If you’ve seen leaf bronzing or tiny brown specs on your plants’ leaves, it is possible that there is too much iron in your garden’s soil. This is a common problem that can have many different causes. Depending on the cause of too much iron in your soil, there are a few solutions.
Too much iron in your garden’s soil needs to be fixed by lowering the iron content available to plants. This can be done by adjusting the soil pH, balancing iron with other micronutrients, or by improving aeration and drainage of water.
In this comprehensive guide, I’ll walk you through everything you need to know about iron in your soil. Read on to learn about the symptoms, causes, and treatments to fix having too much iron in your garden’s soil.
See also: How to Lower Manganese Levels In Soil
Micronutrients and Their Functions
Iron is one of seven micronutrients that plants need to grow. Other micronutrients include copper, cobalt, boron, molybdenum, manganese, and zinc. These nutrients are found in varying small amounts in soil all over the planet and in trace amounts in water sources.
These minerals are called micronutrients because they are only needed in small quantities by the plant. Other nutrients such as nitrogen, phosphorus, or potassium are called macronutrients and are needed in much greater amounts.
Micronutrients serve important functions that are needed for plants to grow and be healthy. Iron is used as a catalyst to synthesize chlorophyll, which is required for photosynthesis.
Without chlorophyll, plants would not be able to produce enough energy to grow. Iron is also used for a plant’s protein synthesis and for nitrogen fixation and reduction.
Other micronutrients play important roles in regulating membranes, controlling nitrogen levels, energy processes, and overall yield.
Iron Excess and Toxicity
Having too much iron in your soil is called “iron excess” or “iron toxicity”. While small amounts of micronutrients like iron are needed for plant growth, having too much of them can be harmful to the plant. A careful balance is needed.
Symptoms of Iron Toxicity
Some plants are particularly vulnerable to iron toxicity, including geraniums, lisianthus, African marigolds, pentas, New Guinea impatiens, and lilies. Before we can think about treating iron toxicity, we first need to identify it.
The plants in your garden may be suffering from iron toxicity if leaves show signs of bronzing (that is, getting darker), if brown spots form on the leaves, or if the plant’s growth becomes stunted.
Bronzing is the most common symptom. While small amounts of iron are needed for chlorophyll production, having too much iron affects the chlorophyll itself, making it darker. This prevents the plant from absorbing sunlight.
Having too much iron also interrupts vital cell processes that produce proteins and energy for the plant. If left untreated, the plant will eventually die.
The seven micronutrients needed by plants need to be balanced. Generally, toxicity or excess of one nutrient leads to a deficiency in others. Almost all micronutrient deficiencies are caused by another micronutrient’s excess.
Having too much iron, for example, interrupts the plant’s ability to take in manganese. This is known as a “nutrient lockout.” Sometimes plants suffering from iron toxicity will also show symptoms of manganese deficiency because of this.
Iron toxicity can also lockout and prevent the absorption of important macronutrients like potassium, nitrogen, and phosphorus. These nutrients are essential for plant growth.
Correlated Symptoms of Iron Toxicity
Iron toxicity most commonly locks out manganese, and chlorosis (yellowing of leaves) occurs as a result. Like iron, manganese is an immobile micronutrient, but manganese deficiency affects both young and old foliage. If manganese is deficient, yellow leaves will be seen at both the top and the bottom of the plant and could be a sign of iron toxicity.
If iron is preventing potassium from being absorbed, leaf tips may curl and show chlorosis. Purple spots may also appear on leaves. Plant, root, and fruit growth will be much slower if the plant does not have enough potassium.
Iron toxicity can also lead to smaller amounts of nitrogen being absorbed from the soil. Without enough nitrogen, plants will grow slower, will show chlorosis, will not grow as many new leaves, and will not grow wider.
If iron locks out phosphorus, the plant will not grow as many roots and will be more vulnerable to cold temperatures.
If your plants demonstrate some of these symptoms, it’s possible they’re suffering from iron toxicity. Later, we’ll discuss testing procedures you can use to make sure that iron is causing the problem and not another nutrient.
Next, we’ll take a look at the causes of iron toxicity and how we can fix them.
Causes of Iron Toxicity
Generally, iron toxicity problems come from the soil. Soil can contain either too much or too little iron. But in reality, the amount of iron in the soil is less important than the amount of iron available to the plant in the soil.
Iron can exist in two different oxidation states, either Fe2+ or Fe3+. Fe3+ is the more common form of iron in the soil, but this form is not available to plants and is immobile. Fe2+, on the other hand, is mobile and available to plants.
The causes of iron toxicity, as we’ll see, are usually because of an imbalance in Fe2+ vs. Fe3+ rather than a lack, or abundance of iron in the soil. This imbalance leads to either too much iron available to the plant (iron toxicity) or too little (iron deficiency).
The ratio of Fe2+ to Fe3+ is most heavily determined by the soil’s pH, which is a measure of acidity. Soils with lower pH are more acidic and will have more Fe2+, while soils with higher pH are more alkaline and will have more Fe3+. A neutral pH – where the soil is neither basic nor alkaline – is around 7.
Usually, plants are grown in soil with a pH between 6 and 7. Some plants prefer even more acidic soil, in the range of 5 to 6, and other plants prefer alkaline soil with a pH between 7 and 8. Differences in soil pH affect the availability of nutrients and micronutrients.
A few factors can influence the soil pH, changing the amount of iron that is available.
Fertilizer Ingredients Affecting Soil pH
Most fertilizers contain large amounts of nitrogen, phosphorus, and potassium, which are the three macronutrients that plants need more than anything else.
The usage of fertilizer can affect the nutrient mixture in the soil and change its pH. Fertilizers that are high in nitrogen can have either high or low pH’s, depending on the form of nitrogen that is used. Ammonium-containing fertilizers will lower the pH of soil, while nitrate-containing fertilizers will raise the pH.
The most common form of phosphorus that is used in fertilizers is phosphoric acid. This increases the acidity of the soil, lowering the pH and making iron more available.
Potassium fertilizers have little-to-no effect on soil pH. Potassium fertilizer does not greatly affect the availability of iron in the soil.
Natural Factors Affecting Soil pH
Two additional natural factors are important in determining the availability of iron in soil.
Aeration of water in soil is important because water can dissolve carbon dioxide, forming carbonic acid. There is more carbon dioxide in stagnant, still water. Carbonic acid lowers the pH of soil, and poorly aerated soils usually have higher iron availability.
The soil itself can have a varying pH. Soils all over the world have pH’s in the range from 5-8, and geographic location or source of the soil can determine its acidity. Different plants prefer to grow in different pH soils, which can lead to compatibility problems.
Other Causes of Iron Toxicity
Iron toxicity is generally caused by having too much iron in the soil. A few other factors can lead to excess iron in soil.
Fertilizer Components Affecting Iron Availability
Manure fertilizers are usually rich in all micronutrients. When used to treat soil, these can greatly increase iron availability to plants.
Some manure fertilizers have been treated with lime and have high pH, though, and these will decrease soil iron availability. Alternatively, if too much fertilizer is used, microbes will produce carbon dioxide and bicarbonate, immobilizing iron in the soil and making it unavailable to plants.
Crop residue fertilizers often contain iron from the decayed plant matter. Adding these to soil can also increase the availability of iron. Like manure, if too much fertilizer is used, microbes will produce chemicals that immobilize iron in soil.
Some supplements contain a form of iron called iron chelate. Adding this to soil greatly increases the availability of iron, and adding too much can lead to iron toxicity.
Plants can only absorb a limited amount of nutrients from soil. Iron toxicity is a problem if iron is being absorbed in a greater proportion than other important nutrients. This can happen because iron is too available, or because other nutrients are not available enough.
If soil is low in manganese or phosphorus, for example, iron will be absorbed in greater amounts and can lead to iron toxicity. Because of this, iron toxicity is closely related to manganese or phosphorus deficiencies.
Zinc, potassium, molybdenum, nickel, and nitrogen deficiencies can also lead to greater uptake of iron into plants and cause iron toxicity.
Carbonates and bicarbonates in the soil can bind strongly to iron and hold it there, making it unavailable to plants as well.
Natural Variations in Climate and Iron Content
Geographic location and climate are important determining factors for how much iron is available to plants.
Rainfall is important because if there is excess water in the soil, iron will become more available. The right amount of water is needed to provide a stable amount of iron, and too much water can lead to too much iron.
Soil also varies widely in iron content and pH. Soils all over the world can have pH as low as 5 or as high as 8.5. Iron content varies greatly from place to place. Soils can contain as little as 0.2% iron or as much as 55%. Soils with higher iron content or lower pH are likely to have more iron availability for plants and are more likely to lead to iron toxicity.
Now that we’ve learned the symptoms and causes of iron toxicity, we can talk about how to deal with it in your garden or on your farm.
How To Fix Too Much Iron In Soil
If you’ve observed the symptoms of iron toxicity in your garden, these next steps will help you to confirm and treat it in your soil.
Testing and Identifying Iron Toxicity
You should always start by making sure that iron toxicity is, indeed, the problem. If you treat your plants for iron toxicity when they actually are suffering from another problem, for example, copper toxicity, you can make the problem much worse and kill your plants.
Soil pH Testing
The first test you should do is a soil pH test. Make sure that your soil’s pH is in a reasonable range, somewhere between 6 and 7 for most plants.
If your soil pH is below 5, it is highly likely that you will have an excess of iron. At these pH levels, your plants will also likely suffer from aluminum toxicity, manganese deficiency, and phosphorus deficiency.
You can test soil pH using a digital meter, a kit, or a soil pH strip. These are available online, usually at low costs.
Soil Micronutrient Testing
If your soil pH is in a reasonable range for your plants, but your plants are still showing symptoms of nutrient deficiency, then the next step is to test the micronutrient content in the soil.
To do a soil test, you’ll usually have to send a sample of your soil to a lab. This can get expensive, but if your plants are struggling, it will likely be worth the effort.
The lab will determine any deficient or excess micronutrients, the expected impact of these imbalances on crop yield, and they’ll give you a plan on how to fix the micronutrient imbalance.
Soil micronutrient test kits are also available for purchase online, such as the LaMotte Professional Soil Test Kit.
If the soil is deficient or excessive in any nutrient – not just iron – this should be treated, as any imbalance in nutrients will affect plant growth and health.
Plant Tissue Testing
Plant tissue testing can also reveal any nutrient excesses or deficiencies. These can be unreliable if performed improperly, but they still can provide useful information regarding the plant’s physiology.
Tissue tests can be either destructive when a plant sample is removed and sent to a laboratory, or non-destructive when the analysis takes place without harming the plant. Fewer non-destructive tests are available for analyzing iron and micronutrient content; instead, these usually determine nitrogen and other macronutrient contents.
A good tissue test will determine which nutrients are available and being used by the plant and which are not. You can adjust your plant’s soil accordingly once you have this information.
Treating Iron Toxicity
If you’ve identified an iron excess in your soil leading to iron toxicity in your plants, there are a few ways to treat it. By following these procedures, you should expect your plants to become healthier after a few days or weeks.
Since iron is an immobile nutrient in plants, the symptomatic leaves will not change for a long time. However, if treated properly, new growth on the plant will be healthy.
To deal with too much iron in your garden’s soil, you’ll want to lower the amount of iron that is available. This can be achieved in a few different ways.
pH Adjustment with Lime
Since iron becomes more available at high acidities (low pH), you can decrease the acidity (raise the pH) of your soil. Make sure to test the pH first, as above, to make sure that it is acidic.
To decrease the acidity of your soil, you can add lime. There are many different types of lime, so be sure to select the one that is best for your application. The placement and time of lime application also can have important effects, depending on the type of lime you use.
Using lime not only makes the soil more alkaline, but it benefits plants by filling the soil with the nutrients calcium and magnesium. By making the soil alkaline, it also protects against aluminum and manganese toxicity.
You can use lime to raise the pH of soil to a neutral range, but if you add too much, a highly alkaline soil can kill the plants. If the soil pH becomes too high, you can add elemental sulfur or sulfate-containing chemicals.
Since iron becomes more available and toxic in stagnant, pooled, poorly-aerated water, one solution is to increase water flow to your plants.
You can follow steps to increase water pressure in your irrigation system, or improve water drainage from your garden.
An alternative is to completely wash the soil bed by running large amounts of water through it. This is a more extreme method, but it will remove almost all nutrients from the soil. Then, once the soil dries, apply fertilizers to get back to the right balance of nutrients.
Reducing Iron Availability by Adding Bicarbonates
Bicarbonates bond to iron in the soil and will prevent the iron from being available to plants. By adding bicarbonate-containing chemicals to the soil, you can reduce the availability of iron to treat iron excess. This works particularly well in soils with a neutral pH.
Increase Other Micronutrient Concentrations
If iron availability is too high, then the plants will absorb more iron than other micronutrients. Instead of lowering the iron content of the soil, another option is to increase the concentration of other nutrients. By doing this, the plants will absorb a more balanced ratio of iron and other important nutrients.
One of the best sources of micronutrients is compost. Compost has a large supply of all micronutrients, and by adding all of them in large quantities, the soil will become more balanced.
Alternatively, you could add specific micronutrient compounds to raise the concentration of zinc or molybdenum, for example. This treatment works best if only one or two micronutrients are deficient. Make sure not to add too much of any single nutrient, leading to other toxicity issues and nutrient lockouts.
Other Nutrient Deficiencies and Excesses
Iron toxicity, while not rare, is much less likely to occur than some other common nutrient imbalances. Here are some of the other most common nutrient imbalances that you are likely to come across. Make sure to test for iron and other nutrient content to be certain what kind of imbalance your plant is facing before you attempt to treat it.
Nitrogen is the most important macronutrient for plants. Symptoms of nitrogen deficiency include chlorosis of leaves. Leaves turn yellow, and if the deficiency is not treated, become more yellow over time.
Necrosis – when leaves die and remain on the plant – does not occur with nitrogen deficiency. Instead, leaves that die from nitrogen deficiency will fall off the plant.
Nitrogen deficiency is the most common nutrient deficiency in plants. It is caused when too much carbon-containing matter is present in soil. Bacteria in the soil use nitrogen to break down carbon sources, and if sawdust or other organic material is added to the soil, not enough nitrogen will be available for plants.
Nitrogen deficiency is most easily resolved by adding nitrogen-containing fertilizers such as ammonium phosphate, calcium ammonium nitrate, or urea. Spraying urea on the leaves can also solve nitrogen deficiencies.
Phosphorus is the second most important nutrient for plants. Phosphorus deficiency leads to chlorosis of leaves, but slower than nitrogen deficiency. Phosphorus deficiency can be easily identified by large, dark patches of necrosis on leaves, particularly near the veins.
Phosphorus deficiency, like nitrogen deficiency, is common in plants. It can be caused by cold climates that slow root growth, dry soil conditions, and unsuitable soil pH’s. Phosphorus is most available to plants at pH between 6-7.5.
Phosphorus deficiency can also be caused by iron toxicity, as too much iron can prevent phosphorus from being absorbed.
Phosphorus deficiency can be treated with inorganic fertilizers containing phosphoric acid, P2O5, or manure. These are rich in phosphorus and can provide the phosphorus that the plant needs.
Potassium is another important macronutrient for plants. Potassium deficiency causes chlorosis in leaves beginning at their tips and slowly moving inwards. Necrosis also occurs at the tips of leaves.
Potassium deficiency is common due to potassium ions being highly soluble easily leach from soils. Clay soils contain colloids, which are more likely to contain potassium, but other soil types do not have as much potassium available.
Potassium deficiency is easily treated by using potassium-containing fertilizers such as potassium chloride. Wood ash is also high in potassium but must be used sparingly because it will raise soil pH.
Other Micronutrient Deficiencies
Most micronutrient deficiencies show similar symptoms and are related to each other. Iron excess can lock out manganese and other micronutrients, and manganese excess can lock out iron and other micronutrients. Both iron deficiency and manganese deficiency appear similar to each other, so identifying the right one can be a challenge.
Symptoms of micronutrient deficiencies include chlorosis, necrosis on new growth, or other irregularities in color (like bronzing) or shape (like deformed leaves).
All micronutrient deficiencies and imbalances, including iron toxicity, can be treated in approximately the same way:
- Make sure all minerals are present in water. Don’t use reverse osmosis water, which has all of the nutrients removed. You can add trace mineral drops to add these micronutrients back into the water if they are not present.
- Test the pH of water and soil. Make sure it is in the 6-7 range, or a range suitable for your plant. Test the micronutrient content of the soil and the plant to make sure all the micronutrients are present and being absorbed.
- Flush the soil with water to remove all nutrients that have built up and to break any nutrient lockout. Wait for the soil to dry before watering the plant again.
Plants only need tiny quantities of micronutrients, and usually, the trace amounts in a water source will supply enough for them. If your plants have micronutrient deficiencies, it is likely because of the water source or a nutrient lock, where an excess of one nutrient prevents another nutrient from being absorbed.
Whether you’re tilling a small garden or running a large farm, you’re likely to come across nutrient deficiencies in your plants from time to time. Iron toxicity is caused when too much iron is available to plants in the soil. By absorbing so much iron, other nutrients are locked out, and the plant becomes sick.
To deal with too much iron in your garden’s soil, you’ll want to raise the pH to a more neutral level, aerate the water, and make sure all other nutrients are available in sufficient quantities.