If the tractor was running recently, it will show if the tractor is a neg or pos ground as ignition coils don't work too good wired opposite of ground.
If your car uses a negative ground and you have a coil with primary and secondary terminals, the primary winding is connected to the terminal on the coil that is marked “+” or “Bat.” This is the terminal that receives voltage from the battery. The primary wire is then coiled around the iron core and its other end is attached to the terminal marked “-“ or “Dis.” The voltage flows from this terminal to the breaker points inside the distributor. (Positive ground is the reverse.)