Can I add BBQ ash to my compost?
DO NOT add hot ash to the HOTBIN, or any other compost bin - it will damage the plastic and void the guarantee. It can ignite a fire if your heap is very dry.
Can I add cold BBQ ash to my compost?
Yes, but too much ash will turn the compost waste strongly alkaline and this will slow down the composting process. Ash (and especially BBQ ash if lots of fat has dripped in towards the end of the BBQ) can also form an impervious layer if not mixed in or too much is added in one go.
How beneficial is BBQ ash to the heap versus directly adding it to the soil?
BBQ ash won't 'breakdown' (ie compost), but it will be incorporated into your compost. BBQ-ash is a mixture of minerals including potash (ie potassium oxide), the source of the second item in NKP fertiliser list. Potash (like any fertiliser) has to be used sparingly. Spread 'thickly' around stems/roots it will 'burn' as it is highly alkaline - so there are some advantages of 'diluting' the potash into your compost.
Can I add BBQ briquettes (broken bits, leftover dust, etc) to my compost?
Yes. We were quite surprised to see a lot of composting help sites advising not to add BBQ briquettes to compost or soil. This really puzzled us – charcoal is a natural, non-toxic, inert form of carbon. It is used widely in agriculture (especially in Japan), is believe to be one of the critical components in Amazonian Terra Petra (Dark Earth, or ATP) and is closely related to Biochar. A huge group of environmental scientists are saying actively add charcoal to soil!
We have noted that many posts state that BBQ briquettes are manufactured using "toxic chemicals"....
We visited some manufactures sites to read up how BBQ coals are made.
Here goes: they take the dust (inert charcoal particles) and press them into briquettes. To stop them falling apart they use a binding agent (starch - that's a potato!)). To get them out of the mould they add a release agent (borax, ie sodium borate - used in a hundreds things around house). To get them to light faster they add an accelerant (sodium nitrate - umm that will be the world's biggest fertiliser!).
Although we cannot be 100% certain of all the chemicals, consider this risk - you tend to stand near/over BBQs and breath in the fumes. Which is more dangerous breathing in a lot of fumes and using BBQs in a domestic environment or composting small amounts where bacteria turn most nutrient into safe forms of plant nutrients.