Many conditions can cause chlorosis (yellowing of leaves) in trees. The most common one, of course, is early leaf coloration changes in the fall when leaves normally begin to change colors due to less available chlorophyll. If the yellow leaves have been prevalent thoughout the growing season, however, the most common causes are various nutrient deficiencies, extreme pH levels, or drought. Sometimes these conditions can combine to create more favorable conditions for chlorosis, and this past August's heat and drought may have contributed to your tree's condition.
Some nutrients that trees need to produce chlorophyll, including iron and manganese, may be deficient or depleted in the soils of some local areas. In other cases, excess potassium or phosphorus may actually limit accessibility of these critical nutrients. A principal cause of nutrient availability, however, is soil pH. Soils can be either acidic or alkaline, typically ranging from extremes of 7.5 (highly alkaline) to 4.5 (highly acidic) on an overall scale of 0 to 14. Urban soils in particular are more likely to be more alkaline, as are areas of the state with limestone bedrock. Sandy soils and special features like bogs are more acidic. In either case, if soils are above 6.5 or below 5.0, the nutrients needed for healthy chlorophyll production may be limited.
The most reliable way to determine current soil nutrient availability and pH levels is to conduct a soil test. Self-mailer soil test kits from the Michigan State University Plant and Pest Diagnostic Lab are available. You can obtain them at the link below. Full instructions are there and also come with the kits. The cost is $25. The Lab will make recommendations for any needed soil amendments. Ideally, samples should be taken in multiple locations around the tree, focusing under the tree’s crown and extending to the drip line (outermost branch tips of the tree). Dig down to at least 4-6 inches with a spade or trowel to collect the soil, since organic matter, roots and other debris can affect the results. Mix the various soil samples in a small bag to “average” the soil conditions, then follow the instructions in the test kit to measure the pH level.
A quick, but less thorough soil test kit can be obtained from your local garden center.
Although chlorosis can be indicative of a nutrient deficiency in the tree, it doesn’t mean that the nutrients aren’t present in the soil. High pH (>7.0) can limit a tree’s ability to access the nutrients in the soil, so adding additional nutrients will not necessarily address the issue entirely.
If the pH level is extreme enough to potentially limit nutrient availability, thus creating chlorotic conditions as indicated by a soil test, the most straightforward actions are to eliminate competition for soil nutrients by removing all vegetation, including grass, around the tree, and replace with a thick layer of composted wood chips or other (natural) mulch material to maximize moisture retention. Also be sure to provide supplemental water during the summer’s extended dry periods.
Most chemical soil treatments for amending pH take some time to be effective. If the pH is high (alkaline), one option is to apply granular sulfur under the tree, all the way out to the drip line in the fall. Be sure to double check soil nutrient test results periodically to ascertain how effective your applications have been.