Thomas Smiley, Ph.D., is an arboricultural researcher at the Bartlett Tree Research Laboratories and adjunct professor at Clemson University. Sharon Lilly is director of educational goods and services for ISA. Patrick Kelsey is a soil scientist at Christopher B. Burke Engineering and also is a Certified Professional Soil Scientist.

Fertilizer Application
Fertilizers can be applied to the soil or foliage, or they can be injected directly into the xylem of trees. Soil application is the preferred technique. Foliar spray or trunk injections should be reserved for rare cases when soil application is not effective or not practical to apply.

Soil surface application of fertilizer is an efficient means of delivering nitrogen to trees or shrubs. Nitrogen is very mobile in the soil; so as long as there is an adequate amount of water moving through the soil, the nitrogen will move to the root area. Dry surface applications are made with carried or wheeled fertilizer spreaders. These types of spreaders provide an even distribution of the material if moved at a constant speed. To avoid loss of fertilizer efficacy, applications should be made immediately before rain or irrigation, whenever possible.
 

Soil surface application is an efficient means of delivering nitrogen.

Liquid surface application can be made with a variety of spray equipment. To achieve an even distribution of the fertilizer, a flooding tip or water breaker nozzle is preferred for surface application.

Although surface application can be effective and inexpensive, there are some use limitations. Where the fertilizer application area is covered with turf, the turf takes up a portion of surface fertilizer. Surface-applying fertilizer on organic mulch increases the breakdown rate of the mulch because of an increase in biological activity. On slopes, surface-applied fertilizers are more likely to run off. Phosphorus may not move into the root area of trees before it is tied up in the soil. In this case, a subsurface application of fertilizer is preferred. Potassium is intermediate in soil mobility; subsurface application is the preferred application technique, but surface application may be effective in many cases.