The horticultural newsletter “HortIdeas” has reviewed several reports on using iron sulfate (green vitriol) to control slugs. “Recent laboratory trials in England support the notion that iron sulfate is rapidly absorbed by slugs which contact it and is highly toxic to slugs. …Iron sulfate is cheap, easily available, and not very toxic to humans.” In fact “it is a widely prescribed iron supplement for people suffering from anemia.”
One of their subscribers in Spain, Brian Lynas, reports great success by spraying or sprinkling (especially following rain) a solution of iron sulfate. “For over a year I have intermittently sprayed iron sulfate solution around lettuces, brassicas [Ed: cabbage family] and any other plants which were under attack from mollusks. The concentration does not seem to be critical. I use four heaping teaspoons in a fivequart sprayer (twice that concentration if using a watering can) on the soil around slug attracting plants. …I’ve sprayed the soil and also sprayed the plants directly. There’s no doubt that either is effective, especially if you can directly spray the mollusks themselves.
"The spray seems to act as a contact poison, so if the animals are wetted or have to cross a sprayed area like a leaf, they die. Unfortunately, when sprayed onto soil, the soluble iron sulfate is quickly changed to insoluble hydrous iron oxides and is inactivated.
"Iron sulfate burns some sensitive (usually young) plants. The damage is minor, and my impression is that the anti-mollusk benefit far outweighs the disadvantage. In fact, ferrous sulfate solution at around 3% strength is often used for correcting iron deficiencies by direct spraying on foliage.
"Regular spraying especially after rains around the plant bases where the creatures hide, as well as generally around the cultivated area, dramatically decreases the mollusk population with almost immediate effect. [In Mallorca] a small conical snail occurs by the hundreds of thousands. A couple months ago these were infesting a patch in which I’d planted small brassicas and lettuces. Sometimes each plant would have 30 or more snails lying around underneath. I sprayed the solution over them, and they evidently all died. What’s more, it seems this killed the eggs also, for even now there are practically no mollusks in the area.”