Yellow leaves can be a sign of iron deficiency. Chlorosis is a common problem of many acid loving trees and shrubs in Minnesota, particularly in urban areas. Chlorosis is caused by a lack of micronutrients, resulting in the inability of the plant to produce chlorophyll, which is essential to the plant’s survival. It can cause the decline of the plant and eventually its’ death. The leaves show the first signs of chlorosis, which will start to turn a pale green or light yellow. It will start out in blotches, progressing to a bright yellow and spreading to the whole leaf, except the veins.
The plant that gets the most attention concerning this problem is the Pin Oak. It is commonly found in trees improperly planted in heavy clay or poorly drained soils which lock up the micronutrients, most commonly iron. The lack of magnesium, zinc, or copper can also cause the same symptoms. Construction damage is also a common cause due to soil compaction and root damage.
Chlorosis is a sign that something is wrong underneath the ground, and not in the leaves. . Planting the tree in the right spot is the best “cure” The soil should be acid, in the ph range of 5-6.5. Avoid planting the Eastern strain of Pin Oak. The northern pin oak is more resistant to chlorosis.
If doing a new construction project, make sure the roots are protected against compaction or damage.
The soil can be acidified, but not all at once. Drill a hole about 2’ apart and 18” deep, and fill it with a mix of soil sulpher and sand. You will need to repeat this treatment in the spring and fall, continuing the treatment even after the leaves regain their normal color, because the sulpher can leach through the soil.
Spraying the leaves with an iron chelate solution will give the leaves a quick green-up, but is not a good long term solution because it doesn’t address the real problem, which is in the soil.