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Found this online:

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Natural cinder of 4-8mm chunks a good stuff to add alongside perlite/pumice for succulents and other soil mix?

EDIT: What about coal cinder?

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    If you have access to perlite, just use that in a pot that contains a drainage hole. Never use cinders if you plan on eating whatever is planted in the pot, due to possible/probably heavy metal contamination. – Jurp Jan 26 at 18:17
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    Coal cinders are mainly oxides of silicon (i.e. sand), aluminium, calcium (i.e. lime), and iron, with vary variable composition depending where the coal was mined. Some can contain as much as 40% lime! There may also be enough contamination from heavy metals to cause groundwater pollution if it is dumped in large quantities. I wouldn't use it in plant pots in any circumstances. – alephzero Jan 26 at 18:29
  • @Jurp Only for succulents and other nn-edible plants. – 4-K Jan 26 at 18:32
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    I used to add vermiculite to soilless mix for non-succulents (forget the amount used, sorry to say), but switched to perlite a long time ago. I found that perlite improves drainage for all types of plants (I change the amount of perlite used within the soilless mix depending on succulent, vegetable, perennial). Note that perlite powder (released when adding it to the mix) can be hazardous to your lungs, so I wear a dust mask when working with it. – Jurp Jan 28 at 16:55
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    Vermiculite does NOT improve drainage but increases the soilless mix's water holding capacity. It also binds to micronutrients and releases them to the plants, which perlite does not do. This makes vermiculite a nice additive for seedlings/plants that like it moist (I had issues with damping off, though, which is why I originally switched to perlite). I would NOT put either additive directly into the soil of my garden. To increase drainage, add water holding capacity, and reduce compaction in the garden I used an organic mulch (wood chips or cocoa bean hulls). – Jurp Jan 28 at 17:09

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