Methods of Application

Fertilizers may be put on either directly or indirectly for trees and shrubs. When sod is fertilized, tree and bush roots that stretch into the sod area absorb some of the fertilizer, and are consequently indirectly fertilized. Sod fertilization rates should be supplemented only if trees and bushes are demonstrating symptoms of nutritive deficiency.

Direct application of fertilizer may involve incorporation into the backfill soil or placement in the planting hole at planting time. However, the most common form of direct fertilizer application, broadcasting, is generally the most effective, especially relative to cost. Simply broadcasting the desired fertilizer over the soil atop the tree and shrub roots and watering it in is usually adequate. Compacted soil should first be aerated or raked.

The most sensible and efficient way to fertilize large trees is to scatter granular fertilizer on the surface of the soil and allow rain or irrigation water to transport the nutrients to the roots. Evenly broadcast the fertilizer over the area to be fertilized - that area covering the outer two-thirds of the distance between the trunk and the drip line and extending at least 50 percent of the crown radius beyond the dripline.

NOTE: Definition: The dripline is the area directly located under the outer circumference of the tree branches. This is where the tiny rootlets are located that take up water for the tree. Trees should be watered here, not by the base of the trunk, or the tree may develop root rot.

An alternative method is to position granular fertilizer into holes in the ground that are four to twelve inches deep. These holes are constructed in a regular pattern at 2- to 3-foot separations, in the same expanse as broadcast fertilizer is applied. Divvy up the fertilizer amongst the holes. This process does not insure homogeneous coverage to all feeder roots, particularly in the upper few inches of the soil surface where the bulk of the roots occur. Strong concentrations of fertilizers in these holes can in addition injure roots located next to the hole.

A commonly used commercial method is to inject liquid fertilizers into the soil. A special injection rod is used and the fertilizer solution is injected under pressure. A comparable probe mechanism called a 'root feeder' is sold at most garden centers. The long probe attaches to a garden hose and water-soluble fertilizer cartridges distribute nutrients and water directly into the tree root zone. The tip of the injection needle should be inserted 4 to 12 inches into the soil at 2- to 3-foot intervals. Fertilizers suitable for liquid injection are typically more expensive per unit of nutrient and are frequently more difficult to apply than granular fertilizers.

Spikes are another alternative for tree or shrub fertilization. These are pounded into the soil with a heavy hammer and can only be used successfully when the soil is moist. The spikes do not evenly distribute fertilizer around the tree's or shrub's major feeder roots. These spikes are an expensive alternative. Their reputation is based on simplicity and ease of application.

Foliar feeding is a quick-fix solution when a nutrient deficiency has been found. The leaves, buds and green wood are capable of a little nutrient absorption. Foliar nutrient sprays are applied with a pressure sprayer or siphon sprayer attached to a garden hose. The greening from foliar spraying is fairly quick but not long lasting. More often than not deficiencies of micronutrients including iron, boron or manganese are corrected by seasonal foliar applications.