Annual tree fertilization in the spring is a good habit to encourage healthy trees. Leaf discoloration or yellowing of leaves might signal that a tree is missing nutrients in the soil. A soil analysis will indicate what nutrients a tree needs, including iron, nitrogen and sulphur. Tree fertilizer can be applied several ways including topical fertilizer application and fertilizer injections into the tree’s vascular system or roots. Pest Management, Inc. is a certified pest control company that also specializes in services for trees and lawns.