Rocket mass heater pipe for Greenhouse

I am installating a greenhouse which is 30meters long and 10 meters wide. I would like to use about 3 rocket mass heaters to heat the soil. What type of pipe to you recommend I use that is fairly cheap Metal, Asbestos, etc? I need about 100meters of pipe, because I want to install 3 mass heaters outside the greenhouse and run 3 pipes under ground to heat the soil surface for the plants on top. How deep must the pipe be? Your advice will be greatly appreciated.

That's an interesting project, surely somebody must be able to add their thoughts?
I have something similar in mind and was wondering the same thing. I was also wondering if a smaller diameter pipe rather than the usual 100mm would mean we could use more of it in length. O would it hinder the burn (less draft?) of the RMH ?