Diatomaceous Earth. GET IT. Provides low levels of silica to the plant, completely organic and non hazardous to you, pets, plants. ONLY thing it hurts is small soft bodied organisms, and i believe the way it kills them would apply to slugs as well. Basically its fossilized Algae that's crumbled up. to us its dust, but on a microscopic level, they are shaped like multi edged razorblades, that on contact with a soft bodied insect like Aphid, mite, etc, it cuts em up and dehydrates em.
got rid of all my mutes and aphids in less than 2 days, and havnt needed to reapply since. worth a look. here is an excerpt from one of the first articles i found on google with a search of "Diatomaceous Earth and slugs"
Diatomaceous earth (DE).
After testing all kinds of slug barriers, Jeff Gillman, author of The Truth about Garden Remedies and professor of horticulture at the University of Minnesota, concluded that DE is the most reliable. "DE is a white powder made from the fossilized remains of diatoms, one-celled algae that have a skeleton made of silicon," Gillman says. "To a slithering slug, this lethal powder is extremely sharp and cuts their undersides, causing dehydration." DE does have to be replenished each time it rains, making it a better choice for climates where it does not rain frequently. (Note: Buy only untreated diatomaceous earth formulated for garden use, and wear a dust mask when applying it. DE made for swimming pools is chemically altered and not suitable for use in any garden, much less an organic one.)
You can buy bags of Diatomaceous earth at
Gardener's Supply or
Planet Natural.