Soot

Spread it over the plot where you are going to plant your spuds the slugs dont like it, if you do it now it will mellow then dig it in before planting.

It's important to let it mellow as PaulW says as there are some by products that need to leach out before use. I have smokeless fuel and I don't think the soot is the same so I use it instead of paving sand when I've been making some paths. It's free and sets rock hard after it's been wetted.