bogart, slugs and earthworms must have very high humidity in their immediate environment. I suspect that ammonia has an adverse effect on this condition and will be harmful to both slugs and earthworms.

Ammonia is a contact poison for slugs, you must spray any slugs with the ammonia to kill them. It has no residual affect on the slugs and once the ammonia is dry it will have no adverse affect at all. Pouring undiluted ammonia onto plant leaves could harm them and is not something that should be suggested.
A good control for slugs is to spray your Hostas with cold, caffienated coffee as often as necessary to keep the slugs from them. The cold, caffienated coffee is a slug repelent. Stale beer placed in shallow saucers around the garden will attract the slugs and they fall in that beer and drown. Iron Phosphate pellets can be purchased and scattered around the garden and that is a poison that alos kills the slugs.

OK - thanks - but would these other methods (iron phosphate)also kill worms? The beer trap is quite gross and I would think spraying with cold coffee would stain plant leaves?

I've sprayed the diluted ammonia on the ground around the plants before the leaves emerge - no direct contact - and this seems to control them but as it seeks into the ground does it also kill worms and useful organisms in the soil. I'm guessing from your responses it might so it's back to the drawingboard.

We're not talking about pure iron phosphate, but specially formulated slug and snail BAIT. These baits are a combination of highly attractive (to slugs) grains and iron phosphate. When applied according to directions, you will not harm any plants.

I have not seen any staining on any of my plants that I have sprayed the cold caffienated coffee on, and all the research I have seen on the iron phosphate says the earthworms will not be afffected at all. Those pellets are, however, pretty expensive and I find that providing a good habitat for my plants also makes for a good habitat for the slugs and the coffee is just much cheaper to use.

Ammonia evaporated só best applied as a diluted spray (1 part ammonia to six parts water) directly to slugs at night by walking around garden with flashlight and spray bottle. This application will not harm the plants.

If you can actually see the slugs you might as well just squash them. Fast, 100% effective and costs nothing. It’s the slugs you don’t see which cause the damage.