What's the best way to feed trees with iron?Generally with older, larger trees, including Eucalyptus, it is very difficult to get the nutrients directly in to the vicinity of the "feeder" roots of the tree, which are the newest roots, furthest away from the base of the tree. Feeder roots are called that for a reason, they absorb all the moisture and nutrients available in the soil.
These roots are in the area of the "drip line" of the tree (or shrub), which if you visually looked straight down from the upper diameter of the tree (or shrub), that is where the drip line is. In some cases you can work and loosen the soil in that area, so the fertilizer can be watered in, and in some cases you cannot. It depends on if you have a lawn or something else obstructing your access. In either case, that is where you want to apply the fertilizer.
Iron does have a shelf life, so if what you have on-hand is much more than 1 1/2 to 2 years old, then I would recommend purchasing a new bag. You want to use a product that has the highest percentage of "water soluble iron", iron which is more readily available to the tree. That information should be available on the bag, or by asking a certified nursery professional at your local nursery or garden center. Something as simple as Iron Sulfate, or anything comparable, will do the trick. Please remember to always follow label directions.
Another method, which in certain applications is much easier and more efficient, especially on larger trees, is a product called "medi caps". You simply drill a hole in to the trunk of the tree, say any tree larger than 6" or 8" in diameter, and insert these capsules. At that point, the active ingredient, in this case iron, is released to be absorbed directly in to the sap of the tree. This product is only available at an independent nursery, such as Harper's or A&P on the east side, and Elgins or Summer Winds on the west side of town. Or check online at acecap-medicap.com
And please remember, all of Arizona's Best fertilizers have a much higher percentage of iron and sulfur, than any other brand on the shelf. Arizona's Best is made locally in Arizona and formulated specifically for the type of soil we have in the southwest.
In closing, think about incorporating Soil Sulfur in to your regular fertilizer program. Sulfur helps to lower the pH of the soil, and in most cases, helps to allow the tree or shrub to absorb nutrients either in the bag of fertilizer, or what might already be in the soil, that they cannot ordinarily get just because the soil is too alkaline (high soil pH).