• Micronutrients are chemical elements that are essential to a tree's normal growth and development, even though only a few atoms out of every million are needed.
• There are 8 known micronutrients. Iron and manganese are the most important for ornamental trees. Crop trees have special needs not discussed here.

• When trees (or shrubs) cannot get the micronutrients they require, they do not grow or develop properly, and slowly die.
• Different micronutrients have different functions. Many commonly affect specific cellular reactions such as chlorophyll synthesis, nitrogen fixation, and respiration.
• Micronutrient deficiencies on urban trees are often due to a high soil pH (7.8-8.2 on soils with significant lime content) that makes the elements unavailable to the plant.

• Lack of iron or manganese in the leaf results in interveinal chlorosis--a yellowing of the leaf blade between green veins--that is worst on the youngest leaves. With extreme deficiency, the leaves become completely yellow. Conifer needles turn yellow-green.
• Recent research has shown that "iron chlorosis" frequently involves more than iron, even with a species like pin oak. Also, manganese is now recognized to be very important.
• BEWARE: diagnosis of micronutrient lack is tricky, because many environmental (e.g., drought) and structural (e.g., girdling root) problems can also produce chlorotic leaves.

• Iron: pin oak and sweetgum. Manganese: white oak, white pine, and red maple. A number of other species show reduced vigor and health.

• Do not plant susceptible species on soils with a high pH.
• Acidify the soil. Place small amounts of an appropriate compound in 1-2" holes drilled 2' apart and 12-18" deep in a band around the dripline. Best results have often come with a mix of a fast acidifier, a slow acidifier, and organic matter. Repeat as necessary.
• Apply a chelate as a soil drench if one micronutrient is involved. The best iron chelate on a high lime soil is EDDHA, found in several commercial products. Repeat as necessary.
• Tree injection works well, but requires multiple and repeated wounding. Spraying is ineffective, because crown coverage is difficult and marginal leaf scorch is easy.

Rich Koenig and Mike Kuhns, 1996, "Control of Iron Chlorosis in Ornamental and Crop Plants," Utah State University Extension. You can get this publication from your regional DEC office, Cooperative Extension, or off the Web (ext.usu.edu/publica/agpubs/ironchlo.htm).