I'd be quite wary about providing an outdoor litterbox. If she won't go in the soil it might not be so much to do with the soil per se but could be that she feels vulnerable doing her business where she is 'exposed' so to speak! She might not feel safe if, for example, if there's a local cat who she perceives as a threat. So rather than solving a problem it could compound an existing one (and could potentially result in little 'accidents' in your home). Your home is your cat's sanctuary! (Please ignore me if she has a run rather than free access).
The smell can be combatted by choosing an 'anti-smell' litter, cleaning it out regularly (although that doesn't always fix the problem!), and by providing a covered litterbox indoors. The last has the added bonus of preventing scattered litter all over the floor! However, wherever you choose to put a covered litter, remember that your cat can smell it more keenly than you can, and litter hygeine will need to be scrupulously observed since the cat gets all the smell in one confined space and might be discouraged from using it. Have you tried making the litter deeper than recommended? I find this helps, as they then dig deeper and bury their doings deeper! (Makes the scattered litter worse, though!)
Hope this helps, and good luck!
