Most trees do not require yearly applications of fertilizer, as they are able to get the nutrients they need through their extensive root system.
For most healthy trees (trees that grow about 15 cm a year), fertilize every 2 to 4 years. The best time to fertilize is in late April and early May, when the ground is workable and the tree has not yet started to grow. If your tree is growing in sandy soil, apply your fertilizer in two separate applications: half in early spring and half in mid to late May. Your local garden centre can help you to determine the amount and type of fertilizer for your tree.