I know that some work has been done on Electric Tractors, but I haven't seen much work done on pneumatic tractors. I have to work out the numbers, but I would expect a 1000 gallon propane tank pressurized to just 300psi should be able to power a tractor nicely. If you added a trailer with an Aux tank, then you extend your range and could be refilling one as you empty another. If you then add some reheating of the output, like running the output of the tank thru a rocket stove, you could create a hybrid that is both sustainable and completely Permacultural. Electric Tractors are interesting, but how long do the batteries last?
This same technology could replace electricity in many places as there are Pneumatic chain saws or as the Amish are using pneumatic kitchen appliances.
Tractors are an interesting case. They benefit from weight. So a pneumatic storage system wouldn't be the best solution but a lead-acid battery powered electric system works well. In normal ground transport, light weight is needed for stops and starts and pneumatics are a good for that.
I looked into air powered cars a while back and basically the numbers don't scale up due to heat losses . Try using a bike pump a lot and see how hot you can get the gas by pumping and blocking the exit.
Living in Anjou , France,
For the many not for the few