Charcoal briquettes are usually made from wood (typically oak), with some petroleum components added to the cheaper brands, including coal.

Wood ash generates lye, which can be in the form of soda lye or potash (potassium lye). Potash is less common, and both forms leach from the ashes in water, which is how lye used to get extracted for making soap. Lye is caustic, so as a soil amendment you wouldn't want to apply it on foliage or the plants will get chemical burns. It is best to mix into bare soil along with organic compost, which is slightly acidic.

Charcoal has other gardening applications not associated with lye, which may be more like what Susan is considering. Hard to say until she lets us know. In any case, lye can be extracted from charcoal ashes as well, so be mindful of that if you are dispersing charcoal into your garden, you will also be likely adding a small amount of lye.