We're on heavy clay and it didn't bother the 1500 we planted, many moons ago - had Norway Spruce, Nordmann, and Noble. Biggest problem was weed/grass control when the plants were small. Used mulch mats, but it was still a struggle. Make sure you leave sufficient room so you can mow between them, too. I'll second the recommendation for Cheviot Trees. I think I also used Buckingham Nurseries.