An iron deficiency that causes chlorosis, or yellowing of the leaves, is a common affliction of oak trees, especially those growing in soil that is pH neutral (7.0) or alkaline (above 7.0). For reasons that elude researchers, iron, a micronutrient, is necessary for photosynthesis. One treatment option is to apply fertilizer that contains a chelated form of iron. Chelated means that the iron is in a form that plants can use.
Iron in Fertilizers
Chelated iron comes in both granular and liquid forms, and is sometimes included in complete fertilizers as a micronutrient. Application rates vary with the brand, and are included on the label. Adding chelated iron to an oak tree should be regarded as a short-term solution. The long-term solution is to decrease the pH of the soil.
Bore a series of 6- to 8-inch-deep holes around the oak tree. The holes should be 1 ½ to 2 inches in diameter and 2 feet apart, beginning 5 feet from the trunk. Expand the holes outward to a distance equal to two-thirds the height of the tree. Slant the holes at a slight angle towards the trunk. Make the holes in concentric circles around the tree. Place the chelated iron at the bottom of the holes, and fill them with leaf compost. If you don’t have compost, fill with soil and cover with a grass plug.
Oak trees will accept iron through their leaves. Spray a fertilizer containing chelated iron just after the leaves expand in the spring. Delaying until mid-season won’t improve leaf color. Use 1 to 1 ½ pounds of chelated iron per 100 gallons of water. If the fertilizer does not contain an ingredient to help the spray stick to the leaves, add 1 pint of dish soap. Foliar treatments must be repeated each growing season.
Chelated iron can be injected into the trunk of an oak tree. This is best done by a professional, who will drill a hole at the base of the trunk and forcefully inject iron into the tree under pressure. The fertilizer travels with the sap, up the trunk, through the limbs and into the leaves. Yellowed leaves can turn green within weeks. A poorly drilled hole or badly calculated dose may prove ineffective or even harm the tree.
Chelated iron in the form of ferric ammonium citrate can be added to a tree by way of a capsule inserted into a hole drilled in the base of the trunk. This hole, larger than that used for injection, can injure the tree and sometimes cause rot. Unlike injection, a home grower can insert an implant; these are available at many garden supply centers.