you need a lot of insulation on all sides. putting it all above ground would be easier because of no risk of groundwater ingress. structural steel container, fire-resistant brick and lots and lots of mineral wool around (we’re talking about insulating red-hot (600C) sand so that it stays hot for months). we’re talking about several tons of dirt suspended above ground and insulated on all sides, including bottom. you could put that heat to better use if it stays contained within another insulated container, that is as a hot water source or for space heating. these things are not free, and this comes before any heaters, pumps, piping, automation needed for it to work

if you want to go for a solution, then all good, but for it to be practical you also need to make sure it’s better than alternatives. i think that getting another tractor would be cheaper and more practical in your situation