Effective nutrient management for trees begins with an understanding of their natural habitat and how trees obtain their mineral nutrients. Built to thrive in nature, trees draw life from nutrient dense soil, plentiful water, and interactions with wildlife. As such, growing trees in an urban environment may be a challenge, as essential nutrients may not naturally occur.
To compensate, homeowners should consult a professional arborist to develop a nutrient management program for their trees, and apply supplements as needed.
First, you need to decide which fertilizer is best for your tree’s nutrition needs.
Regular application of fertilizer might be necessary to ensure your trees have adequate nutrition. Fertilizers may be natural or synthetic, and aim to provide trees with proper nutrients.
Common objectives of fertilization include:
- Overcoming a visible nutrient deficiency
- Eliminating a deficiency not obviously visible that was detected through soil or foliar analysis
- Increase vegetative growth, flowering or fruiting
- Increase the vitality of the plant
Fertilizer and Soil pH Levels
Professional arborists practice “prescription fertilization,” which means they only apply nutrients found to be deficient. As the medical saying goes, prescription without diagnosis is malpractice; liberal fertilizer application can ruin your soil’s pH balance. An unbalanced pH will affect the availability of many nutrients.
While nutrients are vital to tree health, they should never be added if they compromise your soil pH levels. To avoid this, measure your soil pH before deciding which fertilizer to use, or consult with a professional tree care company.
Measuring pH Levels
The pH scale measures how acidic or basic a substance is, and ranges from 0 to 14, with 7 being the neutral value. A pH less than 7 is acidic, while a pH greater than 7 is basic.
Homeowners can take a soil sample and send it to their local university cooperative extension service.
A professional tree care service or your local university cooperative extension can make custom recommendations based on the results of the soil sample analysis. Caution: pH levels can change over time, so be sure to conduct follow-up tests and adjust your soil accordingly.
Choosing a Fertilizer
Professional tree care services have access to slow-release fertilizer formulated for your tree’s health. Often, professional slow-release fertilizers reduce the need for repeated treatments over the course of the growing season.
When choosing a fertilizer appropriate for trees, homeowners should select a fertilizer with the following qualities:
- Features at least 50% slow-release
- Has a salt-index of less than 50 (salt isn’t good for tree health!)
- Does not have high ratios or potassium and phosphorous. Trees don’t like 10-10-10 fertilizers.
Fertilizer Application Methods
Once you’ve selected your fertilizer, it’s time to apply the fertilizer to the planting site.
If possible, fertilizers should be applied to the soil prior to planting. However, as your tree grows, you will need to develop alternate methods for fertilizer application:
- Surface Application. This works best when there is no turf or ground cover over the roots. 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. Dry fertilizer can be used, but needs to be watered-in. Do not use a surface application where runoff can occur.
- Subsurface Application. This method requires drilling holes 2-4” wide to a depth of 4-8” and pouring a specific amount of fertilizer into each hole. There should be at least 2 inches between the top of the fertilizer and the surface of the soil. The fertilizer should be equally distributed among all holes. Drill holes in a grid pattern, with holes spaced 12 to 36 inches apart. Caution: This method is labor-intensive and can damage roots. A professional tree care company can provide a subsurface liquid injection with slow-release fertilizer as an alternative.
- Foliar & Trunk Application. Fertilizers can also be applied to foliage, or they can be injected directly into the tree. Foliar spray or trunk injections should be reserved for rare cases when soil application is not effective or not practical to apply. These are advance fertilizer application techniques, and are best performed by a professional arborist.
- Application Amount. If you correctly select and apply a slow-release fertilizer formulated for your tree, you should only need to apply 2 to 4 pounds of nitrogen per 1,000 square feet per application. The total application for a growing season should not exceed 6 pounds of nitrogen per 1,000 square feet.