If you have just recently planted a new fruit tree, I think it is safe to assume you are not yet an expert on the subject.
More fruit trees die in their beginning years due to poor care habits than any disease or pestilence. Therefore it is vital that you understand how to care for trees in a way that will ensure their immediate success as well as future good health.
During the first stages of the tree’s life, the roots, trunk, and branches have not yet fully developed to a self supporting strength. Therefore if your tree is growing fruits, occasionally the combined weight is enough to snap off an entire branch. If this is the case, you should provide external support for your branches – prop them up with boards, or tie them to something at a higher altitude. As long as you can provide your tree the support it needs in these early years, it should grow to be independent in no time at all.
Proper nutrition is not only necessary for the production of healthy fruits, but is also necessary for the tree to survive longer than one season. The exact specifications vary with the area, climate, and type of tree, but I’ve found that there is no better source than a nursery employee.
Maybe they’re just eager to sell you the right type of fertilizer, but in my experience they are almost never wrong.