One if the tractor is a 12 v then you need to isolate the 24V from the 12V to begin with.
The alt if only used to power say a aux 24Vdc motor then you should still have 2 dual batteries (even if you use small lawnmower/motorcycle type batteries) the batteries need to be series together. 12+ 12- 12+ 12- then you will have 24V across the first 12+ to the last 12- points.
keep this separate from the tractors normal 12 volt system just to be safe, you can tie the ground of the tractor frame to the neg of the 24V without a problem but you should keep the 12 & 24 systems separate to avoid issues.
mark