Like water, fertilizer or added nutrients are needed in greater supply by turfgrasses than trees in the landscape. If mulched properly and growing on decent soils, the majority of newly planted trees don’t need any supplemental fertilizer. In fact, applying fertilizer soon after planting can be counter-productive, especially if it contains nitrogen as well as phosphorous and potassium.
Overall, the goal for the first year after planting is to establish a healthy, lateral root system that is able to mine the soil for water and nutrients as well as anchor the tree. Applied fertilizers generally encourage growth of the leaves and shoots in favor of roots.
If concerns exist about the existing nutrient levels in the soil, taking a soil test can be helpful in determining which elements may be lacking. Information about the pH of the soil, salt content, bulk density as well as micro-elements can be obtained from a soil test as well.
Finally, the site itself should be considered in determining the need for added nutrients. If the tree is by itself or in an ornamental bed with shrubs and perennials, the need may be greater than if in the middle of a lawn, as fertilizer pellets are commonly tossed into the mulch from prescriptive lawn fertilization in such scenarios.