Perhaps you've just planted your first tree and you're wondering how to fertilize it. Gardeners tend to worry unnecessarily about fertilizing trees. Most deciduous and evergreen trees don't require much fertilizing and are more likely to be harmed by overfertilizing. Dropping, yellow leaves, slow growth or scorched leaves are often a sign of disease, water stress or poor soil. Fertilizing your tree won't solve these problems. In fact, overfertilizing can promote disease, cause too much growth and pollute groundwater.
Conduct a soil test. Fill the test vials with soil from several areas of your garden and mail to your county extension office. Your soil analysis report will come in a few weeks and will contain information about the type of soil you have, as well as the pH level and any nutrient deficiencies. The test will make recommendations for fertilizer. Use these recommendations as the basis for your fertilizing plan.
Purchase a slow-release, granular fertilizer based on the results of your soil test analysis. You probably won't find a fertilizer specifically labeled as "tree fertilizer." Look instead for one labeled 16-4-8, 10-6-4 or 10-10-10. These numbers indicate the amount of nitrogen, phosphorus and potassium in the fertilizer. Find these at any good nursery or garden supply store.
Fertilize your deciduous tree every 2 to 3 years for most species. Some trees, such as pecans or fruit-producing trees, need more frequent fertilizing. Measure the diameter of the tree at 4 1/2 feet from the ground. Apply 3 to 5 lbs. of fertilizer per inch of trunk diameter. Broadcast the fertilizer in a circle under the tree, from 12 inches out from the trunk, extending to the outer edges of the limbs. Apply fertilizer from late fall (after the first frost) to early spring when new growth appears.