MGOCPM is a classic that is confirmed to be 'aquarium safe'. might be tough to find though. you don't want to use just any soil from the store because a lot of them contain organic matter like chicken poop or other harmful ingredients which can break down and get nasty in your tank
This Ecoscraps doesn't have poop of any kind, but yes, "harmful ingredients" is what I am trying to look for, but not having luck.
I agree with klibs. If you go with yard dirt I think the risk is low unless you take it from a river or lake. I baked mine the first time then after that said heck with it. Any organics that breakdown will be beneficial as they will help cycle the tank. You will have no issues with an ammonia spike using yard dirt. As long as you know the soil hasnt been treated then its really the way to go in my humble opinion. If I was to go with dirt again thats what I would use.
I would actually use a blend of yard soil mixed with all clay cat litter dumped over a sprinkling of osmocote+ capped with bdbs. With decent lighting and some metricide it will grow plants like mad and your carpet will be killer. Very cheap. Very effective.
There's two reasons I don't want to use dirt from my backyard, so correct my understanding if it's incorrect, one I use fertilizers in my yard, and there's not enough dirt in the frontyard, I would rather not destroy my lawn just for the hell of it. Secondly, I had heard that you would have to bake the soil, I would rather ignore this step - there's actually another option, which is to soak it, but if I were to do DSM, it would yield similar results anyway and I'd have it in my tank rather than soak and bask in the sun.
If I don't find what I'm looking for locally, I'll probably stick to some sort of aquasoil, though I do want to try dirt but depending on how much I put it, might not matter as much as it might need to be rejuvenated later on, same goes for any substrate I suppose.