First, ask the arborist if they are applying fertilizer in compliance with the American National Standards Institute standard for tree fertilization, called ANSI A300 - Part 2 Fertilization.
(Click the title above to see specific information about this standard.)
Arborists who adhere to industry standards of practice are more professional than those who do not. In the arborist industry in the U.S., there are two voluntary standards related to arborists' activities: The ANSI A300 Standard and the ANSI Z133 Standard.
ANSI stands for American National Standards Institute, the accrediting organization for the committees that write the standards.
This standard requires that certain types, rates, and application methods be followed and sets the standard definitions for terms the arborist will use in your estimate. Estimates for fertilization should be written in accordance to ANSI A300 Part 2 Fertilization standards. Properly written ANSI A300-Part 2 Fertilization work estimates should include, at a minimum, the following information:
- a statement that says all work will be performed in accordance with ANSI A300 Part 2.
- a statement with a clearly defined objective for the fertilization,
- the type of fertilizer to be applied,
- the rate of fertilization,
- the timing of the fertilization,
- the method of the fertilization,
- the fertilization area
Commonly, the preferred type of fertilizer is slow-release fertilizer with a nutrient ratio of 3:1:1 or 3:1:2 for shade trees, and 3:1:3 for palms. The preferred rate of fertilizer for slow release is between 2 to 4 pounds of actual nitrogen (N) per 1000 square feet. Usually, it is recommended to avoid applying over 6 pounds of nitrogen in a single year. The fertilization area is basically the root zone of the tree. The fertilization area is generally considered to be from near the trunk to near or beyond the shade canopy (dripline) of the tree. Roots can grow much further from the tree, however, so the arborist may recommend adjusting the fertilization area to account for larger root zones.
There are a number of fertilization methods such as a granular surface application, sub-surface dry application, and sub-surface liquid fertilizer injection. Generally, when a turf or ground cover is growing in the fertilization area, a sub-surface fertilization method is preferred. Your arborist may deviate from these common practices for a number of reasons, such as to correct a nutrient deficiency or to correct other plant health problems. The objective of the deviation should be clearly stated in writing on the work estimate.