Just like any other plant, trees need certain nutrients in order to survive, nutrients that they draw out of the soil with their roots. Although fertilizers are often associated with promoting plant growth, maintaining proper soil nutrients not only helps your trees to grow but also protects them from environmental stressors, pest infestations and disease.
As the University of Missouri Extension cautions, fertilizers will not solve each and every problem that your tree experiences, and overapplying fertilizer can have disastrous consequences, both for the plants in your yard and your local ecosystem. A soil test is the best way to assess need for a fertilizer, but tree growth also indicates soil fertility, according to the University of Minnesota Extension. If your tree grows more than 6 inches in a single year, it is likely receiving adequate nutrients. If it grows fewer than 2 inches, fertilizer may be warranted.
In their natural environment, trees receive nutrients from a constant supply of decaying leaves and other plant matter that collects under their canopies. Many homeowners remove leaves and grass clippings, making another source of nutrients necessary. However, if you leave fallen leaves and clippings in place or if you use natural mulch around your trees, Mother Nature is likely taking care of the tree's nutritional needs for you. If you regularly apply fertilizer to the lawn around the tree, you likely won't need to fertilize the tree as well.
Trees typically experience a growth spurt in the spring, followed by a slower, more continuous rate of growth through the summer and fall. Good nutrient availability helps trees through their growth spurt, causing the University of Minnesota Extension to recommend applying fertilizer in the spring, before growth begins. For sandy soils, split applications between the spring and fall, and fertilize at any time to correct a nutrient deficiency.
Three types of fertilizers provide suitable nutrient sources for trees. Fast-release inorganic or synthetic fertilizers provide a high dose of nitrogen all at once, with the disadvantage that these easily leach or run off, causing pollution. Slow-release inorganic fertilizers, while more expensive, are safer for use around trees, according to the Clemson University Extension. Organic fertilizers behave similarly, slowly releasing nutrients over time. Because the nutritional content of organics tends to be lower, you will need to use more to achieve the same effect. Avoid using lawn fertilizers that also contain herbicides, as these may damage the tree.
Trees can receive up to 4 lbs. of nitrogen per 1,000 square feet of root area, according to the Clemson Cooperative Extension. However, take care not to overapply fertilizer, which may damage surrounding plants or the environment, and always follow the instructions on the package. Apply fertilizer to the entire root zone, which typically extends half of the tree's diameter beyond the tree. Younger trees require more nutrients than older, established trees.