Hi davisjames, welcome to TBN!!! Like Mark said, first check all of your connections, especially the ground and the + connection to the starter. If you have a bad ground, or a loose connection, it will cause the symptoms you are describing. The connections will also get VERY HOT!! I've been burned before fiddling around with a slow moving starter trying to figure out what was going on, so be careful. Good Luck, and let us know how you make out.
By the way, you might want to fill out your profile so people know what part of the world you are from /forums/images/graemlins/smile.gif Which makes me think, if you live in a very cold climate, that might be another reason the starter is turning slow. Does your tractor have a decompression lever?