Firstly, your mixture is too strong to spray directly on plants. It will burn the leaves of Hosta and other plants. Secondly, if you are suggesting using this as a soil drench, you will essentially kill all macro and micro organisms in that area, not just slugs. Lastly, while a 10% ammonia and water solution will kill slugs on contact, there is no residual effect. The soap is useless because the ammonia will evaporate from the leaves and there will be no effect on slugs that aren't directly sprayed.
