Rain breaks these baits down into iron and phosphate. While both of these elements are generally useful fertilizer, these baits are not intended to be a fertilizer. And as with all chemicals and pesticides, read and understand the labels. Remember to keep all chemicals and pesticides away from childern. And another downside is cleaning up the dead slugs and snails.
Some gardeners rely on stale beer traps.
These trap are simple to make and will work. Select a protected area where the slugs will frequent. Fill a shallow flat pan with beer. Raising the rim above ground level will help to prevent benefical insects like predatory ground beetles from falling in. The traps work by drowing the slugs after they crawl in and drink the beer.