Have you tried adding just a little bit of water?
I think that it ought to be OK to add just a small amount, 1 tablespoon or so, of MTS so long as it is surrounded by the planter gravel.
In the picture below you can see how I had situated the MTS so that it came right up against that void in the back of the planter cup and the screen. That must be how the MTS got out, because when I have inspected the planters I am not going to plant like that again.
