Iron and manganese deficiencies in woody plants
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The roles of iron and manganese in plant growth
Iron and manganese are two of the seventeen essential elements that are the building blocks used to create the thousands of organic compounds that make up plant tissue or drive growth processes. These nutrient elements are called "essential" because they are irreplaceable and absolutely necessary for most plants to grow and reproduce. For example, iron is crucial to plants because it is a component of certain enzymes and proteins that are used during photosynthesis (where energy is captured and stored by plants) and in respiration (where stored food reserves are broken down to release energy needed for plant functioning). Manganese is essential to photosynthesis too; it is an enzyme activator during chlorophyll production and it is a structural component of the chloroplasts where photosynthesis occurs.
Causes of iron and manganese deficiencies
Iron (Fe) and manganese (Mn) deficiencies are two of the most common nutrient deficiencies seen in woody landscape plants in Minnesota. Along with 12 other mineral nutrients, they are absorbed by plants as ions (electrically charged particles) dissolved in water within the soil. When the levels of iron or manganese are low in soils or when these two elements are in chemical forms that make uptake by plants difficult or impossible, nutritional deficiency within plants occur. Without correction, these nutritional deficiencies result in plant starvation, decline, and eventually death.
Plant nutrients in soil exist in different forms or pools that impact their availability to plants. One pool consists of nutrients dissolved in soil water and in this form nutrients can be taken up readily by plants. As roots absorb these dissolved nutrients, a second pool of nutrient ions that are weakly bound to soil particles quickly replenishes the soil water with new nutrients. A third type of pool is made up of nutrients that are strongly bound to soil particles or are in insoluble forms that are not readily available for uptake by plant roots. This is the pool that iron and manganese are part of in the neutral and high pH soils commonly found in Minnesota.
Figure 1. The effect of soil pH on nutrient availability; the wider the band, the greater the nutrient availability.
Soil pH indicates a soil's acidity or alkalinity and has strong effects on a nutrient element's chemical form, its solubility, and its availability for plant uptake (Figure 1). Iron and manganese are in soluble forms that are readily available to plants when soil pH is between 5.0 and 6.5. As soil pH approaches 6.5, iron and manganese begin to change into insoluble forms that are unavailable for root uptake and utilization by plants. As pH increases above 6.5, iron and manganese unavailability intensifies. Vulnerable plants growing in these soils will start to show deficiency symptoms as iron and manganese become unavailable.
Figure 2. Soil pH gradients in the Great Lakes and Plains States.
Natural Resources Conservation Service STATSGO Database
In Minnesota soils, iron and manganese are typically present in adequate concentrations. However, where soils are neutral or alkaline, these elements are in insoluble forms and are less available for plant uptake. In general, soil pH increases from east to west in Minnesota (Figure 2). Iron and manganese deficiencies are especially common in the high pH soils of western Minnesota, particularly in the Red River Valley.
Iron or manganese deficiencies may also occur across Minnesota when roots grow into an area of localized high soil pH in undisturbed soils. Deficiencies may also occur when roots grow into disturbed soils where pH increased when sidewalks, gravel parking lots, driveways, and building foundations were constructed. At new construction sites, iron or manganese deficiencies may occur when topsoil has been removed so that planting occurs in exposed subsoils with higher pH.
Figure 3. Preferred soil pH of acid-loving plants.
Figure 3. Preferred soil pH of acid-loving plants.
Iron and manganese deficiencies also occur when plants native to acidic soils are planted in neutral or alkaline soils. Azaleas, rhododendrons, blueberries, winterberry, rugosa roses, pin oaks, river birch, red maples, and eastern white pine are the plants that most commonly exhibit iron or manganese deficiency symptoms in Minnesota. These plants are native to a range of acidic soils with pH values between 4.5 and 7.0 (Figure 3), have poor abilities to take up iron and manganese, and often are deficient in one of these elements when planted in soils with higher pH values. Other less susceptible tree and shrub species may also exhibit signs of iron or manganese deficiency when soil pH increases to 7.0 or higher and include sugar and freeman maples, northern red and white oaks, magnolia, amelanchiers or juneberries, hydrangea, and spirea.
Other factors can intensify pH-related iron and manganese deficiencies. Iron and manganese deficiencies often occur in compacted soils or soils with high clay content. The poor drainage and high soil moisture common to these soils can elevate soil pH even higher to make iron and manganese deficiencies more extreme. In addition, these soils often have low oxygen levels and are often so dense that they restrict or prevent root growth. This can cause a decrease in the growth, vigor, and health of root systems which then limits a plant's capacity to absorb iron, manganese, and other nutrients. Similarly, mechanical injury to or diseases of roots can decrease a plant's ability to absorb iron and manganese.
Diagnosing iron and manganese deficiencies
Figure 4. Chlorosis on a Rugosa rose.
K. Zuzek, UMN Extension
Figure 5. Comparison of a healthy leaf (top) and a chlorotic leaf (bottom) in river birch.
K. Zuzek, UMN Extension
The first symptom observed when trees and shrubs are deficient in iron or manganese is chlorosis (yellowing) of the newer leaves at branch tips (Figure 4). This chlorosis has a distinct pattern, with leaf veins remaining green while leaf tissue between the veins turns a pale green or yellow (Figure 5). Chlorosis affects the appearance of landscape plants but it also indicates a severe health issue that without treatment will shorten a plant's life. Chlorosis occurs because the production of chlorophyll, the green pigment that converts light energy into usable plant energy during photosynthesis, depends on adequate levels of iron and manganese. If levels of iron and manganese are low in plants, chlorophyll production and photosynthesis are reduced, foliage turns yellow, energy reserves in plants decrease, and plant begin to starve.
Figure 6. Chlorosis on oak (left) and a non-chlorotic green ash (right).
K. Zuzek, UMN Extension
As iron and manganese deficiencies worsen over the growing season or in subsequent years, foliage throughout larger portions of the plant canopy may turn chlorotic (Figure 6). As deficiencies intensify, foliage will often turn ivory in color, leaf size will be reduced, necrotic spots may occur between veins and at the margins of leaves, and leaves will die and drop prematurely. Dieback of shoots can occur. In severe or untreated cases, shrubs and trees may decline and die over several years' time.
In iron or manganese deficient conifers, a general yellowing of needles occurs, followed by browning of needles, needle drop, and twig and branch dieback as deficiencies increase.
When chlorosis or other symptoms are observed, confirmation of iron and manganese deficiencies should be confirmed with both soil and tissue tests. Soil testing is available to the general public and horticultural professionals through the University of Minnesota Soil Testing Lab and will provide information on the levels of iron, manganese, and other essential elements in your soil. Soil pH will also be provided and will indicate whether an elevated pH levels may be the source of iron or manganese deficiencies. Soil test results that show adequate levels of iron and manganese and soil pH levels in the neutral or alkaline ranges indicate that iron or manganese may be in insoluble forms that are unavailable for plant uptake.
Deficiency symptoms of iron and manganese are very similar and tissue analysis will provide definitive information on which nutrient is deficient in a tree or shrub. Leaves from both healthy and chlorotic plants of the same species should be analyzed to compare iron and manganese levels. Tissue analysis is available only to horticultural professionals through the University of Minnesota Research Analytical Lab. The gardening public should work with a horticultural professional when tissue analysis is needed so that test results are correctly interpreted and the proper corrective measures are implemented.
Correcting iron and manganese deficiencies
When iron or manganese deficiency due to elevated soil pH is confirmed, there are several treatment methods available. The only permanent solution is to replace species susceptible to iron and manganese deficiencies with species better adapted to the soil and its pH. This is the only solution if acid-loving plants (rhododendrons, blueberries, etc.) have been planted in western Minnesota where soils have a high lime content and pH values above 7.3. Decreasing the soil pH of these soils and maintaining a low pH level is extremely difficult and often results in soluble salt levels that are high enough to damage plants.
If an established plant suffering from iron or manganese deficiency must remain in the landscape and symptoms are not too severe, there are several treatment options. These include foliar application of iron or manganese, trunk injections or implantations, fertilizer applications to soil, and soil acidifying treatments. All of these treatment methods need to be repeated at regular intervals. Treatment methods for both trees and shrubs are available. Some treatments can be applied by homeowners while others require the services of a professional applicator (Table 1).
Table 1. Treatment options for iron or manganese deficiency in trees and shrubs.
|Treatment||Can be applied by homeowners?||Applied by professional arborists?||Recommended for large trees?||Recommended for small trees and shrubs?||Effects usually seen within:||Effects of treatment last:|
|Chelated iron or manganese||X||X||X||1-2 weeks||1 year|
|Ferrous or manganese sulfate||X||X||X||1-2 weeks||1 year|
|Medicaps||X||X||X||1 month||At least 2 years|
|Mediject||X||X||1 month||At least 2 years|
|Mauget||X||X||1 month||At least two years|
|Nutri-ject||X||X||1 month||At least two years|
|Pressurized injection||X||X||1 month||At least 2 years|
|Chelated iron||X||X||X||X||1 month||Variable|
|Elemental sulfur||X||X||X||2-4 months||Variable|
|Iron sulfate||X||X||X||2-4 weeks||Variable|
Foliar sprays of chelated iron or ferrous sulfate in the case of iron deficiency or chelated manganese or manganese sulfate in the case of manganese deficiency give a very quick but temporary greening of foliage. Chelated iron and manganese are water soluble forms that are readily available for plant uptake at higher pH values and are generally more effective than sulfates in foliar sprays. Foliar sprays only affect treated leaves and not those produced later in the growing season. The resulting green-up of foliage usually occurs within one to two weeks but lasts at best one growing season; repeated applications will be necessary to maintain green foliage. Foliar sprays are most easily applied to shrubs and small trees and can be done by anyone with a handheld sprayer. Large trees are more easily treated with alternative methods. Applying sprays during hot weather can damage leaves and cause them to blacken. Iron sulfate can cause rust stains on hard surfaces such as driveways, sidewalks, and siding and should be rinsed off of any surfaces it reaches immediately after the application.
Trunk implants and injections of iron or manganese
Several types of trunk implants and injections are available for treating iron and manganese deficiencies. All of these systems use small holes drilled into the base of a trunk or stem to move iron or manganese up into the tree or shrub canopy through the tree sap. Spacing of implant or injection sites is determined by trunk diameter. The effects of these treatments are usually seen within a month's time when applied in spring and last for at least 2 years. Late spring or early summer treatments are usually recommended; the effects of late summer or fall treatments will be seen the following spring.
Figure 7. Medicaps
USDA Forest Service Archive, Bugwood.org
Medicaps (Figure 7) are trunk implants that allow a powdered form of iron or manganese to be transported through tree sap. The nutrient powder is contained in a gelatin capsule enclosed in a small plastic structure with slotted sides that protects the capsule when it is inserted into the tree. Implants are tapped into small drilled holes at the base of a tree or shrub. As the gelatin capsules dissolve, the iron or manganese is moved systemically in the xylem sap up through the tree. The implants remain in the tree and are calloused over. This technique requires no specialized equipment and is a treatment method available to homeowners.
Figure 8. The Mauget system
D. Cappaert, MI State University, Bugwood.org
Figure 9. Pressurized systems move solutions from a reservoir through the tubing up into the tree canopy.
R. Billings, TX Forest Service, Bugwood.org
There are numerous injection methods available to professional arborists that can be used to treat iron and manganese deficiencies. Medi-ject is a non-pressurized commercial system developed to treat iron chlorosis and uses gravity to move an iron sulfate solution directly into the base of a tree trunk from an elevated container using a network of tubing and tees inserted into holes drilled into the root flare. The Mauget (Figure 8) and Nutri-Ject microinjection systems are examples of pressurized capsules or microinjection units containing plant-available forms of iron and manganese compounds that are placed into small holes drilled into the base of a tree. Another technique used commonly by professional arborists is a pressurized tree injection system that uses a network of tubing and tees placed in the root flare to quickly move liquids containing plant-available forms of iron or manganese from a reservoir up into a tree canopy (Figure 9).
Chelated iron fertilizers provide iron in a soluble plant-available form in neutral and high pH soils. Annual application of these products incorporated into the top 1-2 inches of soil around plants in spring is another treatment for iron chlorosis. Another method that distributes fertilizers deeper into the root zone involves applying chelated iron uniformly across many 2" diameter holes drilled 12-15" deep into the soil using a soil auger. Starting 2-4 feet from the tree trunk, holes are placed 2-3' apart and cover the area under a tree canopy to a point just beyond the dripline of the tree. A third method of application is to dissolve chelated iron in water and inject it under pressure into the soil.
Three commonly used chelating agents are EDTA, DTPA, and EDDHA and they vary in their ability to provide iron over a wide range of soil pH. EDTA maintains iron in a soluble form up to a soil pH of only 6.3, DTPA is effective up to a pH of 7.5, and EDDHA maintains iron in a soluble form from pH 4.0 to 9.0. It is important to read product labels carefully and to know the pH of your soil in order to choose the right iron chelating product. These products are available in both solid and liquid forms at garden centers.
Soil applications of chelated manganese products are not recommended when manganese deficiencies occur from elevated soil pH. Chelates have a greater affinity for iron than manganese. When chelated manganese is applied, exchange of soil iron and the chelated manganese occurs, iron is taken up by plants instead of manganese, and the manganese deficiency in plants is intensified.
Foliar sprays, trunk implants and injections, and soil applications of iron chelate all treat the symptoms of iron or manganese deficiencies. Soil acidification treats the cause: elevated soil pH. Information on soil type, current soil pH, and the desired pH are used to determine the amount of acidifying agent to be applied to soil. As with other treatments, soil acidification treatments are temporary and will need to be repeated at regular intervals as soil pH values creep back up to their original values over time.
Soil acidifiers are incorporated into the top 6-12 inches of soil and can be difficult to incorporate into soil around existing plants without damaging their roots. Soil acidification is most practical for shrubs and small trees. Tree root systems are typically 2-3 times the width of the tree canopy and it is difficult, if not impossible, to acidify large areas of soil such as those found within a large tree's root zone.
Elemental sulfur is the most economical material used to lower soil pH but takes at least 3-4 months for full effect. Iron sulfate is a second product that lowers soil pH and acts more quickly (3-4 weeks) than elemental sulfur. It requires higher rates of application and is more expensive. Table 2 provides recommended application rates for elemental sulfur and iron sulfate that lower soil pH by one unit (i.e. from pH 6 to pH 5) for sandy and loam soils.
Table 2. Elemental sulfur and iron sulfate application rates to lower soil pH by one unit1
|Soil texture||Elemental sulfur amount||Iron sulfate amount|
|lbs./100 sq. ft.||lbs./1000 sq. ft.||lbs./100 sq. ft.||lbs. 1000 sq. ft.|
|sand, loamy sand, sandy loam||0.8||8.0||4.8||48.0|
|loam, silt loam||2.4||24.0||14.4*||144.0*|
|1 Adapted from Soil Test Interpretations and Fertilizer Management for Lawns, Turf, Gardens, and Landscape Plants, http://www.extension.umn.edu/garden/yard-garden/soils/soil-test-interpretations-and-fertilizer-management/.
*To avoid damaging levels of soluble salts, do not apply more than 9 lb/100 sq. ft. or 90 lb/1000 sq. ft. in a single application.
Ammonium sulfate, a third soil acidifying agent, is also a nitrogen fertilizer. Ammonium sulfate should not be used to make initial large changes in soil pH because this will result in excessive nitrogen applications. It can be used as a nitrogen fertilizer and to maintain pH at desirable levels after pH has been lowered by initial applications of elemental sulfur or iron sulfate.
Aluminum sulfate is another acidifying agent but should not be used to lower soil pH because it has the potential to create aluminum toxicity to plant roots, especially on soils with low levels of organic matter.