Hi, I am an Englishman living in Northern Germany. I have found the book by Ernst Stahl on Google books and will have a read through and see what I can extract. I have also been working the the problem SLUG!

That with the reduced iron is interesting. I have a clay soil with a very high level of OXIDIZED iron. I think however that copper may also play a role. I heard a longtime ago that snails are a dynamic accumulator of copper, seeming to contain more copper than the environment they live in can give. I assumed it landed in the shell. But I recently stumbled on a site with nutritional values and snails have an extraordinary high copper levels (2980 µg per 100g – I assume they are not including the uneaten shell). Perhaps slugs are similar. Coffee (which also has a very high level of copper) is often quoted in gardener’s forums as deterring slugs…. Mangan is also another interesting nutrient. When slugs are introduced to an environment the amount of available mangan can increase by 270% (*ON THE ROLE OF THE GASTROPOD SLUG
LAEVICAULIS ALTE
IN THE ENRICHMENT OF SOIL NUTRIENTS, M. Kavitha)

I would like to start a whole thread on the problem SLUGS… Who’s with me…