Chlorosis (yellowing) refers to leaves that are light green or yellow--not a healthy, dark green. Often, leaf veins remain dark while the rest of the blade is lighter. This condition is common on pin oaks in Illinois but also is found on silver maple, red maple, sweetgum, and birch.
Illinois soils typically have a high (alkaline) pH level, which may cause problems such as nutrient-deficiency chlorosis. You have likely seen this on pin oaks in the form of iron chlorosis. In alkaline soil, minor nutrients are often bound within the soil chemistry, making them unavailable to the tree. Iron or manganese seems to be the most limiting nutrient in a high soil pH system in Illinois. Symptoms of either are similar, with manganese deficiency most likely if symptoms are worse on older leaves and iron deficiency more a problem on new leaves first. An Extension report discussing iron and manganese chlorosis (Report on Plant Disease, RPD, no. 603) can be accessed at. http://www.ag.uiuc.edu/~vista/horticul.htm
Recently, the Plant Clinic has received many chlorotic samples of red maple and river birch. In wet periods or under root compaction or injury, symptoms are intensified. Clay subsoils aggravate the problem.
The question is how to treat trees that are deficient because of the soil pH. You might want to test the soil pH at a local lab. This provides a reference for retesting after treatment. Find out the pH level preferred by your tree. Michael Dirr, in Manual of Woody Landscape Plants, often lists the desired pH for trees and shrubs discussed. Soils with a pH below 6.7 seem ideal for red maples. Birches thrive at 6.5 or below. Illinois soils usually have a pH of about 7.4.
Changing the soil pH around an established tree, however, is not a quick process. If the tree is severely affected, consider spraying the foliage with a chelated-iron or manganese product available at garden centers. If you spray a small area, you may determine which element is limiting by watching for a response. During the growing season, the leaves should become darker green in a week or so if the limiting element is applied. This treatment works only on the leaves sprayed, so the effect is temporary and benefits only the leaves currently expanded. New leaves will not turn darker green.
The treatment that is effective the longest, but which is also slowest to take effect (and most time consuming), is to acidify the soil. This is done by adding sulfur to holes dug 12 to 15 inches deep and at 2- to 3-foot intervals in a series of parallel lines 2 feet apart under the complete spread of the branches and extending just beyond the drip line of the tree. Details are discussed in Report on Plant Disease (RPD), no. 603 (see Web site mentioned earlier in this article). This process takes several years to change the soil pH.
A short-term alternative for a couple of years is to inject chelated iron or manganese into the soil in the holes you dug for the sulfur. Combining the two processes may give you the longest control. Chelates can be injected directly into the trunk, but many horticulture specialists would rather avoid wounding the tree in this manner.
Soil treatment is best done when the soil is moist in April, May, or early June.