I may have solved the mystery of why coffee grounds repel slugs. Maybe.
I put down some coffee grounds around my broccoli plants. We had a spell of
sunny and warmish (for end of Autumn May) weather. Last night we had a few
mm of rain. Today I have found several dehydrated slugs on the coffee
grounds. My deduction is that the coffee grounds dehydrate the slugs as they
crawl across it much as cigarette ash does. If so, I protect my plants AND
add some organic mater & fertiliser to the soil. Perfect.