Spring is an excellent time to fertilize your trees. A complete fertilizer containing Nitrogen, Phosphorus and Pottassium would be fine (10-10-10, 12-12-12 for example). You could feed monthly until July 1st if desired. Do not exceed 4 lbs. of actual nitrogen per 1000 square feet or you will burn the lawn. If you fertilize your lawn, count the nitrogen as part of your total. Applications should be within and past the canopy 10-12 feet. Application within 4-6 feet of the trunk are not needed.