can i keep a common garter snake on plantion soil

[Spankenstyne]

the potting soil we get here is not coconut husk, and doesn't say that it contains it either. I can also see other things in it, of unknown origin. So I will continue to stick with an alternative I know is safe here.

"Plantation Soil" I mean, not potting soil.

Yeah I'd also steer clear of potting soil with unknown stuff. It's more the fertilizers & additives I would be worried about than anything. If you get fertilizer & additive free stuff many folks have used it successfully.
Lotsa ways of keeping these awesome snakes but if something's already working really well for ya why change up?