How to Use Chelated Iron in the Garden
Although gardeners often apply nitrogen and potassium fertilizers to plants, they sometimes overlook micronutrients like iron. Join farmer Briana Yablonski to learn about iron fertilizers and how chelated iron can help your plants.
Contents
Although you may think of iron (element Fe) as a metal, it’s also an essential plant micronutrient. Plants of all types need a small amount of this element in order to remain healthy and grow well. However, it can be challenging to determine how to apply this nutrient to your garden.
If you quickly browse through information on Fe soil amendments, you’ll see there are handfuls of options. And if you dig a bit deeper, you’ll find that not all of these fertilizers work the same! Since Fe availability greatly depends on factors including soil pH and aeration, adding this nutrient to the soil doesn’t necessarily mean it’s available to your crops.
Applying chelated Fe is one way you can remedy this availability problem. This fertilizer is available to crops when other fertilizers aren’t, making it a useful product to have on hand. In this article, I’ll explain more about this fertilizer, why it’s beneficial for crops, and how to use it in your garden.
What Is Chelated Iron?
Iron (Fe) is an element that’s necessary for both human and plant health. Although plants don’t require large amounts of this trace nutrient, it’s essential for forming the chlorophyll molecules where photosynthesis occurs. It also helps maintain chloroplasts, the structures that house chlorophyll, and is a component of many vital enzymes.
Iron can bond with other elements to form numerous structures. For example, ferrous sulfate is a common type of synthetic fertilizer. However, since the Fe molecule is exposed to the surrounding environment, it can readily interact with other elements and change forms.
Chelated iron is one iron molecule surrounded by a chelating agent. The chelating agent acts as a shield and prevents the iron from reacting with other substances in the environment. This stabilizes the nutrient so it remains soluble in water.
Types
Chemists have realized that you can use numerous substances as chelating agents. Although all of these substances surround the Fe to form a chelate complex, they have slightly different properties.
Ferric EDTA
A widely available product, FDETA works well in acidic soils and as foliar sprays. However, if your soil pH is above 6.0, it’s not the best option. By the time the pH reaches 6.5, almost half of the iron has broken away from the chelating agent and become unavailable to your vegetables and fruits.
Ferric DTPA
This chelate works well at neutral and slightly acidic pH. At a neutral pH of seven, almost all of the iron is still held by the chelating agent. However, when you hit a pH of eight, the majority of the micronutrient is unavailable to plants.
Ferric EDDHA
EDDHA is the strongest of these three commonly used chelating agents. It continues to hold iron molecules past a pH of nine. Therefore, it’s the best option if your garden needs a boost of the micronutrient ASAP and the soil pH is strongly alkaline. However, you should note that plants growing in soil with a high pH will experience other issues even after you add FEDDHA.
Why Use Chelated Iron in the Garden?
Although plants need less of this micronutrient than nitrogen or phosphorus, this metal is still a key nutrient for plant health. Adding chelated Fe ensures your crops can access this micronutrient and take advantage of the benefits it offers.
Role of Iron
Although Fe isn’t a component of chlorophyll molecules, it’s essential for chlorophyll production. This molecule is where photosynthesis occurs, capturing solar energy and using it to produce sugars. Without enough chlorophyll, plants don’t have the sugars they need to build complex structural molecules and complete processes like cell division and flower production.
This element is an important part of electron transport chains within plants, which are essential for photosynthesis and cellular respiration. Iron serves as a cofactor for enzymes, meaning it helps the enzymes complete their jobs. Plants that are low in this nutrient may experience difficulties completing photosynthesis and respiration.
Deficiency Symptoms
By the time your plant has developed signs of low iron, it’s sometimes too late to remedy the problem. However, properly diagnosing these symptoms allows you to remedy the problem and grow healthier crops the next time around. Here are some common signs your plant may lack this vital micronutrient.
- Yellow leaves with green veins: Since this is an immobile nutrient, it can’t move from old leaves to new leaves. Therefore, young leaves turn yellow before old leaves. If there are nitrogen or magnesium deficiencies, the older leaves will turn yellow first.
While sulfur and manganese deficiencies also cause newer leaves to turn yellow, some notable differences exist. Those low in sulfur form yellow leaves with yellow veins. And those low in manganese have more of a splotchy yellow appearance.
- Stunted growth: Crops may appear shorter than normal and have small leaves. However, since a wide range of factors can cause this symptom, it’s difficult to use it to diagnose deficiency.
- Burnt leaf margins: The edges and tips of leaves may develop a brown, burnt look. This symptom usually appears in extreme cases after the leaves have turned yellow.
Availability in Soil
Many soils contain a large amount of iron. In fact, it’s the fourth most common element in the Earth’s crust! However, plants can’t access all of these nutrients. Therefore, it’s possible for plants to become iron deficient even if there’s plenty of it in the soil they’re growing in.
You can look at the different forms of this element to better understand when this nutrient is available. This element will generally take one of two forms: ferrous (Fe²⁺) or ferric (Fe³⁺). Both the presence of oxygen and the amount of water in the soil influence the state in which this element enters.
Crops can take up both forms of this nutrient, but the ferrous version is easier for plants to absorb. The main issue with availability occurs when this element binds to other compounds. High pH and/or aerobic conditions cause iron to undergo oxidation (the loss of electrons) and become ferric. This form of iron readily combines with oxygen to form ferric oxides, which are unavailable for plant uptake.
While lowering the soil pH can help make the element more available, acidification is often a slow process. Adding sulfur will lower pH, but it can take a year or more for real change to occur. During this time, your garden will benefit if you provide soil with another source of iron.
Types of Fe Fertilizers
You can find various Fe fertilizers, including the commonly used ferrous sulfate. While adding synthetic ferrous sulfate fertilizer can be effective, it presents a major challenge. When the soil pH is above 6.0, the Fe molecules readily react with positively charged ions and precipitate out of the solution. When this happens, plants cannot take it up. Therefore, ferrous sulfate isn’t a great way to add this nutrient unless you’re growing in acidic soils.
That’s where chelated iron comes in. The chelating agent surrounds the molecule and prevents it from coming into contact with other ions. This allows the nutrient to stay in solution and remain available. You can think of chelated iron as table salt that readily dissolves in water and imagine unchelated iron as sand that stays separate from the water. Chelated iron is also available as an OMRI-approved product suitable for organic gardening.
As you’ve read above, there are various chelating agents, and they all work differently at various pH levels. Choosing a product based on your soil pH will allow the nutrient to remain in solution and available.
When to Apply Chelated Iron
The best time to apply this fertilizer is before your garden needs it. That’s not to say you should overapply, but ideally, your crops will have access to this nutrient as soon as they need it. If you wait until there are signs of a nutrient deficiency, you’ve likely waited too long.
However, it’s important to recognize that chelated Fe is available to plants as soon as you apply it. Therefore, there’s no need to apply it to the soil before you have seedlings in the ground. With all this in mind, the best time to apply chelated iron is just before you plant your seedlings or sow your seeds.
Applying this fertilizer to the soil once a year is generally sufficient. That’s because this form of the nutrient stays in the soil for about a year. If you find signs of deficiency after applying fertilizer to the soil, you can make foliar applications. Complete these applications every month.
How to Apply
There are two main ways to apply this fertilizer. First, apply it directly to the soil and wait for crops to take it up via their roots. Second, use a diluted solution to foliar feed.
Soil Application
If you’re dealing with soil with a high pH or low iron, there’s a good chance all of your plants need more of this micronutrient. When this is the case, it makes sense to apply chelated iron to the soil. However, since this fertilizer is expensive, you should only apply the necessary amount.
Most types come in liquid form. This makes it easy to dilute the product, so you apply a small amount evenly across your garden. Since plants require a small amount of this micronutrient, you’ll only have to apply a few ounces of it to your garden.
Application rates vary depending on the species you’re growing, the availability in the soil, and the nutrient content of the fertilizer. Many liquid products contain between 3-5% iron. Therefore, you can apply one ounce of liquid fertilizer per 100 square feet of vegetable or flower garden. You should mix the fertilizer with at least one gallon of water to allow for even application.
After you spray the surface of the soil with the nutrient solution, gently rake the top few inches of soil. You can water the soil after raking, but it’s not necessary.
You can also find granular forms of chelated Fe, although these are less common. Dissolving these granules in water makes it easier to evenly apply them across the soil surface. In most cases, you can expect to apply one to four ounces of product per 100 square feet. However, follow product instructions to apply the correct dose.
Foliar Application
Applying liquid chelated iron to your plant’s leaves allows you to target individual plants and allows them to take up the nutrient immediately. When you foliar spray, make sure to avoid over-applying the nutrients, as this can have a negative impact. Application instructions vary depending on the type of product you’re using, so make sure to read and follow product instructions.
However, most liquid products instruct you to apply one-half to one ounce of product per 100 square feet of crops. Try to spray on a dry, calm day to avoid drift and runoff. Set your sprayer to a fine mist and spray the foliage until the solution is running off the leaves.
Be Aware of Stains
Chelated iron will stain clothing, stone, and other materials it contacts. Therefore, you should be careful when applying it. If you accidentally get it on a stone patio or brick stairs, rinse it off with water immediately. It may still stain, but it will be less pronounced if you quickly dilute it.
