Question
I have a tree farm in Heber City Utah, evergreens. I am growing them in contianers placed in holes in the round to facilitate harvesting in a few years.  Some are double potted with gravel in the bottom to facilitate drainage. Suggestions on soil prep, fertilizers.  They are watered by sprinkler system.

Answer
Buy the best soil you can afford. The key to healthy container trees is that they have the nutrients and even water level to sustain their long growing period. A good strong root system in the container, means a healthy big tree, when it is finally planted.

If you have several trees, then it might be economical to find a bulk soil business near by. I have a couple of places near where I live in Oregon, and find that I can save considerably from the bagged soil prices. I mix in some Osmocote time-release fertilizer in each container, and some Moisture holding Polymers.
Check the drainage. The water should drain free within a few minutes. Never leave the containers in standing water. In hot weather check the moisture level often, before they dry out. But, don't over water either. Let the oxygen return to the top soil, between each water application. The trees need to breathe, and will drown if they are not allowed to dry enough.
The concern I have about the sprinkler system is that it causes the trees to be wet. This can often encourage disease, especially if it doesn't dry quickly. It also causes sun burn in hot sun conditions.
It is best to just water the soil, and not the plant. So I suggest that you watch for problems, and correct them, before they lead to serious damage.