I never heard of using a soda bottle for clones that way. I've been using beer cups, and a soda bottle wouldn't fit over one. But I like that idea, use the bottom of the bottle for soil and slip the top over it. Brilliant! Imma try that next time fo sho.
And the salt thing...
Hemp will have to give you the definitive answer here, buy I think the use of Rapid Rooters or rock wool cubes like Grodans is what most of us use not soil, but some do go that way. Not soda bottles, they are usually colored, water bottles are what he is referring to.(Like from bottled water)
If you have yours in beer cups find a bottle cut in half that will slip inside the cup, it doesn't necessarily have to surround or cover the outside of the cup.