You can apply fertilizers directly to plants or indirectly. If trees or shrubs show symptoms of nutrient deficiencies, fertilizer rates for turf should be increased.
Direct tree and shrub fertilization could require placement in the backfill soil or the planting hole. Broadcasting is the most common method of fertilizer application. It is also the least expensive. Broadcasting fertilizer on top of the bush roots or tree is sufficient. Watering in the fertilizer should also suffice.
First, aerate or rake the soil.Spreading granular fertilizer over the soil is the best and most efficient way to fertilize large trees. Rain or irrigation water can then transport the nutrients to their roots. Spread the fertilizer evenly over the fertilized area – the area that covers the two-thirds distance between the trunk & the drip line, and at least 50% of the crown radius beyond this drip line.
Another way is to place granular fertilizer in holes 4-12 inches deep in the soil. These holes should be dug in a consistent pattern, at 2- to 3 foot intervals. Divide the fertilizer between the holes. This method does not guarantee uniform coverage, particularly in the areas where most of the roots are located. These holes can also cause damage to roots that are located near the hole.
Injecting liquid fertilizers into soil is a common commercial procedure. The fertilizer solution is injected using a specialized injection probe.