Aluminium sulphate is effective as a molluscide, killing spanish slugs. Aluminium sulphate is considered to be a relatively environmentally friendly treatment for slugs compared to the more toxic conventional products like metaldehyde.
HOW TO USE ALUMINIUM SULPHATE TO KILL SLUGS
Aluminium sulphate is effective for killing slugs by desiccating them (drying them out by absorbing water from the slime producing glands). As well as killing young slugs it acts as a deterrent keeping them away from plants.
Apply the aluminium sulphate close to and around plants to be protected especially those most favoured by slugs, like Hostas. Apply the crystals at a rate of around 1kg per 40 square metres. Avoid touching the plants with the solid as this can cause them harm.
Note: Kibbled aluminium sulphate contains some very large crystals and these should be broken up into smaller pieces by placing inside a towel or other material and carefully hitting with a hammer.
Another method for applying aluminium sulphate is as a solution. This is highly favoured by growers of delphiniums and is best applied in the winter and early spring. Make up a solution by dissolving around 100g of aluminium sulphate in 5 litres of warm / hot water. Allow to cool. Apply around the base of the plants by saturating the soil. Be careful to avoid spraying directly on plants. Repeat every few weeks. As aluminium sulphate acidifies the soil it may be necessary to apply some lime on treated soil to raise the pH again.