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We are often asked 'Can I add ash from my fire pit, chiminea or BBQ to my compost or soil?' Yes, with care and knowledge and this blog explains why and how.
Most importantly - DO NOT put ANY hot ash into a compost pile or compost bin. Hot ash in a BBQ or fire pit can be 1000C and even after many hours can still be 100C, which is hot enough to melt plastic and/or start a fire in dry plant waste. It is always best to ensure that ash is cold.
'Ash' is the compound left after combustion. We can have ash from lump wood charcoal, BBQ briquettes, wood fire ash and coal ash. The composition of coal ash is different to wood charcoal and wood fire ash and is not covered in this blog.
When we combust organic matter the carbon, hydrogen, oxygen and nitrogen are converted to hot gases (carbon dioxide, steam (water) and nitrogen dioxide) and go up in the smoke! Everything else (non-combustible) ends up in the ash. This means that all the nutrients in the wood (except nitrogen) are in the ash. Ash contains potassium, calcium, magnesium, iron, etc which are vital for healthy plant growth. These metal atoms are in the oxide form (Calcium oxide etc). Rarely does unprocessed wood contain any significant (unsafe) levels of toxic metal oxides (lead, arsenic, cadmium).
Typical wood ash contains
(* note if you look up wood ash composition on the internet - you will find no mention of carbon and all the other oxide figures being about twice the level above. This is because the method of oxide analysis requires all the carbon to be combusted away at 550C before the oxides are tested. The analysis report is on the ash remaining after complete combustion of the ash from the fire).
You need to take care in what you burn because anything on the wood will end up concentrated in the ash. Example: you would not make charcoal from wood fence posts treated with the old CCA (Chromium, Copper Arsenate) timber protection to BBQ charcoal. Nor would you burn them on an open fire. However, CCA was banned years ago and no charcoal maker of any worth would ever add contaminated wood to their charcoal process.
is made from logs of wood. Good makers will only use managed woodland (E.g. FSC certified) and use a clean burn retort (not a ring) kiln. The charcoal will be clean, light easily and produce no problematic gases. (This is not true for poorly prepared lump charcoal). Lump wood charcoal does not have any chemicals added.
are made from compacted carbon powder. Think of this powder as the leftovers from making lump wood charcoal. It is very difficult to stick the powder back into a lump (i.e. briquette). It needs a binder (glue). This can be starch (expensive glue) or clay (inexpensive). The problem with clay is that it does not burn, which means the briquettes are very ineffective, resulting in a poor burn and lots of ash.
Yes, in certain circumstances. A little knowledge goes a long way....
"Ash" contains many vital plant nutrients - calcium, potassium, iron, magnesium, etc. All ashes are highly alkaline (pH 10-14), so unless you know the pH of your soil, the pH of the ash, and the exact ratios you need to add to keep your soil pH healthy, then it is not worth the risk of adding directly to your soil.
If you do have the knowledge and expertise in pH levels for your garden, then spread thinly and rake in. Never have a thick layer of ash in one place. Never let it drift onto leaves (ash consists of tiny particles, and when dry it blows everywhere). When ash is wet it forms sludge and is hard to spread.
For these practical reasons, for most gardeners, adding ash direct to soil is probably unwise and to be avoided. However, remember that ash contains lots of highly beneficial items therefore we need to get it into the soil in a different way - via compost.
YES - adding it to compost is good. The composting process will remove the high pH (finished compost is nearly always pH 7, i.e. neutral). The nutrients are diluted into the waste which is good as there is then less risk of too much concentration, and as you spread the finished compost you are returning vital nutrients back into the soil ready for the plant roots to use them.
YES
What about the rumour of 'nasty chemicals' added to briquettes? You are burning the BBQ briquettes, so how many companies are going to add a dangerous chemical to something you cook with?
The chemicals added are:
- things to help ignition. These are no more or less dangerous than anything else you use to ignite/start fires. Such chemical 'fire lighters' will not be in ash because they will have burnt off and gone in the BBQ smoke.
- Clays (bentonite, binders) - your soil is 50% clay so no problem adding them back into the compost!
YES. Disregard all the advice to the contrary re charcoal is not biochar. If you want the full story, jump to this blog.
Adding charcoal/biochar (carbon) to soil when done in the right way (i.e. add to compost then add compost to soil) will enhance your soil. It will also sequester (lock) the carbon in soil for hundreds if not thousands of years.
