Markct,
True, many tractor tires have tubes. BUT (ain't there always a but?) usually when the fill is with antifreeze or windshield wiper fluid, they don't put in a tube. Saves money and those fluids won't corrode the wheel like CaCl2 will. Also, from what I've been seeing many new tractors don't come with tubes anymore, they just count on the rim sealing.
Yeah, a plug won't hold up forever on a tractor, but it will probably get you by until the season slows down and you can do the repair on your schedule.