• Yellowing of pin oak leaves is a possible sign of chlorosis, which may be treated with chelated iron.Hemera Technologies/PhotoObjects.net/Getty Images
  
  Oak trees, and pin oaks in particular, are susceptible to chlorosis, a potentially deadly condition. Chlorotic trees result from conditions that diminish the lack of available iron to the tree. These include soil compaction or growing conditions that restrict the tree's roots; alkaline soils where the pH is 6.5 or higher; and insufficient iron in the surrounding soil. Symptoms of chlorosis on an oak are most visible on the leaves, which transition from green to yellow as the condition worsens. The veins of the leaves, however, remain green. Severe cases result in death of tree limbs and eventually, of the tree itself. Treatment of chlorosis commonly calls for the application of chelated iron, which is sold at most garden centers.
  
  

What Is Chelated Iron Fertilizer?

• Chelated iron, which is sold in granular, powder or liquid form, is simply iron mixed with a chemical known as a chelate. Chelates assure that the iron applied will be available to the tree rather than becoming locked in the soil. Although the product is most effective when injected into the soil, it is also applied as a foliar spray, or injected into the trunk. Chelated iron fertilizer is sold with a number of different chelating agents; deciding which one to use depends upon the pH of the soil.
  
  

Chelated Iron as a Foliar Spray

• When used as a foliar spray, chelated iron provides only temporary relief and is not effective on leaves that emerge later in the growing season. This means that spraying is required repeatedly throughout the growing season, a formidable task if the tree is large. Because foliar spraying works better in some regions of the country than in others, it pays to consult with a local arborist or county extension professional to find out how effective this method is in a given area.
  
  

Chelated Iron Applied to the Soil

• Chelated iron has longer lasting impact when applied to the soil. The application is most effective when made from April to early June, although it is possible to apply it at other times of the year, including when the tree is dormant. The easiest way for a homeowner to use the product is in liquid form with a root feeder, an implement commonly sold at garden centers. It is also possible to drill a series of holes of approximately 12 to 15 inches deep, spaced 2 to 3 feet apart, continuing these until the limits of the oak's canopy. Drill the holes no closer than 2 feet from the tree's trunk. Mix the chelated iron according to directions, pouring the required amount--usually a gallon--into each hole, and then fill the holes with soil.
  
  

Chelated Iron Trunk Injections

• Injecting chelated iron into the trunk of the tree is a method that greens the tree within a month of application, and while not as effective as soil application, has longer lasting effects than foliar spraying. This method is best left to an arborist, as it requires drilling holes into the trunk without damaging the tree and using injection capsules that are not commonly sold in garden centers.