What Is Chlorosis? Treating Iron Deficiency in Plants

After spending months tending to your plants, it’s alarming to watch healthy green leaves fade to shades of yellow. You know something is wrong, but what?

Although yellowing can occur as a natural part of aging, it’s often caused by a lack of chlorophyll in the leaves. When that’s the case, the yellowing is known as chlorosis.

There are many different causes of this type of yellowing, so it can be difficult to understand what’s behind the discoloration. But once you determine the cause, you can get to work fixing the issue. I’ll introduce you to some common causes of chlorosis and then discuss how to treat yellowing caused by iron deficiency.

The Short Answer

Chlorosis occurs when plant leaves turn yellow due to a lack of chlorophyll. This phenomenon can be caused by factors including compaction or waterlogging, high pH, drought, or nutrient deficiencies. Iron deficiency is one of the most common causes of chlorosis. Plants may lack iron if the soil is low in this micronutrient or if the soil pH is too high.

If the yellowing is caused by a deficiency, you can solve the issue in a few ways. Lowering the soil pH will make the iron already present more available. If the ground lacks iron, you can add fertilizer. However, be aware that many types of iron become unavailable to plants when the pH is above 7. You can also spray an iron fertilizer on the leaves.

The Long Answer

Both environmental and chemical issues can lead to this yellowing, so determining the cause is your first step in remedying the issue.

What Is Chlorosis?

Chlorosis occurs when leaves lack chlorophyll, the compound that gives plants their green color. Chlorophyll is an important component of plants because it’s where photosynthesis occurs! If they don’t have enough chlorophyll, they won’t be able to capture the sun’s energy and use it to produce sugar. Therefore, chlorosis often leads to problems other than yellow leaves.

Not all yellow leaves indicate chlorosis. As leaves grow older and die, they naturally change color, a process known as necrosis.

Causes

Although all cases of chlorosis occur due to a lack of chlorophyll, this deficiency has multiple causes. Many nutrients are involved in the production of chlorophyll, so nutrient deficiencies often lead to yellow leaves. However, it’s possible for these nutrients to be present in the root zone but unavailable to plants. That means both environmental conditions and a lack of nutrients in the soil can cause chlorosis.

Here are some of the most common causes of yellowing.

Improper pH

Soil pH can range from 0 to 14. Zero is extremely acidic, 7 is neutral, and 14 is extremely basic. Most plants grow best at a pH between 6 and 7, although there are some exceptions to this rule. For example, plants in the Ericaceae family, like blueberries, cranberries, and rhododendrons, prefer a pH between 4.5 and 5.5.

The pH greatly impacts nutrient availability. Each nutrient is most available at a specific pH, with some nutrients being most available at high pH and others at low pH. Therefore, most gardeners aim to maintain a slightly acidic pH so all nutrients are available.

If your garden has a pH above 8, most of the manganese, copper, zinc, and iron will become unavailable to plants. A pH below 6 leads to issues with calcium, magnesium, and phosphorus availability. Therefore, your plants will begin to show signs of nutrient deficiencies, even if there are plenty of these elements present. Fortunately, you can take steps to raise or lower the pH, although it’s often a slow process.

Compacted Ground

Well-aerated soils have numerous small air pockets that allow water to infiltrate the surface and move down through the various layers. These air pockets also provide space for gas exchange and root growth.

When ground becomes compacted, these small air spaces disappear. Therefore, soils often become waterlogged and/or anaerobic, and microbes may become unhappy. This is all dependent on what species you’re growing and where you’re growing them.

Many of the microbes in our gardens play a major role in making nutrients available to plants, and studies show compaction often leads to lower nutrient availability. This may not be a problem for a plant growing in its native habitat where compacted ground is normal. But in a garden, compaction also limits root growth of cultivated crops, and makes it more difficult for them to access necessary nutrients and water. When roots don’t obtain the necessary nutrients, the leaves may develop chlorosis.

It’s difficult to improve aeration while crops or ornamentals are growing in the ground. However, loosening with a broadfork, growing deep-rooted cover crops, and double-digging the ground between seasons will all improve aeration, lessen compaction, and improve nutrient uptake for the next round of crops.

Drought

Water is essential for all living things, including plants. When they can’t access the water they need, they can’t complete essential processes like photosynthesis, respiration, and cell division. Adequate moisture also allows roots to uptake nutrients and move them throughout the plant.

Drought conditions limit both the uptake and movement of nutrients. If the root zone remains dry for more than a few days, crops (especially those you’re cultivating outside of their native range) may experience chlorosis due to a lack of nutrients.

Nitrogen Deficiency

Nitrogen (N) is one of the three plant macronutrients, so plants need a large amount of it to remain healthy. The ideal amount of nitrogen varies by plant type. For example, annual fruiting crops like corn and tomatoes require lots of nitrogen, but perennials like coneflowers and rudbeckia can thrive with less nitrogen.

This nutrient serves many vital roles. It’s a component of proteins, including enzymes that drive reactions and structural proteins that maintain rigidity and form. It also helps with cell division and is a part of chlorophyll molecules. Therefore, a lack of nitrogen leads to chlorosis.

Nitrogen is a mobile nutrient, meaning they can send the nutrient throughout the roots, stems, and leaves. Therefore, nitrogen-related chlorosis will first show up in older leaves.

You can correct this problem by adding nitrogen-rich fertilizers and ensuring environmental conditions allow for proper nutrient uptake. Remember, aerated soil, adequate moisture, warm temperatures, and proper pH improve nutrient availability and uptake. 

Magnesium Deficiency

Magnesium is a secondary nutrient that makes up the middle of chlorophyll molecules. Therefore, a lack of nitrogen leads to a lack of chlorophyll and yellow leaves. Since magnesium is a mobile nutrient, yellowing will appear in older leaves first. However, the yellowing often appears patchy rather than consistent.

Adding a magnesium-rich fertilizer will improve soils lacking this nutrient.

Sulfur Deficiency

Sulfur is another secondary nutrient needed in moderate amounts. This nutrient is a component of some proteins and helps with disease defense. It’s also involved in chlorophyll production, so a deficiency can lead to chlorosis.

Sulfur is an immobile nutrient, so deficiency symptoms first appear in young leaves. Leaves become yellow and may develop green and yellow stripes.

Iron Deficiency

Although iron is a micronutrient that plants only require in small amounts, it’s essential for functions like photosynthesis, respiration, and DNA formation. Although it isn’t a component of chlorophyll molecules, it’s crucial to chlorophyll formation. Therefore, a lack of this micronutrient is often associated with chlorosis.

Iron is an immobile nutrient, so yellowing first occurs in young leaves. The leaves turn light green and then yellow while the veins remain green. Although the soil may have this nutrient, plants often experience this deficiency when a high soil pH makes it unavailable.

How to Determine the Cause

Before you can begin treating yellow foliage, you need to determine which factor is responsible for the yellowing. Follow these steps to pinpoint the issue.

Conduct a Soil Test

Since numerous nutrient deficiencies can cause chlorosis, I recommend taking a soil test to learn the nutrient levels and soil pH. Collect samples from the top six inches of the soil and submit them to a laboratory. If the results show your soil is low in a specific nutrient, this deficiency is likely the cause of the chlorosis. A high or low pH is also a possible cause of the yellow leaves.

Remember that even if a nutrient is present, it’s not necessarily available to plants. Some nutrients constantly change forms, and some of these iterations are not available for uptake. Additionally, a high or low pH will make nutrients unavailable to plants.

Observe the Soil

If your soil test results show the pH is in the ideal range (6.0-7.0) and all necessary nutrients are present, examine the soil. Even if nutrients are present, dry, saturated, or compacted soil may prevent plants from taking up nutrients. Rather than adding unnecessary nutrients, focus on improving soil composition and moisture.

Iron Deficiency: A Common Cause of Chlorosis

So, how do you determine a lack of iron is causing chlorosis? First, look at the symptoms. An iron deficiency will always affect young leaves first, and the discoloration will be even rather than spotty. The affected leaves’ veins will also remain green.

You can also look at the soil test results. A low level of Fe guarantees your crops lack this nutrient, but an adequate amount of the nutrient doesn’t necessarily mean they have all the iron they need! A high pH limits them from accessing this nutrient, so a high pH coupled with adequate nutrient levels means iron deficiency may still be the cause of chlorosis.

Availability

Just because iron is in the soil, it doesn’t necessarily mean it’s available. This element often changes forms and combines with other compounds to become unavailable to plants. Simply put, it generally exists in a ferrous (Fe²⁺) or ferric (Fe³⁺) state. Although plants can absorb both forms, the ferrous version is easier for them to take up.

The real problem occurs when the ferric version combines with other elements. High pH and/or aerobic conditions can cause Fe to become oxidized and enter into the ferric state. Then, it can then combine with oxygen to form ferric oxides, which are unavailable for plant uptake.

How to Correct Soil pH

If iron deficiency is caused by high pH rather than a lack of the element in the soil, you should focus on lowering the soil pH. Although this is often a slow process and will not benefit the plants that are currently growing, it will prevent iron deficiency in the future.

The most reliable way to acidify and lower the pH is to add sulfur. You can add sulfur in the form of elemental sulfur, iron sulfate, or aluminum sulfate. However, if you are worried about iron or aluminum levels, it makes sense to add elemental sulfur. The amount of sulfur to add depends on how much you want to lower the pH.

As sulfur is oxidized, hydrogen ions are released, and the pH drops. However, since sulfur oxidation is a biological process reliant on microorganisms, it’s slow to occur. Cold temperatures and dry soil slow the process even more. Expect to wait at least a few months to see any noticeable change in pH.

Types of Fe Fertilizers

Although high pH is often the cause of iron deficiencies in plants, it’s possible that the ground lacks this micronutrient. This is especially true if you’re growing in a raised bed filled with a potting mix. If that’s the case, adding Fe fertilizer can allow plants to take up the nutrient and limit chlorosis.

Even if the bed contains an adequate amount of iron, your plants may still benefit from the addition of an micronutrient-rich fertilizer. The trick is choosing a product that’s available to plants even at a high pH.

Chelated Iron

If your pH is high, most of the Fe is unavailable to plants. If you add fertilizers like iron sulfate, the nutrient will quickly transform into an unavailable form, rendering the fertilizer application practically useless.

That’s where chelated iron comes in. These fertilizers consist of iron molecules held in the “claws” of larger molecules. The outer shell prevents other molecules from interacting with the iron and keeps it in a plant-available form. Therefore, plants can access the nutrient even if the pH is high.

You can apply chelated iron to the ground by spraying it over the dirt surface. No matter which product you use, follow product instructions to determine the ideal application rate. In most instances, using one ounce of product per 100 square feet is a good place to start.

Foliar feeding leaves on a cloudy, cool day is another way to provide your plants with this nutrient. Start by applying a half ounce of chelated iron per 100 square feet of plants. Make sure to mix the fertilizer with at least one gallon of water to allow for even application.

Iron Sulfate

If your pH is below 7 and the ground lacks Fe, you can apply this fertilizer to your garden. However, if the pH is above 7, don’t bother. That’s because the nutrient will quickly change into a form that’s unavailable to plants.

Although iron sulfate typically doesn’t make sense for topical applications, it is helpful for adding iron to plants via foliar sprays or trunk injections. To foliar spray, wait until a cloudy day when the temperature will remain below 90°F (32°C). Mix 1.25 ounces of iron sulfate with one gallon of water and spray plants until wet. A trunk injection can help treat chlorosis in trees, but a certified arborist should complete it.