I'm not disagreeing with what the other guys have said, but here's what I would do: First, check with a little water (soapy is best) to see if the valve core is leaking. If so, replace it and you're through. If not, take the tire and wheel off the tractor and get it thoroughly wet (bubbles may show up at that time). If you still don't find bubbles, I keep a small spray bottle of water and dishwashing detergent (about 10 to 20% detergent is plenty) and spray everything with that to see where they bubbles are. If it really is a tubeless tire and no one has added a tube, and the leak is in the tread, I use a plug to repair it myself. If you don't have the equipment and know how to do that, take it to a tire shop to either be plugged or patched on the inside. If the leak is along the bead, then as already stated, break it down, clean the bead and re-seat it. And once again, if you've never done that, you may need to take it to a tire shop to have it done. And don't forget when applying soapy water, the leak may be around the valve stem in the wheel. Personally, I never use the little "fix a flat" cans, but Tractor Supply Co., other ag equipment companies, and some tire dealers have a variety of "stop leak" stuff which can be installed in the tire in quantities of a quart to a gallon or more per tire that will stop or prevent most small leaks. And if I use any of those, I make sure it is a "water soluble" formula so it can be washed out if necessary in the future.
Bird