If a soil test reveals that your soil is lacking iron, then you are probably looking for a way to supplement this important nutrient in your garden. Luckily, there are many options for high-iron fertilizers.
So, what fertilizer is high in iron? Fertilizers that are high in iron include iron sulfates, chelated iron, iron supplements, blood meal, compost, and manure. Some of these will affect soil pH, so keep this in mind when you apply them, and get a soil test before doing so.
Of course, you can use a mixture of any of these sources of iron, depending on what you have available.
Let’s get into more detail about each of these sources of iron.
What Fertilizer Is High In Iron?
Here is a table with some of the best fertilizers with high iron content. You can find more detail about each type of iron fertilizer after the table.
|Fertilizer||Percent Iron |
|Iron Sulfates||14 to 23|
|Iron Chelates |
|5 to 10|
|Iron Supplements||2.5 to 4.5|
Iron sulfates contain between 14% and 23% iron by weight. They also contain sulfur, which is another important nutrient for your garden soil.
Iron sulfates include ferric sulfate (23% iron by weight), ferrous sulfate (19% iron by weight), and ferrous ammonium sulfate (14% iron by weight). Ferrous ammonium sulfate will also add nitrogen to your soil.
Remember that iron sulfates will lower soil pH when added to your garden. In fact, iron sulfates lower pH faster than elemental sulfur.
There are a couple of drawbacks to using iron sulfates. First of all, iron sulfates are not chelated, so they may not work in high-pH soils. Even if they do work, the iron will not remain available to plants for very long.
Another drawback of using iron sulfates is that they are expensive to use at scale. Your best bet is to use iron sulfates to treat iron deficiencies in a small area or for a small number of plants.
Unless your soil has a serious iron deficiency, you should consider using compost or manure instead. Both of these will contain some iron, and they are much more cost effective than iron sulfates.
Chelated iron (iron chelates) contain 5% to 10% iron by weight. Chelated iron is often applied to the foliage (leaves) of plants as a liquid spray.
One advantage of chelated iron is that it remains available to plants for a longer time than iron sulfates. Often, iron deficiency in plants is caused by low iron availability, not by a complete absence of iron in the soil.
In addition, chelated iron works well in soils with high pH. Therefore, it is your best bet if your garden has a naturally high lime (calcium carbonate) content, or if you overused lime in your garden at some point.
There are several specially formulated fertilizers that will supplement nutrients that are lacking in your soil. To add iron to your soil, some options include Ironite and Milorganite.
Ironite is a mineral supplement use primarily to add iron to soil. It contains 4.5% iron by weight.
Ironite is often used to treat iron chlorosis in lawns, which causes yellow grass. For more information, check out Ironite on the Home Depot website.
Milorganite is a fertilizer made from recycled wastewater. It contains 2.5% iron by weight.
Milorganite looks like small pellets, which are actually dried microbes. In addition to providing some iron to your soil, it adds nitrogen, which is one of the “big three” nutrients for plants.
For more information, check out this article on the Milorganite website.
Blood meal is a powder made from the dried blood of animals (often cattle or hogs), and like bone meal, is often a by-product of slaughterhouses. Blood meal contains 0.2% iron by weight.
Blood meal also contains 12.5% nitrogen by weight, making it much better than manure and compost in terms of percentage nitrogen by weight. Blood meal has a medium release time, and is effective for 6 to 8 weeks.
Blood meal also contains 1.5% phosphorus and 0.6% potassium by weight. Thus, blood meal is a good all-around fertilizer to supply a variety of nutrients to your garden soil.
For more information, check out this article on blood meal from Wikipedia.
Compost is made from kitchen scraps and yard waste, such as banana peels, orange rinds, grass clippings, and raked leaves.
Compost contains small amounts of iron, depending largely on the materials used to make it. Compost is a slow release fertilizer, and also contains nitrogen, phosphorus and potassium.
The best part about compost is that you can make your own in your backyard. For more information, check out my article on how to make compost.
There are many different types of manure, including cow, horse, pig, and chicken. All of these manures have a medium release speed, and can be effective for up to two years.
Manure, like compost, contains small amounts of iron, depending largely on the diets of the animals and the type of livestock that the manure comes from.
Manures also contain small amounts of phosphorus and potassium as well, making them good all-around fertilizers. Just make sure to decompose manure completely before using it on your garden, to avoid burning your plants!
For more information, check out my article on manure.
Are Rusty Nails Good For Plants?
No, rusty nails do not help plants. While it is true that rust (iron oxide) contains iron, there is a catch.
Iron in the form of rust is water-insoluble, meaning that it is not available to plants in a form they can absorb.
This is especially true in soils with a high pH, which is often part of the cause of low iron availability for plants.
What Does Iron Do For Plants?
Iron has many important functions in plants. For one thing, it is used in chlorophyll production (chlorophyll is the compound that makes plants green).
Iron is also used in photosynthesis, meaning that it helps plants to produce energy.
Iron is an immobile nutrient, meaning that a plant cannot easily move iron between its tissues. That means that an iron deficiency will result in chlorosis (yellowing) of younger leaves (the ones higher up on the plant).
For more information, check out this article on fertilization from the Penn State University Extension.
What Causes Iron Deficiency in Plants?
There are several possible causes of iron deficiency in plants. The two basic reasons are lack of iron in the soil, and inability of plants to absorb iron in the soil. Let’s take a closer look at some of the reasons for iron deficiency, and how you might avoid them.
Soil pH is Too High
One of the main reasons that plants cannot get enough iron from soil is that the soil pH is too high. As soil pH rises, iron availability decreases.
To see an illustration of this, check out this chart from Research Gate on the effect of soil pH on nutrient availability.
High soil pH can occur if you use too much lime (calcium carbonate) in your garden. This may occur if you fail to measure and use the proper amount based on your soil type (clay, loam, sand, etc.), current soil pH, and desired soil pH.
High soil pH can also occur if your garden is irrigated over a long time period with water that has a high pH.
One way to decrease the pH of soil is to add sulfur. You can also use iron sulfate, which will add iron to your soil at the same time.
Nutrient Imbalances in Soil
Gardening is all about balance – too much of one nutrient can prevent plants from absorbing another. More specifically, too much phosphorus in your garden will prevent plants from absorbing iron – even if there is plenty of iron in your soil.
Excessive phosphorus in your soil will kill fungi that the plant needs to absorb iron. Too much phosphorus also prevents plants from producing phytochelates, which help to absorb iron.
When choosing a fertilizer, look at the nutrient content (NPK) and choose one that has a lower phosphorus content (phosphorus is the middle number, represented by P). For more information, check out my article on NPK ratios and what fertilizer numbers mean.
Lack of Iron in Soil
A lack of iron in soil can occur if vegetables are grown without using crop rotation, and if nutrients are never replaced by use of compost and fertilizers.
Over watering can also leach away nutrients from the soil, especially ones that are easily dissolved in water. Iron deficiency is more likely to occur in a growing season after a cool, damp spring.
Any of the fertilizers mentioned in the table above contain iron, and they will act as good supplements for your garden.
Also, remember that certain plants, such as azaleas, rhododendron, and blueberries, are highly susceptible to iron deficiency. For more information, check out this article from the Arizona State University Extension on iron deficiency.
Can Plants Get Too Much Iron?
Yes, plants can get too much iron. This condition is known as iron toxicity, and it can occur when there is too much iron in the soil.
Too much iron in the soil can prevent plants from absorbing other nutrients, even when they are present. You might see iron toxicity in plants if you add too much iron without knowing the cause of chlorosis or other nutrient deficiency symptoms.
The moral of the story is this: always get a soil test before adding any supplements to your soil. Make sure you really do have an iron deficiency before adding supplements!
A soil test will also tell you if your soil is too acidic (low pH) or too basic (high pH), which can help you to decide which supplement to use.
For more information, check out my article on how to do a soil test.
By now, you have a much better idea of which fertilizers (both natural and man-made) have high iron content by weight.
I hope you found this article helpful – if so, please share it with someone else who can use the information. If you have any questions about fertilizers that are high in iron, please leave a comment below.