Fertilization is not recommended at the time of planting. After the third growing season, a slow-release fertilizer can be applied if the soil has nutrient deficiencies. Fertilizer that contains N, P, K are known as complete fertilizers. In different proportions, you have to be careful with want you want. For fertilizing trees, the best sources are slow release. Remember you’re dealing with a plant that lives year after year and doesn’t need a lot of fertilizer at any one time, but needs it throughout the growing season. You sprinkle slow release fertilizers on the ground, using no water, directly to a specific tree. They gradually drain down and release the fertilizer throughout the 3 or 4 months of the growing season. This is better for the tree. You don’t need to use very much. A small jar would last a couple of years for all the trees in your yard.