A 50 pound bag of "Earth Gro" organic topsoil is $1.99 at Home Depot. I've used this brand on numerous occasions. The key is finding one without pesticides, fertilizers, or manure.
As far as your 'local' soil, didn't you collect the salamander locally? If so, then it's been exposed to this soil already... If you're really worried, you can take soil and bake it in the oven to kill bacteria. However, most soil microbes are not pathogenic. They're just that, soil microbes.
The definition of amphibian is an animal that usually spends part of its life in an aquatic stage (egg, juvenile), and then metamorphoses into another stage (in salamanders, this second stage is usually terrestrial). The tail of an adult tiger salamander is not made for swimming. It is large, heavy, fat, and not paddle-shaped in any way to facilitate swimming (as opposed to the larval tails). There is a reason tiger salamanders are found on land, and not in the water. However, I'm sure you know much better about its needs than what the salamanders know in the wild. How dare lowly creatures such as salamanders dare live on land when we think they should live in water!
You plant flora, not fauna. Fauna = animals. Keeping a semi-large water area is what you are trying to avoid (what with the drowning and all). All they need is a simple water dish. This should be rinsed and refilled daily.
Frankly, if you think we don't know jack, why bother coming here? We have told you what to use, and you accuse us of attacking you, and go on about how you know so much better than we do. So why bother coming here? Go ahead and keep your salamander aquatic and hopefully your dad's vet clinic is good enough to revive a drowned salamander.
.
