Can I use soil from the backyard?

[kahuli]

Or is it better to buy plain potting soil? 

[witchpig]

I use backyard soil and my snails are fine, but i'm no expert. Just to be safe you should buy plain potting soil or topsoil.

[Liguus]

Both options could be carrying some bacteria/microorganisms that might be detrimental to the health of the snails (In an extreme case I found a predatory flatworm in the substrate I collected in florida). The best option is to buy coir/coconut fiber soil, or bake the normal soil for a while to kill anything in it. After that the snail waste should re-populate the soil with beneficial bacteria from their guts. As a side note I also put relatively clean leaf litter into my terrarium (not sterilized since there won't be much on them other than fungi and bacteria)  and the snails seem to be healthier and more encouraged to breed when it's present.

[Cashell]

I strongly agree with the advice above. Never use wild soil from outside, because it's full of too many hazards like chemicals, parasites, diseases etc.

[kahuli]

OKay, thanks! I'm going to get some coconut fiber tomorrow (my snails just arrived today)!

ETA: I just realized I posted this in the aquatic snail section, oops!