Using Iron Phosphate
The active ingredient in the products listed below is iron phosphate which is an organic compound that is found naturally in the soil. If the iron phosphate bait is not consumed by a slug, the material breaks down becoming fertilizer for your soil.
Iron phosphate products are used the same as other bait materials. Simply scatter the granules over the soil’s surface in the evening where snails and slugs feed. It is not broken down by rain or watering the garden. Once the slugs have eaten the bait they immediately stop eating and crawl back under the plants to die. Taking 3 to 6 days to pass on to slug heaven, Iron phosphate is slower acting than the synthetic metaldehyde but is active on the soil surface longer that other baits and when ingested the slug stops damaging your plants - which is the whole idea.
Iron phosphate baits can be used in vegetable and berry gardens up to the day of harvest.
Iron phosphate baits will be most effective if used in early spring when the baby slugs appear from the eggs that had wintered over in the soil. When you see the first leaves of a hosta plant peeping up from the soil, that is the time to cover the area affected by slugs with the granules. Then make another application again in a month and then again in early August. With this approach you short circuit the three to five generations of slugs that appear every season. You will still have slugs in your garden but they will be in small numbers so no damage to your plants is apparent.
For more information about killing these slimy critters see Controlling Slugs and Snails