Well I live up state from you in Brandon. I have a
L2800 4WD and have laid about 600' of 1" line in my irrigation project. The sub soiler will work and I have used a middle buster. Run the sub soiler down the path a couple of times and it will remove roots etc. I put a cap on the pipe then tied it to the subsoiler and away I went. It works okay at best in my opinion. Seems like the pipe wants to rise up and does not really get down like you would think it would. However, in our case the water line doesn't need to be much over 6 to 9 inches deep for freezing purposes. The reason you want it deeper is when you run over the line with your tractor it does not break it. If it is too close to the surface you will have problems, though freezing will not be one of them.
After doing several hundred feet of irrigation pipe like this I found a Ground Hog T4 18" trencher and bought it. In about a month when I finish my water lines I will sell the trencher on Craig's list. The trencher is my preferred way to go.
The guy above, grsthegreat, is in nothern Idaho. That's pretty close to the artic circle I think and he must dig pretty deep to keep it from freezing.