Choosing the best tree fertilizer is not as easy as it may seem. While most fertilizers will offer some benefit to trees, the best product for your particular situation may not only involve soil testing, but also a consideration of the types of trees being fertilized. Any tree fertilizer may be able to provide at least some nutritional value, but you should be able to get better value for your money with a little research.
One of the first things to consider is the amount of available nutrients in the soil. Typically, fertilizers provide three main nutrients that all, or nearly all, land plants need: nitrogen, phosphorous, and potassium. On fertilizer packaging, nitrogen, phosphorous and potassium are listed in that order and may appear on the packaging at N-P-K. These nutrients are likely already available in the soil and a general purpose fertilizer typically provides them in equal, or nearly equal amounts. This should suffice in most cases, but may not always the best choice.
Using a home soil kit, or possibly taking a sample of your soil into a local extension office are good ways to find out exactly what your soil both contains and lacks. For example, if you soil already has adequate nitrogen, adding more through the use of a general purpose fertilizer could be counterproductive because the tree fertilizer can cause too much growth and actually weaken the plant. While growth is sometimes indicative of good health, that is not always the case.
You may even be able to customize your tree fertilizer applications with fertilizers that contain only one or two of the major three nutrients. This may help provide only the nutrients you need, but you may also wish to have the soil tested periodically to make sure there are no major changes over a period of time. Further, using a supplement may help the soil recharge, especially if other organic material is added from time to time.
In general, a tree does not need as high a nitrogen level as smaller leafy plants, which have shallower root systems and must take all of their nitrogen from the topsoil. Therefore, if you are looking at a tree fertilizer, the best ones may have a N-P-K ratio of something like 1-2-1. It is important remember this is somewhat dependent on the soil conditions.
Another thing to consider when using a tree fertilizer is a time-released product. Otherwise, the tree may absorb too many nutrients too quickly, or cause you to have to reapply it many times during a growing season. Trees are prime candidates for time-released fertilizers because there is no need to speed up growth during a growing season as may be necessary for annuals such as vegetables and some flowers.