The energy density of solar is a little low yet. However, tractors seem to have three major characteristics: torque, weight, and reliability. Electric does pretty well on all three (think diesel-electric locomotives).
The main issue is whether batteries can equal the energy density of chemical fuels, and the answer as yet seems NO. Diesel has about 36 megajoules of energy per liter of volume, and lithium batteries have about 8% of that, and lead-acid another tenfold lower than that.
I could see a smallish tractor with a 15 horse DC electric motor driving each rear wheel, or, better yet, as part of each wheel, and about 1500 pounds of battery in the not-too-distant future. No need for hydro for continuously-variable transmission; no frictional losses in clutch, tranny, differential....
I wouldn't scoff at folks thinking down these lines, even if a pilot implementation seems unworkable.