The best way I've seen is to produce crops grown with solar power and digest them for alcohol or press them for oil to fuel a conventional powered tractor.
As far as solar power directly for the tractor, I suspect you would need a large electric tractor that would dock to a battery bank and huge collector that has been charging all day. Could work, huge traction motors exist already and tractors shed of engine and tranny would have rooms for lots o batteries. Of course it wouldn't be so good for harvesting at night and such like the custom operators do.
