Iron deficiency in trees


Grapevine showing chlorosis of the leaves.

Our hollies are showing signs of iron deficiency. What should we do? -Paul

Iron deficiency, also called iron chlorosis or lime chlorosis, starts with a yellowing of the leaves in between the dark green veins, giving the leaves a spidery look. Over time, the leaves become whitish and start to die back, eventually resulting in stunting and dying back of the entire plant. Unfortunately, these symptoms can also be caused by other deficiencies and conditions, so it’s important to make sure you have the right diagnosis.

The very first step in diagnosing and treating iron chlorosis is to do a soil test. Your agricultural extension center can help with soil testing, and they may also be able to test a leaf sample to determine which mineral is missing. While you may find that your soil is actually lacking in iron, the problem may well be caused by:

  • Alkaline Soil: When soil pH gets above 7 or so, many plants are unable to absorb iron as well. This sometimes happens accidentally when gardeners over apply lime around acid-loving plants. Correcting the soil pH will improve nutrient uptake and may be all the fix you need.
  • Mineral Imbalance: Too much of a good thing can also cause problems. Your soil may have too much of some minerals, and not enough of others, which makes the solution a lot more complex. Simply adding iron won’t help unless you correct the overall mineral balance in the soil. This can particularly be a problem in clay soils, where nutrients are scarce and may not have enough organic material and microbes to be absorbed properly.

Hollies are particularly susceptible to iron chlorosis, along with other acid-loving plants like azaleas, blueberries, oaks, and rhododendron. I would begin with a soil test, then follow all the recommendations given to correct your soil. If the soil test does indicate an iron deficiency, you can find iron supplements at your local garden center. You’ll have a couple of choices:

  • Treat Plant: Liquid iron (iron sulfate or chelated iron) is available in a liquid form that you spray directly on the plant foliage. This quick fix doesn’t have lasting results, but it can help get your plant back on track while you work on a better solution.
  • Treat Soil: Powdered or granular chelated iron is the best option for soil amendment. Sprinkle it around the root zone of the plant according to package instructions. Phosphorus overload can contribute to iron chlorosis, so if your supplement also contains fertilizer, make sure it’s phosphorus free.

Iron supplements are usually applied in spring, and although the plant will start to look better in a few weeks, it’ll need TLC over the next year or so to stay healthy. Be sure not to spill any of the supplement on your sidewalk – it’ll stain!

Further Information

  • Soil Test (video)
  • Straight Talk About Iron Deficiency and Plants (Texas A&M Extension)

How to Prevent Iron Deficiency in Iron Inefficient Greenhouse Crops

Iron deficiency is a relatively common problem among bedding plants and other spring greenhouse crops. The main symptom of iron (Fe) deficiency is chlorosis, which usually starts at the shoot tips, but more often occurs throughout the entire plant. Iron deficiency symptoms appear as interveinal chlorosis of the youngest foliage when new leaves expand before iron can be taken up by plant roots, because iron is a necessary component of chlorophyll synthesis. In all but the most extreme cases, iron deficiency can be easily mistaken for nitrogen or magnesium deficiency, so a soil or tissue test is necessary to confirm a suspected case of iron deficiency. In extreme cases, the leaves of some plants turn almost white.

Susceptible Crops

Some plants are inefficient at taking up iron from growing media, these include: Petunias (all types), Brachycome Daisies, Diascia, Nemesia, Scaevola Fan Flower, Paper Daisies, Argyranthemum, Calibrachoa, Scaevola, Ivy geranium, Piggyback Plant (Tolmiea menziesii), Basil, Pansy, Snapdragon and Catharanthus (vinca).

Deficiency Prevention

There are three main considerations for preventing Fe deficiency: pH control of the growing media, optimizing phosphorus fertilization, and the use of iron chelate treatments.

  • pH Control: The key practice for promoting iron availability to plants is to maintain pH values in the acidic range, and\ the target pH range for iron inefficient crops is fairly low: 5.5 to 6.0. Most commercial soilless media have default pH values that approximate this range and in most cases, the use of an acid-forming fertilizer with a balance of ammonium and nitrate will be enough to maintain an appropriate pH level. A major exception to this rule occurs when the irrigation water is alkaline; in that case acid injection might be needed to counter the effects of excess alkalinity on pH. Also, if sphagnum peat-based growing media is being mixed in-house, then dolomitic limestone should be added at a rate of no more than 5 lbs/yd. Too much limestone is an aggravating factor that can result in elevated pH values and contribute to iron deficiency.
  • Optimizing Phosphorus Fertilization: In certain circumstances, phosphorus (P) and Fe can react together to form insoluble iron phosphates. This reaction ties up iron because the iron in iron phosphate is not available to plants. The chemistry of this reaction is well understood in field soil, but has not been adequately studied in soilless media. To help ensure sufficient iron availability and also minimize the potential for adverse environmental impact, excess P fertilization should be avoided, especially for iron sensitive crops. Other useful practices to this end include not mixing superphosphate into growing media, use of an an acid other than phosphoric acid if acid injection is practiced, and, if possible, use of water-soluble fertilizer that supplies no more than 10% P (5% is a good rule-of-thumb) as the main fertilizer source.
  • Iron Chelate: Fertilizing iron sensitive crops with Fe from time to time is probably the least complicated and most proactive way of preventing Fe deficiency.

Note that there are different forms of iron. Chelated forms are more available to plants at higher soil pH levels compared to iron that is in an unchelated form. Chelated forms of iron are bound to an organic molecule that helps keep the iron dissolved in the soil solution. As long as the chelate holds on to the iron, it is soluble and available for uptake by plant roots. There are also different chelates available, with varying characteristics. Studies have shown the following ranking of iron forms from most effective to least effective as a corrective drench at high pH:

  • Iron-EDDHA which is effective at media pH above 7.0 (high solubility);
  • Iron-DTPA is next in effectiveness;
  • then Iron-EDTA;
  • then Iron Sulfate (not a chelate, but an iron salt containg approximately 20% Fe).

All three chelates are soluble at media pH values of 4.0 to 6.0.

If iron is applied as a drench in a form that is not soluble because of high pH, then most of the nutrient will not be available to plants until media pH is lowered. In studies, iron sulfate tended to be less effective at increasing chlorophyll and resulted in more burning than iron chelates.

Sprint 330™ which is a commonly used product, contains 10% chelated DTPA iron. It performs best in slightly acid media with a pH of up to 7.5. Sprint 138™ is 6% chelated EDDHA iron for media with a pH over 7.0.

Verdanta OFE™ is an organic-based, water soluble, foliar iron fertilizer with seaweed extract that contains 3% iron.

Follow label directions for the product you are using. Sprint 330™ and similar Fe chelates are applied as a soil drench or as a foliar spray at the same rate of 4-8 oz./100 gal. (½-¾ tsp gal). The chelate is also soluble enough to make a concentrated solution for injection, but it must be mixed and injected by itself. The company advises not to pre-mix Sprint with pesticide or fertilizer concentrates. Soil drench is the safest method of application; foliar sprays should be tried experimentally first to check for injury or residues. At the recommended rate, Fe chelate can be applied every 3 or 4 weeks if desired. Rinse excess iron off foliage and flowers because concentrated iron can cause brown or black spots. Iron chelate is fast acting, and a response is typically apparent within a few days after treatment.

Special Considerations

The information in this fact sheet is intended only for those crops known to have a special requirement for Fe. Some growers struggle with spring crops which are susceptible to Fe toxicity rather than deficiency. These crops include marigolds, zonal geraniums, and seed geraniums and their nutrition should be managed to minimize plant-available Fe; essentially the opposite of the practices described above. This is done by keeping pH in a higher range and avoiding to much Fe fertilization. Applying the treatments described here to plants susceptible to Fe toxicity would be disastrous!

Prepared by:
Dr. Douglas A. Cox
Plant and Soil Sciences
University of Massachusetts
Amherst

2000; updated 2012 Tina Smith, UMass Extension; updated 2019 Jason Lanier, UMass Extension

Links to Further Resources on the Web

Understanding Iron

Knowledge will help you avoid problems with this vital micronutrient

Iron, a vital element for crop production, is like some people—laced with contradictions. Depending on how soils were formed and their geographic location, they contain 70,000 lb. to 200,000 lb. of iron per acre (1 acre of soil 6″ deep)—more than any other micronutrient. A 200-bu. corn crop needs to take up only 0.3 lb. per acre. To compare, a 200-bu. corn crop needs to take up about 300 lb. of nitrogen.

So supplying sufficient iron to corn or soybean plants should be easy—right? Yet Farm Journal Field Agronomist Ken Ferrie has seen iron deficiency chlorosis cost growers 3 bu. to 20 bu. per acre in soybeans and 5 bu. to 10 bu. per acre in corn.
Like anything else, the more you know about iron, the easier it is to manage and prevent problems. Let’s start with some basics:
Iron—abbreviated Fe—is a catalyst required for chlorophyll formation. That’s why symptoms of iron problems show up as changes in the plants’ color. “Iron also is essential for synthesizing proteins within the plant,” Ferrie says. “It’s important for making enzymes that aid in respiration and for energy transfer within the plant.
“Iron is called a micronutrient not because it isn’t important, but because only a small amount is required by plants,” Ferrie adds. “Because plants need only a small amount, we may encounter toxicity problems when iron is in oversupply, as well as deficiency issues when plants run short. Deficiencies—called iron deficiency chlorosis—are the most common problem because toxicity usually occurs only in certain soils and in certain localities.”
In corn, iron deficiency results in striping between the veins, running the entire length of the leaf. “That’s also the symptom of magnesium deficiency,” Ferrie says. “But iron deficiency shows up on the newer leaves and magnesium deficiency on the older leaves. That’s because magnesium is mobile inside the plant and iron is not. But if iron deficiency becomes severe, the entire plant might turn white.”
In soybeans, deficiency symptoms are similar. But, while yellowing occurs between the veins, the veins themselves stay noticeably green.
Iron deficiency shows up faster in soybeans than in corn because soybeans, and some other plants, have to convert iron into a usable form before their roots can take it up from the soil, Ferrie explains. “They do this by excreting acids that convert the iron in the soil solution from the Fe(III) form to the Fe(II) form, which plants can use,” he adds. “Corn plants, on the other hand, can take up the Fe(III) form, but then they must convert it to Fe(II) inside the plant.”
The two forms of iron explain why deficiencies can occur, despite all the iron in the soil solution. Most of it is in the unavailable Fe(III) form. Deficiencies result when conversion to the Fe(II) form slows or stops. That tends to happen in alkaline soil.
“Soil pH is one of the main drivers of iron availability,” Ferrie says. “The higher the pH reading, the more carbonates and bicarbonates in the soil. They, especially bicarbonates, cause iron to become fixed and unavailable.
“The soil environment—wet, dry or cold—is also a factor. Normally, we would expect iron to be available if soil has a pH up to 7.0. But in a neutral soil with a pH of 7.0, crops will have trouble taking up iron if the soil gets saturated with water. The water traps carbon dioxide and bicarbonates, so the iron becomes fixed and unavailable. That’s why we see iron deficiency symptoms show up in new leaves two or three days after a saturating rain.”
In acid soil, the opposite happens. If the soil is saturated with water, and aeration is poor, too much iron might become available. That can result in iron toxicity.
In poorly drained soils, where water ponds in low areas, you might see a ring or halo of iron deficiency chlorosis develop around the ponded areas several days after a saturating rain. That’s because water is being pulled upward through the soil as it evaporates from the surface. The water carries bicarbonates that have built up in the saturated soil. The bicarbonates are deposited around the edge of the pond as the water evaporates.

Soil and tissue testing might help predict iron availability, but they’re not perfect indicators. “The most common test for iron availability is the AB-DBTA extraction test,” Ferrie says. “It is geared to predict the potential amount of iron available for plant uptake. Typically, we expect there is sufficient iron available if the reading is above 5 ppm. You might experience a deficiency if it’s below 3 ppm.
“There’s a problem: The AB-DBTA test is a snapshot in time, and iron availability is keyed to the soil environment—bicarbonate level, oxygen level and temperature. If the soil is cold, plants grow slowly and are less able to convert iron from the Fe(III) form to the Fe(II) form. If soybeans are green and growing well, a soil test will show we have adequate available iron. But if we test soil when the beans are showing chlorosis symptoms, we’ll get a low reading because the soil environment has changed and iron has become less available,” Ferrie says.
Tissue testing is difficult because samples must be free of dust, which could trigger a high reading. If you attempt to wash off the dust, you might distort the test.
Another problem is the iron measured by tissue testing might be in the Fe(III) form. The test might show a high reading for iron, but the iron is not usable by the plant.
“I’ve seen situations where we knew there was an iron deficiency, but the deficiency did not show up in the soil or tissue test,” Ferrie says. “Although the crops showed visual symptoms, the tissue test result said plant levels were high. Yet, when we made a foliar application of iron, both crops turned green.”

In corn, iron deficiency symptoms include striping between the veins, running the length of the leaf. If the deficiency becomes severe, the entire plant might turn white.

Some practices can cause iron deficiency. “One of the most common man-made causes is high amounts of crop residue or manure,” Ferrie says. “If you apply and incorporate high amounts of manure, especially bedding, microbes release carbon dioxide and bicarbonates as they decompose it. If the soil gets saturated with water during this process, bicarbonates might get trapped, triggering an iron deficiency. Another common cause is overliming.”
In soybeans, excessive soil nitrate levels might cause an iron deficiency. “This occurs because of the way nutrients are exchanged by the soybean roots,” Ferrie says. “To take in a negatively charged nutrient ion, roots must kick out another negatively charged ion to maintain equilibrium. If they take up a negatively charged nitrate ion, they release a bicarbonate ion, which also is negatively charged. The bicarbonates cause iron in the soil solution to become unavailable.”
Once nitrate gets inside a plant, it must be converted to ammonium. The conversion process raises the pH level inside plant cells, which can block the Fe(III) form of iron from converting to the Fe(II) form.
Nitrate levels might become high from overapplying manure, from nitrates carrying over in the soil after a drought or because some soils produce a lot of nitrate. A less common cause is high phosphorus levels, which also can fix iron.

The opposite problem from iron deficiency chlorosis is toxicity, caused by excess available iron in the soil. “Muck soils might have high levels of available iron,” Ferrie says. “High iron levels reduce the availability of manganese. Almost all micronutrient problems are caused by an excess of some other nutrient.”
In severely acid soils, with pH in the low 5 range, iron levels (along with aluminum levels) might become excessive. “Excess iron can tie up phosphorus and manganese,” Ferrie says.
In our next article, we’ll explain how to avoid or fix iron problems, whether caused by man or Mother Nature.

Getting to the root of how plants tolerate too much iron

Despite dozens of attempts in the last two decades to uncover the genes responsible for iron tolerance, these remained elusive until recently. Now, Salk scientists have found a major genetic regulator of iron tolerance, a gene called GSNOR. The findings, published in Nature Communications on August 29, 2019, could lead to the development of crop species that produce higher yields in soils with excess iron.

“This is the first time that a gene and its natural variants have been identified for iron tolerance,” says Associate Professor Wolfgang Busch, senior author on the paper and a member of Salk’s Plant Molecular and Cellular Biology Laboratory as well as its Integrative Biology Laboratory. “This work is exciting because we now understand how plants can grow in stressful conditions, such as high levels of iron, which could help us make more stress-resistant crops.”

In plants such as rice, elevated soil iron levels cause direct cellular damage by harming fats and proteins, decreasing roots’ ability to grow. Yet, some plants appear to have inherent tolerance to high iron levels; scientists wanted to understand why.

“We believed there were genetic mechanisms that underlie this resistance, but it was unclear which genes were responsible,” says first author Baohai Li, a postdoctoral fellow in the Busch lab. “To examine this question, we used the power of natural variation of hundreds of different strains of plants to study genetic adaption to high levels of iron.”

The scientists first tested a number of strains of a small mustard plant (Arabidopsis thaliana), to observe if there was natural variation in iron resistance. Some of the plants did exhibit tolerance to iron toxicity, so the researchers used an approach called genome-wide association studies (GWAS) to locate the responsible gene. Their analyses pinpointed the gene GSNOR as the key to enabling plants and roots to grow in iron-heavy environments.

The researchers also found that the iron-tolerance mechanism is, to their surprise, related to the activities of nitric oxide, a gaseous molecule with a variety of roles in plants including responding to stress. High levels of nitric oxide induced cellular stress and impaired the plant roots’ tolerance for elevated iron levels. This occurred when plants did not have a functional GSNOR gene. GSNOR likely plays a central role in nitric oxide metabolism and regulates the plants’ ability to respond to cellular stress and damage. This nitric oxide mechanism and the GSNOR gene also affected iron tolerance in other species of plants, such as rice (Oryza sativa) and a legume (Lotus japonicus), suggesting that this gene and its activities are likely critical in many, if not all, species of plants.

“By identifying this gene and its genetic variants that confer iron tolerance, we hope to help plants, such as rice, become more resistant to iron in regions with toxic iron levels,” says Busch. “Since we found that this gene and pathway was conserved in multiple species of plants, we suspect they may be important for iron resistance in all higher plants. Additionally, this gene and pathway may also play a role in humans, and could lead to new treatments for conditions associated with iron overload.”

Next, Li will be starting his own laboratory at Zhejiang University, in China. He plans to identify the relevant genetic variants in rice and observe if iron-tolerance variants could increase crop yields in flooded Chinese fields.

High Levels of Iron

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Chlorosis

Chlorosis is a yellowing of normally green leaves due to a lack of chlorophyll. Many factors, singly or in combination, contribute to chlorosis. In northern Illinois, some of the most common causes among trees and shrubs include nutrient deficiencies related to soil alkalinity (high pH), drought, poor drainage, and compaction of the soil. Common tree species exhibiting chlorosis are pin oak, red maple, white oak, river birch, tulip tree, sweet gum, bald cypress, magnolia, and white pine.

SYMPTOMS OF CHLOROSIS

Chlorotic plants may only show symptoms on one or two branches, or the whole plant may be affected. The first indication of chlorosis is a paling of the green color of the foliage, followed later in the season by a general yellowing. In mild cases, the leaf tissue is pale green, but leaf veins remain green. In moderate cases, the tissue between leaf veins is bright yellow. In advanced cases, leaf size is stunted and the leaf tissue is pale white to pale yellow. The leaf margins may become scorched or develop brown, angular spots between the veins, and the leaves may wither and drop prematurely. In conifers, an overall yellowing of the needles occurs. If severe, the needles progressively turn brown and drop, and twigs and branches may die back.

CAUSES OF CHLOROSIS

Availability of plant nutrients from the soil varies with soil pH. Most urban soils in northern Illinois are alkaline, especially the disturbed soils of neighborhoods developed since the late 1940s. The soil’s pH is an indicator of soil acidity or alkalinity (on a scale of 1-14, 7.0 is neutral, below 7.0 the pH is acidic, above 7.0 the pH is alkaline). If you do not know the pH of your soil, consider having a soil test done.
A common cause of chlorosis is a deficiency of iron or manganese, both of which are present but unavailable in high pH soils (pH>7.2). Iron and manganese are needed by plants to form chlorophyll and to complete photosynthesis. With most plants, the micronutrients iron, manganese, copper, and zinc are most available when the pH is between 5.0 and 6.5; a soil pH range between 6.0 and 6.5 is considered optimal for nutrient availability. Excesses of potassium, magnesium, and phosphorous also contribute to chlorosis. When present in excess, these elements cause some trees, particularly oaks and maples, to take up inadequate amounts of the micronutrients iron and manganese. If iron or manganese deficiency is suspected, there are both long-term and short-term treatment strategies, but a soil test will determine the pH as well as the availability of nutrients that cause chlorosis. Stressors, such as temperature extremes, drought, poor drainage (which limits soil aeration) or restricted root growth, further limit nutrient uptake in plants sensitive to chlorosis.

MANAGEMENT

Prevention is the best control. Do not plant chlorosis-susceptible trees in soils having a high pH or in soils low in organic matter. If practical, replace species that are susceptible. When the leaves of plants become chlorotic, always determine the primary cause through a soil test. Then, take the necessary steps to prevent further damage.

Short-Term solutions

Soil treatments, spraying applications of micronutrients to foliage, and trunk injections merely treat the symptoms and not the basic causes of chlorosis.

Soil fertilization treatments produce the best results, but are usually the slowest to respond. Soil treatment is best done in early spring through mid-May. For mildly chlorotic trees, fertilize with a nitrogen or nitrogen- and sulfur-based fertilizer. This will provide some acidity for the treatments below.

  • Incorporate chelated iron (found in garden centers under various names) into the top two inches of soil. A root feeder may be used, but follow label instructions for rates. Water in well.
  • Apply manganese sulfate or iron (ferrous) sulfate at a rate of 0.25 pound per 100 sq.ft., watering in well.

Foliar spray usually has a faster green-up response time, but is only a temporary solution, lasting 60 to 90 days. Spray treatments will only correct leaves that are treated but will not benefit leaves that are produced later in the season. Thoroughly spray the foliage in late spring or early summer when leaves are expanding in size. Repeat applications are usually needed. Large trees may require a professional arborist or landscape service for adequate coverage. Use one of the following:

  • Iron chelate, following label directions and rate.
  • Iron sulfate should be used at a rate of 0.50 pound of iron sulfate per 100 sq.ft. (follow label directions for soluable solution rate). Iron sulfate may cause rust stains to sidewalks, buildings, or spray equipment and should be washed off immediately.

Trunk injection is another method of applying iron or manganese-containing compounds to chlorotic trees. Recovery is often quick and treatments are effective for two or three years. Trunk injections should be made by a commercial arborist for safest and best results.

Long-Term solutions

The most lasting results are obtained by improving the tree’s rooting environment.

  • Remove existing grass from under the tree.
  • Apply a one-to-two inch layer of organic compost (acidic leaf mold), followed by three-to-four inches of organic mulch (composted woodchips). This will reduce weed competition, prevent soil temperature fluctuations during the winter and summer, and help to create more favorable soil conditions for roots to grow.
  • Water during dry periods to minimize stress.
  • Avoid fertilizing chlorotic plants with potassium and phosphorous unless a soil test indicates a deficiency, and avoid nitratecontaining fertilizers, limestone, and limecontaining materials.

In early spring, apply 3 pounds per 100 sq.ft. of granular sulfur to the soil beneath the crown of the tree out to the drip line (distance to end of branches). If possible, apply 1.5 pounds per 100 sq.ft. beyond the drip line. Water thoroughly after chemicals have been applied since sulfur can cause chemical burn to turf grass, especially in dry soil.

Or, in the fall, apply 3 pounds per 100 sq.ft. of granular sulfur beneath the crown of the tree out to the drip line. This treatment may also be broken into two 1.5 pounds per 100 sq.ft. treatments, one applied in the late fall and the other in the early spring. Sulfur should be watered in at the time of application or applied immediately before rain is expected.

Leaf Chlorosis And Iron For Plants: What Does Iron Do For Plants

Image by Mk.tham via Q&A

Iron chlorosis affects many kinds of plants and can be frustrating for a gardener. An iron deficiency in plants causes unsightly yellow leaves and eventually death. So it is important to correct iron chlorosis in plants. Let’s look at what does iron do for plants and how to fix systemic chlorosis in plants.

What Does Iron Do for Plants?

Iron is a nutrient that all plants need to function. Many of the vital functions of the plant, like enzyme and chlorophyll production, nitrogen fixing, and development and metabolism are all dependent on iron. Without iron, the plant simply cannot function as well as it should.

Symptoms for Iron Deficiency in Plants

The most obvious symptom of iron deficiency in plants is commonly called leaf chlorosis. This is where the leaves of the plant turn yellow, but the veins of the leaves stay green. Typically, leaf chlorosis will start at the tips of new growth in the plant and will eventually work its way to older leaves on the plant as the deficiency gets worse.

Other signs can include poor growth and leaf loss, but these symptoms will always be coupled with the leaf chlorosis.

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Fixing Iron Chlorosis in Plants

Rarely is an iron deficiency in plants caused by a lack of iron in the soil. Iron is typically abundant in the soil, but a variety of soil conditions can limit how well a plant can get to the iron in the soil.

Iron chlorosis in plants is normally cause by one of four reasons. They are:

  • Soil pH is too high
  • Soil has too much clay
  • Compacted or overly wet soil
  • Too much phosphorus in the soil

Fixing Soil pH That Is Too High

Have your soil tested at your local extension service. If the soil pH is over 7, the soil pH is restricting the ability of the plant to get iron from the soil. You can learn more about lowering soil pH in this article.

Correcting Soil That Has Too Much Clay

Clay soil lacks organic material. The lack of organic material is actually the reason that a plant cannot get iron from clay soil. There are trace nutrients in organic material that the plant needs in order to take the iron into its roots.

If clay soil is causing iron chlorosis, correcting an iron deficiency in plants means working in organic material like peat moss and compost into the soil.

Improving Compacted Or Overly Wet Soil

If your soil is compacted or too wet, the roots do not have enough air to properly take up enough iron for the plant.

If the soil is too wet, you will need to improve the drainage of the soil. If the soil is compacted, oftentimes it can be difficult to reverse this so other methods of getting iron to the plant is usually employed.

If you are unable to correct the drainage or reverse compaction, you can use a chelated iron as either a foliar spray or a soil supplement. This will further increase the iron content available to the plant and counter the weakened ability of the plant to take up iron through its roots.

Reducing Phosphorus in the Soil

Too much phosphorus can block the uptake of iron by the plant and cause leaf chlorosis. Typically, this condition is caused by using a fertilizer that is too high in phosphorus. Use a fertilizer that is lower in phosphorus (the middle number) to help bring the soil back in balance.

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PHOTO: astrid westvang/Flickrby Lynsey Grosfield July 13, 2016

I heard a piece of folk wisdom from an Irish gardener. Her sapling apple tree was struggling, and a local woman told her to bury a piece of scrap iron at the base. By the next year, the tree was thriving.

Stories and superstitions like these from gardeners and farmers abound. They’re obviously not based on hard science, but they often come from observations that have been passed through generations.

In this case, what an old piece of scrap iron can add to the soil at the base of a seedling apple tree is rust or iron oxide, which is an under-appreciated plant micronutrient. Iron deficiency, also called iron chlorosis, causes a yellowing of the leaves, and overall lack of vigor. It is often especially prevalent in acidic soils or soils with an excess of copper, manganese or phosphorus.

Iron is a necessary component for the formation of chlorophyll, so it impacts a plant’s ability to harness the energy of the sun. Iron also plays a role in the respiratory function of plants, where they convert carbon dioxide to oxygen. Soils can have a lot of iron, but plants growing in them can still be iron-deficient. Red soils, for example, are typically iron-rich, but often it is in the insoluble form of the mineral.

Iron is one of the most common micronutrients that plants can be deficient in, which is probably why the older Irish lady suggested burying a piece of scrap iron beside the apple tree. Even without a comprehensive soil test, a diagnosis of iron deficiency is often right on the money.

Adding iron to the soil can be as simple as using scrap pieces of iron, but sharp, rusted pieces of metal in the garden carry their own risks. A popular choice as a soil amendment is powdered or granular chelated iron. Correcting other soil imbalances, like pH and the other aforementioned nutrients, can also go a long way toward preventing iron chlorosis.

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