Trees were traditionally fertilized by creating holes in the root zone and pouring in dry fertilizer. Some arborists use probes to inject liquid fertilizer into the root zone area. Some pros also use Mauget injection, where holes are drilled in the trunk, and fertilizers are introduced using capsules. Another method is hammering hardened, pointed rods (called "spikes") of fertilizer into the rootzone.
Recent tests have shown that surface applications of fertilizer, at the right time of year, can be as effective as deep feeding methods. These surface apps should be timed for late fall or early spring, when tree tops are still dormant yet roots are active. Roots remain active until the soil drops below 40-degrees F and this period includes several weeks after leaf drop in fall, and a few weeks before spring bud break. Therefore, these applications would be considered dormant feedings. Most trees like a 2-1-1 fertilizer analysis, such as a 20-10-10.
Placement of fertilizers need to be where the tree can get them, so don't place them too deep --- most tree roots are within 12" to 18" of the lawn surface. Tree size and fertilizer analysis will dictate the exact amount of fertilizer you should end up applying. To prevent groundwater contamination, don't apply fertilizer when tree roots aren't actively growing, since this will allow the nutrients to leach from of the root zone area. Chemical fertilizers shouldn't be put in the planting hole when a tree is first planted, since this can cause root burn. We would recommend using an organic fertilizer -- a slow release fertilizer with little burn potential -- in with the soil backfill. Just remember that a "little bit" is always better than too much.
Since it's difficult for phosphorus to move through the soil, this is your best chance to introduce phosphorus. An organic source of phosphorus used when planting spring bulbs is bone meal. Triple super phosphate (0-46-0) is a chemical source. Phosphorus promotes rooting, as well as blossoms, in trees that flower.
Below is a bird's eye view of a tree's root zone illustrating where most of the "feeder roots" are located. The green line represents the tree's branch tips, also known as the "drip line." Most feeder roots on a tree are located just inside, and just outside of the drip line, as indicated by the zone surrounded by dashed red lines below. Therefore, focus your tree fertilization efforts in the area between the two dashed red lines, where most of the feeder roots are located. (Tree research also indicates most tree roots are located on the north side of a tree).
KEY TO DIAGRAM ABOVE
BLACK = Tree trunk and roots
GREEN = Tree canopy drip line
RED = Area between the dashed red lines represents the highest concentration of feeder roots. Fertilize in this zone.
Fertilizer bags have three numbers on the label, indicating the fertilizer analysis, or "percentage by weight" of nitrogen, phosphorus and potassium, in that sequence.
A 50-pound bag of fertilizer labeled 10-5-5 would contain 10% nitrogen (5 pounds), 5% available phosphates (2.5 pounds), and 5% soluble potash (2.5 pounds). Here's the math:
10% nitrogen (.10 x 50 lbs = 5 lbs)
5% available phosphates (.05 x 50 lbs = 2.5 lbs)
5% water soluble potash (.05 x 50 lbs = 2.5 lbs)
This fertilizer product would be considered a "complete" fertilizer since all three nutrients are present in the bag.
Fertilizers are also rated with "ratios" that indicate the proportion of nutrients to one another. For example, a 10-5-5 fertilizer is a 2-1-1 ratio, while a 20-10-5 fertilizer is a 4-2-1 ratio fertilizer.
A fertilizer ratio of 2-1-1 is what most trees prefer, which translates into a fertilizer analysis like 20-10-10 or 10-5-5.