I have heard this rumour too, but I don't think baking soda will do a damn thing to help you pass a meth test. Drug testing is pretty advanced these days, so a little bit of baking soda won't fool anyone!
I would also say that, if you're on probation and your freedom is at stake, I think you should be focusing getting clean and not worrying about whether baking soda will help you fluke pass a drug test.
It just doesn't seem sensible. You need to get off the the methamphetamine dude, it's no good for you, TRUST ME