If a soil test reveals a lack of 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, iron chelates (chelated iron), greensand, 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 your budget and what you have available.
In this article, we’ll talk about the various fertilizers you can use to add iron to your soil. We’ll also answer some common questions about iron and its use in the garden.
Let’s get started.
Hey – you can get our weekly gardening newsletter here!
Join 1000+ gardeners to get access to news, tips, and information.
Delivered right to your inbox – once per week.
What Fertilizer Is High In Iron?
The table below includes with some of the best fertilizers with high iron content.
You can find more detail about each type of iron fertilizer after the table.
| Fertiilizer | Percent Iron By Weight |
|---|---|
| Iron Sulfates | 14 to 23 |
| Iron Chelates (Chelated Iron) | 5 to 10 |
| Greensand | 5 to 12 |
| Iron Supplements | 2.5 to 4.5 |
| Blood Meal | 0.2 |
| Compost | Varies |
| Manure | Varies |
iron and their iron content by weight.
Iron Sulfates
Iron sulfate is an excellent source of iron for plants. According to the Utah State University Extension, iron sulfate contains 20.5% iron by weight.
Iron sulfate also contains sulfur, which is another important nutrient for plants.
There are a few different types of iron sulfates, including:
- Ferric sulfate (23% iron by weight)
- Ferrous sulfate (19% iron by weight)
- Ferrous ammonium sulfate (14% iron by weight)
Note that this last type (ferrous ammonium sulfate) will also add nitrogen to your soil.
Remember that adding iron sulfates to your garden will also acidify your soil. In fact, iron sulfates lower pH faster than elemental sulfur.
According to the University of Maryland Extension, iron sulfate is a good choice if the soil has a high pH. The iron sulfate will add iron and also lower soil pH, which will make iron more available.
Iron sulfate is also relatively in expensive. In addition, it lasts a long time (2 to 4 years according to the Utah State University Extension).
However, there are some drawbacks to using iron sulfates:
- Iron sulfates work slowly – This means that the iron may not become available to plants fast enough to prevent a problem with this year’s crops.
- Iron sulfates are expensive – This means it will not be cost efficient to use them in large amounts. 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)
Chelated iron is another good source of iron for plants. According to the Utah State University Extension, chelated iron is formed when an organic molecule bonds with iron, making it available to plants.
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.
Chelated iron boasts two distinct advantages:
- Iron chelates makes iron available to plants quickly – This is especially true when it is used as a foliar spray.
- Iron chelates resist oxidation – when iron oxidizes, it cannot dissolve in water, and it becomes unavailable to plants.
However, there are also some disadvantages to using chelated iron:
- Some chelated iron becomes unavailable to plants when pH rises above 6.0 – they may become almost completely unavailable to plants at a pH of 7.0 or higher (you can learn more in this article from the Michigan State University Extension).
- Chelated iron is short-lived – an application may only last one season, whether it is mixed into soil or sprayed onto leaves.
If you use chelated iron as a soil additive, work it into the soil in spring. Keep it in the plant’s root zone to make it most effective.
If you use chelated iron as a foliar spray, apply it to the leaves. Avoid application when fruit is present or it may cause staining.
Greensand
Greensand is another good source of iron, containing 5% to 12% iron by weight. Greensand has a green appearance due to its glauconite content.
Image courtesy of:
Brocken Inaglory via:
Wikimedia Commons: https://commons.wikimedia.org/wiki/File:Green_sand_
close_up.jpg
Greensand has a few advantages as a soil additive:
- Greensand improves water retention – greensand holds 10 times more moisture than ordinary sand, and it can hold up to 1/3 of its weight in water.
- Greensand improves soil texture – greensand loosens up heavy, dense clay soil and binds loose sandy soil.
- Greensand contains potassium – in addition to its iron content, greensand also contains up to 7% potassium by weight.
You can learn more about greensand in my article here, and you can learn more about fertilizers with high potassium content in this article.
Greensand also has some drawbacks:
- Greensand contains iron in the form of iron oxide – this means that the iron may not be readily available to plants.
- Greensand is expensive – it might only be worthwhile to use it for high-value crops.
If you do decide to use greensand in your garden, buy a 50-pound bag in bulk, rather than several smaller bags. This will save you a few dollars.
Iron Supplements
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
Ironite is a mineral supplement used 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
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
Blood meal is a powder made from the dried blood of animals (often cattle or hogs). Like bone meal, blood 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
Compost is made from kitchen scraps and yard waste, such as:
- banana peels
- orange rinds
- grass clippings
- 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.
Hey – you can get our weekly gardening newsletter here!
Join 1000+ gardeners to get access to news, tips, and information.
Delivered right to your inbox – once per week.
Manure
There are many different types of manure, including:
- cow
- horse
- pig
- chicken
All of these manures have a medium release speed, and they 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?
Adding rusty nails or rusty water to soil does 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.
Rusty nails also put you at risk of tetanus, so avoid them if at all possible!
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 first (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.
Iron Deficiency Caused By High Soil pH
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
- 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 (mentioned earlier), which will add iron to your soil at the same time.
Iron Deficiency Caused By 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.
For more information, check out this article from the Colorado State University Extension on fertilizing your vegetable garden.
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.
Iron Deficiency Caused By A Lack Of Iron In Soil
A lack of iron in soil is more likely if vegetables are grown without using crop rotation. It is also more likely if nutrients are not replaced by using compost and fertilizers.
Over watering can also leach away nutrients from fertilizer, 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 Too Much Iron Kill Plants? (Is Too Much Iron Bad For Plants?)
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.
According to this article from NCBI, symptoms of iron toxicity (too much iron) in plants include:
- Leaf Discoloration (Bronzing)
- Stunted Root System
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.
Conclusion
Now you have a much better idea of which fertilizers (both natural and man-made) have high iron content by weight. You also know the answers to some common questions about the use of iron in the garden.
I hope you found this article helpful – if so, please share it with someone else who can use the information.
To find books, courses, seeds, gardening supplies, and more, check out The Shop at Greenupside!
Hey – you can get our weekly gardening newsletter here!
Join 1000+ gardeners to get access to news, tips, and information.
Delivered right to your inbox – once per week.
If you want to read some of my most popular posts, check out the “Best of GreenUpSide” page here. Enjoy!
~Jonathon