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I always have some ash left in the barbecue. Would mixing it in the potting soil be a good or a bad thing for my pot plants? The ash is only from charcoal so nothing nasty in it.

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You didn't use lighter fluid or those fast-starting coals? Also, take a look at some previous questions about using ash –  Niall C. Oct 17 '13 at 17:40
    
aaah I did use firelighters though which does have kerosene –  Neil Meyer Oct 17 '13 at 17:56
    
Also the bbq briquettes can include a number of nasties. Okay they can't be too bad because we cook from them, but some of the manufacturing plants can leave some nasty pollution such as dioxins (depends on the fuel and the binder being used). Burning may concentrate some of these in the ash. –  winwaed Oct 18 '13 at 13:51
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up vote 5 down vote accepted

It will definitely make a difference if you add enough, but probably not a useful one.

Ash is high in potash (compounds of potassium) and usually also high in calcium salts. This invariably raises the pH of soil. Ash unfortunately does not contain nitrogen, which is a more important plant nutrient than potassium.

Given that potassium is the third major plant and crop nutrient after nitrogen and phosphorus, ash is an excellent fertiliser in cases where you have a potassium deficiency and/or soil that is too acidic.

I wouldn't use it in potting mix because usually potting mix, by definition, already contains optimal nutrients to nurture young plants. I would consider using the ash for other purposes in the garden. There are several, given the key properties of ash, but I won't go into that here.

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"I would consider using the ash for other purposes in the garden" -- A very good statement, but remember to not overdo this. I give my vegetable garden a small layer every 2 months more or less and the plants seem to love it. –  Rudolph Oct 18 '13 at 6:05
    
At a minimum, I would definitely get a soil pH test done before using ash. Probably would be an "ok" addition to the compost pile –  JGurtz Nov 1 '13 at 1:14
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