yes I did mean liquid n2. I too worked with cryo juice. also worked with, liquid HE,O2,Co2,N2,Ar,propane and a ammonia. N2 is something more then -3oo deg.
So what I think happens is. When you pour the N2 on the ground it will ony freeze so deep.
The rest gases off. It's not the N2 that freezes it's the moisture in the dirt. I also don't know what it does to the plant. I do know when we used it on the yard plants they were close to dead. after pouring the N2 they turned very green and grew about a foot then droped over dead. It was just a thought for some mass growth at the end.