Before recommending a fertilizer, the arborist may note that other problems need to be addressed. An improper soil pH often makes it more difficult for a tree’s roots to absorb needed nutrients, even when those nutrients are abundant in the soil. Generally a pH range of 5.2-6.2 is the most desirable. Acid soils are sometimes limed to raise the pH, but more commonly, basic soils are treated with sulfur to adjust the pH downward. Maintaining a soil pH range of 5.2-6.2 for shade trees generally insures that essential plant nutrients will be available to the tree.
What is a nutrient deficiency and why does it need to be corrected? What is iron chlorosis?
Just like people, trees can have nutrient deficiencies that lead to problems. The deficiency may cause the tree to exhibit predictable symptoms that the arborist can use to diagnose the problem. Other times testing and analysis may be required to determine the exact nature of the problem.
Here is an example of a common nutrient deficiency. Many trees suffer from iron deficiency induced by poorly drained or compacted soils or by soils with a high calcium content. These soils are characterized by pH levels above 7.5. The unavailability or non-utilization of iron commonly causes a condition called iron chlorosis, and is characterized by yellow leaves, slow growth, and branch dieback. Pin oak and silver maple frequently show this condition. Chlorotic trees exhibit partial or complete yellowing of the leaves. The yellowing first occurs between the veins, leaving a network of darker green veins on a greenish-yellow background More severe symptoms include smaller than normal leaves that turn pale yellow and develop angular brown spots between the veins. Leaf margins may turn brown. The leaf eventually curls, dries up, becomes entirely brown, and falls. Tips of affected branches may die, especially in the tops of broadleaved trees. If the condition is not corrected, it can cause poor root development, severe stunting, and plant death. Weakened plants also are very susceptible to other diseases and insect infestations. On junipers, pines, and other evergreens, chlorosis usually develops as an overall yellowing of needles.
Cause of Iron Chlorosis
Under alkaline conditions (soil pH above 7.0), iron changes to insoluble forms that are less available to growing plants. Iron is more soluble and most readily absorbed from the soil at a pH range of 5.0 to 6.5. Lack of available iron is aggravated by: 1) low temperature and high soil moisture, 2) relatively large amounts of copper, manganese or zinc, and 3) excessive phosphorus applications.
Correcting Iron Chlorosis
There are three methods of applying iron salts to correct iron chlorosis in plants. They are: 1) applying a mixture of dry ferrous sulfate (iron sulfate) and sulfur to soil, 2) spraying the foliage with a solution of ferrous sulfate or iron chelate, or 3) introducing iron salts into the main stem or trunk of affected plants by implantation or injection. This is a case where the arborist might recommend a foliar spray or trunk injection to save the tree in the short term, and then recommend other treatments to correct the real cause, the high soil pH, in the long term.
