What are micronutrients?
Micronutrients are mineral elements that are essential for growth of plants, but they are only needed in very small quantities. Micronutrients include iron, manganese, zinc, copper, boron, silicon, and molybdenum. These are in contrast to macronutrients (nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur), which are required in much larger amounts and make up a larger proportion of the plant tissue.
What are symptoms of micronutrient deficiency?
The most common micronutrient deficiencies seen in landscape plants in Iowa are iron deficiency of pin oak, river birch, and several other species, and manganese deficiency of maples. Both deficiencies are marked by pale green or yellow leaves, with veins that remain green. As the deficiency continues, new growth is stunted, branches may die back, and the margins and interveinal areas of leaves may become brown. In severe cases, the entire tree may decline over several years and die.
What causes micronutrient deficiencies?
Micronutrient deficiencies, most commonly iron or manganese, are not caused by a lack of these mineral elements in the soil. Usually the soil has plenty of these elements, but at high (alkaline) pH, they are in a form plants are not able to take up through their roots. Micronutrient deficiency symptoms usually indicate the soil in that area is too alkaline for the plant. Micronutrient deficiencies also may occur when a plant's root system is compromised, such as by injury, disease, poor site conditions, drought, or saturated soils.
How can micronutrient deficiencies be treated?
The best way to avoid poor plant health caused by micronutrient deficiencies is to test the soil before planting a tree and choose trees that are tolerant of the soil conditions, especially pH.
In the case of an established tree suffering the effects of a micronutrient deficiency, several strategies can be used to improve the health and vigor of the tree, although none are easy or permanent. Effectiveness of these strategies is variable, and tree professionals disagree about their value.
Foliar application of the deficient micronutrient is a short-term treatment that can help trees appear more green and healthy in the current season. The element must be applied after the leaves have emerged, and the positive effects will only appear on leaves that directly receive the application. Effects last only for the current season. Application of micronutrients to the foliage during hot, dry weather can cause a toxic reaction, turning the leaves black.
Micronutrients also may be injected into the trunk of the tree, using implants or a feeder system. These treatments help leaves to turn green within a week or two, and can last up to two years. However, injections require injuring the tree, which may allow the entry of fungal decay organisms.
Soil treatments also may help alleviate symptoms of micronutrient deficiencies. Chelated iron may be applied directly to the soil in the spring, which will result in greener foliage for a few months. However, this treatment may yield inconsistent results, since it does not address the core problem, high pH.
The most permanent solution is to acidify the soil in which the roots are growing. This can be accomplished by applying elemental sulfur or iron sulfate to the soil, covering the area one foot from the trunk to twice the distance from the trunk to the dripline. Two to ten pounds per 100 square feet can be added, depending on the original pH (see table below). Aluminum sulfate should not be used for acidifying soil, as it may cause aluminum toxicity to the plants.
Treatments that acidify the soil or provide additional elements to the plant will be ineffective if the main problem is injury to the trunk or roots, rather than simply high pH.
The decision to treat established trees for micronutrient deficiency symptoms depends on the value of the tree in the landscape. Treatments may be costly, and may have unsatisfying results. A careful diagnosis of the problem is important in choosing the best management option.
Pounds of sulfur needed to lower soil pH
|Material||pH change||Pounds per 100 square feet|
|Sulfur||7.5 to 6.5||1.5|
|8.0 to 6.5||3.5|
|8.5 to 6.5||4.0|
|Iron sulfate||7.5 to 6.5||12.5|
|8.0 to 6.5||29.0|
|8.5 to 6.5||33.2|