The ideal time to fertilize fruit trees varies by climate region. Usually, you can fertilize about six weeks before your fruit trees start blooming so the trees can absorb and use the nutrients when they produce fruit. You need to understand the nutrient levels in the soil around your fruit trees in order to properly fertilize them.
Buy in a soil testing kit that can test for levels of phosphorus, potassium and nitrogen in your soil instead of a single element tester. Knowing your soil's nutrient content helps to avoid buying the wrong combination of--or too much--fertilizer.
Dig a small number of core soil samples with a shovel, about 8 in. deep; take a few samples from around each tree. Combine the soil samples of a tree into an individual Ziploc bag and mix it. Test the soil mix in the bag using the kit. Repeat this process for each fruit tree in your yard.
Note the results of the soil samples in a notepad. Keep track of your findings carefully and save them for comparison in future years. This gives you a good snapshot of your soil's overall condition and the changing needs of your fruit trees.
Buy the fertilizer that is right for your kind of fruit tree and soil condition. Fertilizers for peach and cherry trees usually should be higher in nitrogen than fertilizers for pear, prune and apple trees. Determine the amount of fertilizer by the age of the tree. For example, a one-year-old apple tree requires about one pound of fertilizer, but a five-year-old tree needs about five pounds.
Apply fertilizer to the soil with a hand-held spreader. Start at the tree's drip line and work inward. Imagine an outer circle under the outermost branches of the tree to identify the drip line for the tree. Imagine an inner circle with the tree's trunk at the center. This identifies the border within which you should not apply fertilizer to help protect tender roots from burns. For a young tree, stop fertilizing about 6 in. before reaching the trunk.
Work the fertilizer into the soil with the rake. Use the tines of the rake to loosen and turn the upper 4 in. of the soil within the tree's drip line. Mix in the fertilizer with the soil.