial and contaminated that causes me concern.

    If we adhered to the what the authorities said we would not be able to grow much if anything at anywhere. The American food chain shows traces of polybrominated diphenyl ether ((PBDE)flame retardant substance) in butter, they cause thyroid trouble, so research is done but not in this country either that or they are keeping it very quiet.

    However we should be aware of the risks of growing any food anywhere and minimise the risk to human or animal health/welfare.

  2. Steve in Salford says:

    As a further word of warning Rumex Acetosa (sorrel) is very fond of drawing up such substances as arsenic and storing it in its leaves.

    It is used for phyto-remediation purposes cleaning up areas which have been used for extracting gold, as the two substances are associated.

    If coal ash contains arsenic then growing Sorrel is not a good idea if you are going to eat it, however putting in to your bin for recycling of green waste is an option but if you are knowingly putting contaminated substances into the system, then you may be prosecuted.

    Just a word of warning. Arsenic is cumulative in fatty/body tissues, it may be consumed by other organisms in the food chain which is where the bio-magnification comes in and if it then becomes part of that food chain, they it bio-accumulates in your body as well.

    There is no research that shows how much or how many or if at all any toxic substances are drawn up into the food chain, there probably is but the powers that be are keeping it all very quiet.

  3. gwyn says:

    Just look at all the old coal spoil heaps ,and you see that
    not much grows on them for years, i would not chance putting coal ash on my land,but the wood ash is very good.

  4. John says:

    For goodness sake, I’m talking about a very small amount not a slagheap! 🙂 Nothing much grows on them because of lack of soil until the reclaimers move in.

    Sadly there is little hard analysis of ash available. However, I go back to accepted practice in the days of smog and coal fires was to use soot on onion beds and to darken soil to warm it faster in the spring. Cinders were used for paths and dug in to break up heavy soils.

    Incidentally, basic anthracite coal is compressed vegetation from millions(?) of years ago. I can’t see why ancient compressed trees would be worse than modern uncompressed but I’m not a scientist!

  5. Steve in Salford says:

    It might only be a small amount but the toxic substances that you mentioned are in the coal ash. They are cumulative in the food chain/environment.

    Time is all these substances need to build up to harmful levels in the environment.

    It is up to you if you choose to use the wood/coal ash on your land or not. Just trying to make you aware of the implications of doing so.

    I used the example of sorrel which is a leaf/salad vegetable if sorrel takes up arsenic among others then other leaf/salad plants will do the same in varying amounts. Be careful that is all…..

    Personally wood ash I would use coal ash I would not use anywhere near my land which was for growing food.

  6. Last year I made a path down the garden using ash from a coal fire and now it is almost covered by the normal perennial weeds you’d expect to find anywhere. They don’t seem to mind it. You are right John, in saying that soil should contain these trace elements like lead, cadmium zinc, etc. They only become a problem where they are excessive. My grandad used soot on his onions and got great crops. I think there is an extreme fringe of environmentalists who believe anything man-made is bad. On top of that you have the paranoia of the health and safety people who see problems with everything. The nice thing about rural living is that people tend to make up there own minds about what is right, usually based on years of observation not a few statistics.

  7. Tracy says:

    This very question was asked at a conferece I was at a few years ago at Garden Organic and the scientific answer given was, at the time the vegetation etc was laid down that has produced the coal we now use the atmosphere in which the vegetation grew was much more toxic than it is now and so we shouldn’t use it.

  8. John says:

    Sorry Tracy but that sounds highly unlikely. I believe the O2 levels and CO2 levels have varied during Cretaceous and other prehistoric periods and, of course, there was the K/T boundary event laying down some minerals from the collision of an asteroid but generally the atmosphere did not contain toxic chemicals. I suspect someone was giving a glib answer that they could get away with unchallenged.

    Some of the organic movement’s advice seems to be knee jerk reaction rather than properly researched (or at least dictated by predicate logic) Unlike the approach of L D Hills which was to expound a theory tempered by a pragmatic gardening background

  9. mike says:

    interesting debate

    my wood ash [potash] goes on the plot and my coal ash goes on the paths, but this is personal choice, presumably theres run off from the coal ash into the soil, ive no evidence that theres a problem with growing beside the path, so how much contaminates are there and how much coal ash would you need to contaminate the land. I went to see a mate oveer xmas who was iced in on his narrowboat, tha ashes from his coal fire were thrown under the hedge and you could see that in a short time the grass around the heap was showing extra growth, I was surprised at this but he says its normal as the ash fertilises the ground. I wasnt aware that grass grew in these conditions but there you are it was definatly looking lush compared to further away from the heap, so I think a small amount of coal ash probably replaces some trace elements that are depleted but a large build up could upset the balance of the soil

  10. mike says:

    I would also add that concerning taking in arsenic which is a concern that has been expressed, Im adecorator that sometimes works on old buildingd where arsenic was used in the paint pigments, and we do take it seriously but arsenic has to be concentrated to do any harm to you, the background trace isnt a problem and is all around us anyway, so you need to ask how much arsenic is there in coal ash and how many tonnes of it would you need to get the concentrated amount

  11. BeardyP says:

    I’m ex-oil/gas industry, and my chemist mate used to be employed at the old British Gas Labs. His job was to regularly measure the amounts of heavy metals and radiation dropping out of the gas inside the gas holders – it’s ‘orrible! But, that is because of the huge volumes involved.
    Coal and oil do have volumes of the nasties – the primeval forests soaked it up out of the ground the way the sorrel does, and due to volcanic actions there was more of it in the soil rather than the atmosphere, then it all got compressed which magnified the inherent levels. Unless you get a high enough burn temperature the heavy metals etc aren’t burnt off. Alternatively, use coke/smokeless not coal which have a lot less of them left in. If you have ash leftover then you are not getting the sort of temperature required – look up rocket stoves/high efficiency stoves (they are low tech, John Seymour describes one in his self-sufficiency book). You will also find these are used to produce charcoal which is much more useful in the garden/allotment for altering acid levels. Again WWW web has info utilising this in third world to improve soils.

    My parents and grandparents had several allotment plots, a commercial greenhouse with coke boiler, and big vegetable gardens. Wood ash was spread on the soil, a limited amount of smokeless coal and wood ash went into the compost, and all other coal related ash, coke clinker etc went into paths not near crops.
    Soot combined with water produces acid so strong it destroys brick mortar!That is why chimneys have to be lined or have a specific grade of stainless steel flexible liner retrofitted. There are better ways to warm the ground!

  12. John says:

    Thanks BeardyP for your useful comment – much appreciated.

  13. TONY TAYLOR says:

    I use to have Koi pond and use to get a report of Drinking water if you get one from your local water Athority and read it, They say drinking water contains Arsenic Ink lead copper all the stuff mentioned, so if that’s ok for Drinking and you water the garden with it what is the diffrence,I think the man made coal performed shapes contain plastics as it does say on most of the petroleum in the make so not sure on that bit, but if you think abut it everything comes from out of the ground, oil coal water even Asbestos so i can’t see any problem my self, i would say if it is not for veg patch ok, but if you want to put a bit around Acidloveing plants and Trees and plants cant see the harm will just give a little as ours is a mix of wood and about 2/3 of a ton of coal, i will give it go in one small place and see what happens, as after the rain has been through it a couple of times it will be well under ground.

  14. Leonardo says:


    Crop growers in Limerick in the old days used human dung, soot and cow or pig blood from the abattoir. It would be great to know what chemicals was in this concoction. The crops were great & Park was renowned all over Munster.

    They never used hedges to divide their land, but used twine. This prevented pests.

  15. Anne says:

    So. I suppose to spread a little ash is fine. It must be better than putting it out in the rubbish bin! Especially now that people are using solid fuel fires much more due to the high price of oil and gas etc.

  16. geoff K says:

    Coal ash includes iron and sulphur. Do you have moss in your lawn? I use ferrous sulphate to kill it, that is iron disolved in sulphuric acid. So by using coal ash in my compost, then seiving it and then applying as a top dressing is surely the same thing.

  17. D C says:

    Coal ash is fine in small quantities – the issue is different coals have different trace elements in – A high grade “hard” anthacite may produce ash which is lower in EVERYTHING that regular wood ash, conversely a low grade “Soft” or bituminous coal may have high levels of arsenic and sulfur. To boot, with hundreds of different sources of coal, their are hundreds of different concentrations.

    Thats the route reason why no one will condone its use, its too variable to know!
    Generally speaking, house coals tend to be higher grade coals, but not always.
    I myself add coal ash to my allotment – and use it to kill lawn moss (very effective if the ash has red-brown tint to it, thats the give away of an iron content in excess of 5%). That said I only add small amounts of it to the land – typically 2 buckets for a full 250sqm plot per year.

    This is about 5-6kg of ash – if you know the type of coal a quick google search will give you the % of weight likely to be each major ellement.

    As we are growing, and constantly removing the trace ellements from our soil some years I will put more, some less than what has been removed. Remember its possible growing spuds to remove in excess of 2kg of sulfur from a full plot in a year….

    Sulfur is often added by farmers, and their was an article (sorry cant find link but may add later) that since FGDS was added to coal plants in england, farmers have had to add more sulfur to their fields to make up for the reduction in rain-supplied sulfur.

  18. Richard.E.Craig says:

    Coal ashes are fine to use, don’t forget many councils now have bins for garden refuse,grass cuttings and ashes that are composted by the council. It should also be pointed out that Pollard,Laurel,Lylandi ashes ect all contain considerably higher amounts of arsnic than coal ash!

    • D C says:

      Arsnic has little bearing on the species, and far more to do with the levels in the soil – all soil contains arsnec but some, such as those in cornwall have naturally higher levels, and thus plants accumulate more. Leylandii does bio-accumulate to an extent so may well have more, but Laurel and Poplar (I assume you mean poplar vs a pollard – pollard being a practise not a plant), are not any more bio-accumulators than most other trees!

      That said the arsenc in ash is more likely to come from the comon CCA or Copper Chromium Arsenate which contains chromium and arsenic, which was used as a wood presevative up until the early 2000’s.

  19. polymerman says:

    Thanks for the interesting article. It totally makes sense that coal ash should be compostable. After all, coal is just million – year – old compost. I know we deal with the problem of high levels of toxic chemicals in some areas from industrial pollution, and we have to be careful about using large amounts of coal ash, but we also deal with the problem of depletion of micro-nutrients – supposedly even arsenic is good for you in small doses, to say nothing of zinc and copper!

  20. Sandra says:

    Interesting discussion.
    I have had a thriving compost heap all through summer and autumn, packed with the extremely active short darker red, slimline worms that I believe to be the right sort for working the heap.
    However we have a multi fuel woodburner and, since late autumn, have put the wood and “smokeless” fuel ash on the compost (only lit in the evenings so not masses). Lo and behold today I dived in and found – no worms, or very, very few of the wrigglers.
    I am wondering if it is coincidence or if they really don’t like the ash since autumn. I seem to recollect noticing this before in the winter but not thought it through till now.
    It may not harm the soil or veg but I feel strongly that it doesn’t suit the hardworking worms.
    Anyone else notice this connection?

    • John Harrison says:

      It strikes me that the ash from your stove will be a large amount relative to the compost heap. Wood ash contains potash and that helps plants but has no food value for worms. Plus, worm activity is suppressed when the temperature falls. They seem to head for the warmest part of the heap and sleep in until things warm up.

  21. Tim says:

    Hi just found the site by looking up ash and coal on the allotment. And found the conversation interested,I have a multi burner and use coal just as a base to start then wood.
    On my allotment the soil is is pretty good and free draining,so maybe my adding the ash will replace the elements? Tim.

  22. Anthony O Grady says:

    So what about Peat/turf for spreading that ash about.Much like coal eh?
    I got a trailer full.But was mixed with slack- that loose powdery brown.
    Supposedly dampens and holds the fire in but more puts it out!

  23. Andy says:

    Thanks for all the info everyone. I’m in the process of getting my back garden productive after years of neglect. Well, what else to do in retirement? Wood will always be my preferred fuel in my stove but I use a fair amount of smokeless fuel nuggets. Coke hasn’t been around since the gasworks closed! Looking at the above comments I feel reasonably confident to carry on riddling my ash onto the garden. I tend to store it until there is a downpour to soak it in. I have planted lots of spuds to choke back weeds with their ground cover and tuber growth, seems like that’s a good idea from one of the comments.

    The mention of brandling worms was good. As far as I’m concerned a compost heap needs them, if it ain’t got ’em it ain’t compost heap. Besides, the worms attract hedgehogs which also enjoy a snack on other pests. Getting a proper heap going is in my plans. The only worry is whether my neighbours complain about the smell on warm afternoons.

    I’ll pop my head around the door again if I have anything to share, but more likely to learn from your correspondents. Thanks again.

  24. Christopher Wortley says:

    Interesting stuff!
    My “cheap” fuel agreement with British Gas has just come to an end and I am starting to use my old open fire with wood and “Trueflame” smokeless ovoids. This produces a lot of ash and like allotment holders I’m reluctant to throw things away and I am thinking of using it on my veg garden. Wondered if any of the contributors to this chain or anybody else has noticed bad effects on the soil or their veg from the use of smokeless fuel ash. Very wet November here in South London/Surrey border- we need all the nutrients we can get. Regards to you all, Chris

  25. Joyce Millington says:

    My compost bin is full of slithering red worms. I don’t think I would put the ashes from my multi fuel burner in there. Neither would I put it in my raised beds but i do spread it under my small copse of pine trees, inherited when we bought the house. It doesn’t seem to harm them. I use the cinders along my box hedge and then a covering of leaf mold and they seem to like that. Regretfully the bulk of the ashes goes in the council green bin. Pure wood ash would be much better but alas, I didn’t think about ashes when I made my choice of burner.

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