Be Careful - it is wood-based charcoal that you must use. Unless you have a woodstove it is probably not cheaper than vermiculite.

Do not use BBQ charcoal (unless it is the expensive compressed sawdust kind)! You can use activated charcoal, but that is expensive. Artist's Vine Charcoal is made from wood (again, expensive).

I use natural urea-soaked charcoal in my raised beds (I heat with a woodstove). I read that charcoal absorbs nutrients from the soil, thereby robbing the plants of those nutrients, so it is best to soak the charcoal in a nutrient-rich solution for 2 weeks so that it will slowly release those nutrients as plants need them.

Producing charcoal deliberately is a MAJOR WASTE of energy! It requires a long, slow, reduction (low oxygen) burn. Many third-world forests are destroyed for charcoal production. It is only efficient to use if it is an already existing by-product, such as that left in a woodstove used to heat your house. High-efficiency stoves produce more ash than charcoal, so aside from forest fires, there probably is no way that any substantial amount of commercial wood charcoal is actually eco-friendly.

Rice-hulls are also used, I believe.

Josh