As mentioned above, the machinery used today (ie conventional "tractors") isn't cost-efficient, but machines in general are (if they are built appropriately).
I was thinking that a purpose-built traction engine, based on the efficient "1902 Ivel Agricultural Motors Ltd traction engine" (see http://en.wikipedia.org/wiki/Talk:Traction_engine#oldest_traction_engine , http://en.wikipedia.org/wiki/Traction_engine ) might be suitable. Like the Ivel, it would best not be steam-powered since the parts required for this would be quite heavy and thus result in a economically unuseful project. The Ivel had instead, a (low-powered; 2HP, see below) internal combustion engine. Potentially, either wood gas could be used, or an emissionless fuel, ... I am still thinking about this though, but it would be made lightweight and as efficient as possible (the use of lightweight, low-cost caterpillar tracks might be incorporated, perhaps based on the DFRobotshop Rover, see http://www.robotshop.com/ )
Regarding wood gas: 5 acres will produce about 25tons of biomass a year (trees), with an energy content of 450GJ. Using this as woodgas in a traction engine would produce 10kW (about 10hp at drawbar), you will be able to run your tractor for 2500 hours/year (6.8hrs/day). However the technology lends it much more aptly to larger stationary machinery than to smaller portable machinery, and it's difficult to run engines of much under 10 hp at all (-->significant problem), because it's hard to draw enough gas through the generator to keep it hot enough to "generate."See http://www.theoildrum.com/node/6287