Fertilizing Sick Trees

Fertilizing Stressed Trees
For the most part trees, especially older trees, do not need to be fertilized. There are some cases where fertilization can help a tree but these are uncommon. Indeed the careless fertilization of a sick tree can do more harm then good. For instance a Hemlock that is sick because it is infested with Woolly Adelgid is likely to die more quickly if it is fertilized, as the Woolly Adelgid reproduces faster when the tree is fertilized.

Some questions you kneed to ask before fertilizing a stressed tree are:

Is it diseased?

Are pests causing the problem?

Is the tree root bound or does it have girdling roots?

Was this tree planted too deep?

Is there too much mulch and is it touching the tree trunk?

Is the tree suffering from soil compaction?

Is the soil PH right for this tree?

Is this the right tree for its environment?

Has something changed, like sudden exposure to more sun?

Is the tree suffering from drought, or too much water?

Have the roots of the tree been disturbed by construction?

Once you rule out other causes of stress, take a soil sample and have it analyzed.
Only a soil test can tell you if and what your soil is lacking for good tree growth.
Here is more on why we don't just fertilize a sick tree.