Just like other types of plants, trees in the landscape require good nutrition and care to achieve their full potential. Individual specimens outside of a naturalized forest setting may require more feeding than trees where leaves are allowed to accumulate and decay naturally. Soil testing, determining proper fertilizer application rates and where to apply fertilizer are key elements of learning how to fertilize trees.
Types of Fertilizer
Many types of fertilizer for use on vegetables, shrubs, trees and flowers are known as "complete" fertilizers, meaning they contain at least a portion of the three elements essential for healthy plant growth: nitrogen (N), phosphorous (P) and potassium (K). The proportion of each element present in the fertilizer mixture is reflected as a percentage of the total weight of the fertilizer. Chemical, or inorganic, fertilizers differ from organic fertilizers in that they are chemically manufactured rather than produced as a side product of something that is naturally high in a given nutrient.
Tree Growth Habits
Most trees and shrubs produce a main flush of growth in the spring, followed by slowed and steady growth throughout the remainder of the growing season. Making sure a plant has adequate nutrition available before this growth spurt occurs will enable the tree to make the most of its natural cycles. In many cases, this means fertilizer should be applied around a tree's drip circle in early spring, just as leaves are beginning to unfurl. The outer edge of the drip circle is the imaginary circle created by the tree's outer limbs, or the edge of where water naturally rolls from leaves during a moderate rainfall. Fertilizers are usually applied from the outer edge of the drip circle to within 2 or 3 feet of the trunk. Mature trees generally require less fertilizer than younger trees.
Trees in Lawns
Trees that are part of a landscape that includes a regularly fed grass lawn are generally in no great need of a separate feeding regimen. Many types of inorganic fertilizers are not completely absorbed by grass roots as it breaks down, and feeder roots from trees extending into the lawn generally pick up adequate nutrition from grass fertilizer. Trees growing in areas that are generally not mowed or where leaves are not collected in the autumn generally do not need to be fertilized, as decomposing organic matter fulfills most, if not all, of the tree's nutritional requirements.
Determining Fertilizer Loads
Before applying any type of fertilizer, a soil test should be conducted to determine the latent nutrient loads present in the native soil. Many university agricultural extension offices perform these tests for a moderate fee and return an analysis report that lists the levels of nitrogen, phosphorous, potassium, trace elements and presence of salts, and recommendations for fertilizer application rates. Rates of application are generally included on the labels of commercially produced fertilizers, as well. In general, a good rule of thumb is to apply 1 lb. of fertilizer per inch of trunk diameter at 4 1/2 feet from the ground (breast height), or 1 to 2 lbs. of fertilizer per 1,000 square feet of soil surface beneath the tree canopy. Avoid applying too much nitrogen, especially with young trees, as this tends to encourage rapid but weak growth.
There are three main methods of applying fertilizer within a tree's root zone. The easiest and best-known method is to broadcast fertilizer directly onto the soil surface above the root zone, allowing weather to naturally break down the fertilizer pellets and incorporate them into the soil where the tree can absorb the nutrients. Feedings should be divided into two or three portions over several weeks' time, followed by a thorough watering after each fertilizer application. A second method is to bore holes with an auger or crow bar to a depth of 12 inches, spaced 3 feet apart, within the circumference of the drip zone. The recommended amount of fertilizer should be divided among these bore holes, inserted and the holes backfilled, followed by a thorough watering. Finally, trees may be fertilized by injecting solutions of fertilizer directly into the root zone, known commercially as feeding needles.