I would not think it should be turning brown. 9 times out of 10 this is from to much or too little water, either will do it. You should be watering a new tree every day but only a little water, just enough to keep the original root ball moist, not damp or wet. remember to get the water into the root ball, not just out away from it. The nursery would have been watering the potted tree ever day.
Slowly, as the tree begins to extend it's roots into the surrounding soil, you will be able to reduce the frequency, eventually to once a week.