Yeah, I have the same concerns as those above. Also I wouldn't use any products that the manufacturer does not provide an msds for.
I just checked their site and they don't provide any real information on reactivity and such.
One thing I have though about is using some polyester (synthetic would work well) like the cushon (spelling?) padding. It is generally the same thing as STG. It holds a little bit of water, but lets excess drain and has enough airspace to let O2 in and let root grow.
Wash the stuff down real well to get rid of any trace from production and you get a nice clean non-reactive neutral material (all definitions to be able to be called polyester). Also a full synthetic polyester may or may not biodegrade (most don't) so you will have a medium that is reusable for a while. Not to mention it is dirt cheap (well probably cheaper than dirt, pardon the pun).
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