Answer: There are several options for fertilizing your trees. Ultimately, you must look at your trees to determine their nutritional needs. If they lack vigor they may need extra fertilization, while overly vigorous trees should be put on a low nitrogen diet!
The traditional approach is to sprinkle some ferilizer on the surface. One cup of a complete fertilizer per inch of trunk diameter is about right. Rake or rototil the area to only scratch the upper 1 inch or so of soil. This approach is only effective if you can get a water hose to the trees to water it in well. The same approach could be used for compost; just spread it about 1 inch thick.
Since you have a drip system set up there is a high tech method which would really be effective. That is injecting fertlizer through the drip lines. You must select a water soluble product for liquid fertilizing, like Miracle Grow, for this application. Your tree's roots are primarily concentrated in the area wet by the drippers. Therefore this is the most efficient area to place fertilizer.
A simple injector with a water hose connection can be purchased at a local garden center or hardware store. Or, you can talk with the company that designed your irrigation system about injectors they may be able to install.
One note of warning. Anytime your use an injector with water lines that are connected to a well or your home water supply you should have a backflow preventer installed before the injector (between the injector and the source) to prevent contamination of the water supply. Some hose end injectors come with a pressure valve to accomplish this purpose.
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