Most mature trees do not need additional fertilizer. They spread their large root system out wide to gather nutrients from the soil. Young trees and saplings growing in containers benefit from an all purpose slow release plant food such as Osmocote Plus. Follow the instructions on the back of the package for correct amount to use. The amount depends on the size of the container and size of the young tree. Slow release plant foods like Osmocote are safer to use on any plant because there  is less of a danger from overfeeding and causing fertilizer burn on plants especially young plants. The fertilizer is released when the plants need it most during an active growth phase. It is released very slowly so one application is all that is normally needed during a growing season. Keep growing, Marianne Binetti