As few as 10 years ago, fertilizers were considered a cure-all. This is no longer so. "Think moderation" would describe today's recommendation. Still, many young, recently transplanted trees are fertilized to prevent nutrient deficiencies and stimulate more rapid growth. Many mature trees are fertilized to prevent nutrient deficiencies and maintain vigor. Frequently, the health and vigor of trees stressed by construction activities can be improved by fertilization.
With the exception of nitrogen, fertilizer applications should be made based on a soil or foliar test. Soil tests are more commonly used unless a micronutrient deficiency is suspected. Both soil and foliar tests and fertilization recommendations based on those tests are available through independent labs. Your local Ohio State University Extension county agent can assist you in locating a laboratory in your area. Nutrients other than nitrogen should not be applied unless a soil or foliar test identifies them as being deficient. Not only is applying unneeded nutrients (fertilizer) a waste of money, but it can damage or kill the plants.
Many trees will respond favorably to moderate applications of nitrogen. Two to three pounds of actual nitrogen per 1,000 square feet per year from all sources of fertilizer is normally adequate. Rarely should more than two pounds of nitrogen per 1,000 square feet be applied in a single application. Allow at least a month between split applications of nitrogen to avoid fertilizer burn. And remember, trees growing in lawns are rooted in the same soil as the grass and rarely require separate nitrogen fertilizer programs. When the lawn is fertilized, the trees are also, since nitrogen is a principle component of lawn fertilizers.
Broadcast applications of nitrogen are preferred. Soil injection is suggested on slopes where soil erosion is a concern or where immobile nutrients such as phosphorus are being applied. Fall is the best time to fertilize with early spring second best.