Granular Iron for Yellow Leaves on Indoor Plants
Yellowing foliage on your indoor potted plants? An iron deficiency could be the problem. Read on to see what the right granular iron can do for those nasty yellow leaves.
By John G. Stamos
Table of Contents
Introduction to Granular Iron for Yellow Leaves on Indoor Plants
The subject of the development of yellow leaves on garden plants has come up here on The Renaissance Garden Guy before. In my 2022 article, “How to Fix Yellow Rhododendrons (and Azaleas and Pieris, Too)”, I discussed one of the most common reasons for yellowing leaves in acid-loving plants – iron deficiency. It’s surprisingly common in plants that prefer acidic soil, and it usually trumps other potential underlying causes of chlorosis (the scientific term for yellowing leaves) in this type of plant as the most likely culprit. That’s not to say that deficiencies in nutrients like magnesium or calcium, or problematic growing conditions like overwatering (surprisingly, a really common reason for yellowing plant leaves), dehydration, or too little/too much sunlight can’t cause chlorosis in acid-loving plants. They can. But I’ll say it again. When it comes to chlorotic leaves on acid-loving plants, an iron deficiency, more often than not, is the cause.
This article, as you’ve probably guessed by its title, is a little different than my 2022 RGG yellow rhododendron article, which outlined my methodology for fixing yellow leaves on plants growing outdoors in my hardiness zone 5B/6A garden. This one’s about identifying and confirming the cause of, and repairing, yellow leaves on indoor plants. And, as you’ve also correctly inferred, a granular iron product is going to be the rescuer of those yellow leaves. It’s still acid-lovers that we’re talking about, but they’re the ones that grow in pots indoors for at least part of the year. What I’m going to do in this article is briefly explain why acidic soil is so critically important for the health of acid-loving plants (and why sometimes acidic soil alone won’t help cure iron deficiency-induced chlorosis), I’ll outline quickly which types of plants fall into the acid-loving category (in this case, a few outdoor varieties, but mainly those which grow in pots indoors during the colder months), I’ll give you a list of exactly two things to check off in order to be sure that it’s an iron deficiency that’s causing the problem, and I’ll give you the fix and a few great product recs, plus a short bibliography/”For Further Reading” list. Here you go…
Iron Deficiency and Yellow Leaves on Plants: The Iron-Acidic Soil Connection
The importance of iron as a plant nutrient. Iron (chemical symbol Fe) is an absolutely critical plant nutrient.¹⁻⁴ It is a core component of a vast number of vital plant enzymes, and it plays a central role in such biological plant functions as respiration⁵ and photosynthesis.⁶ Iron is assimilated by plants in two basic ionic forms, ferrous (Fe²⁺) ions and ferric (Fe³⁺) ions. Of these two ionic forms, Fe²⁺ is more soluble in soil and more absorbable by plants.¹⁻⁴
Acidic soil and nutrient uptake. Acid-loving plants are defined as those who need substantially acidic soil in order to effectively assimilate nutrients, particularly iron, through their root systems. These plants, on average, require a soil pH range of 4.5 – 6.5, and develop iron deficiencies once soil pH rises above these levels and the soil acidity drops. The solubility of the Fe²⁺ irons in soil decreases in these conditions as does the plant’s ability to absorb it.¹⁻⁴ The more acidic the soil, the more readily the acid-loving plant can assimilate iron and other nutrients.
Iron levels and the great outdoors versus the not so great indoors. Usually, when an outdoor plant is suffering from chlorosis resulting from an iron deficiency, it’s not because there’s not enough iron in the soil. Iron makes up 5.6% of all elements in the Earth’s crust, and is generally amply present in garden soil.⁷ More often than not, if an outdoor acid-lover is suffering from an iron deficiency, it’s not because there’s not enough iron in the dirt, it’s because the soil’s acidity levels are not high enough to allow the plant to assimilate the iron. But with indoor, potted plants, this isn’t always necessarily the case. Sometimes the soil in which a potted plant grows is plenty acidic. If this is the case, and the plant’s leaves are iron-deficient yellow, then the problem is a lack of iron in the soil. This happens more frequently with potted indoor acid-loving plants as the soil in their pots gradually is drained of nutrients and minerals. Year after year, that same soil sits in the plant’s pot and becomes depleted of iron. And a lot of times, fertilizing alone won’t help. Not all indoor plant fertilizers contain iron, and the ones that do generally contain them in secondary, micronutrient form. Sometimes a shot of granular iron for those yellow leaves is just what the doctor ordered – particularly if that plant’s soil is sufficiently acidic. But I’m getting to that…
Acid-lovers: The Short List
Outdoor acid-loving plants. Since we’re concentrating on potted, at least partly indoor plants, I’ll keep the list short. In my hardiness zone 5B/6A garden, evergreen bushes and trees are the main acid-loving group, and it’s the group most susceptible to an iron deficiency. Rhododendrons (including azaleas and pieris), gardenias, and hollies, all of which grow in my garden, are three common groups of outdoor acid-lovers in my hardiness zone.
Indoor acid-loving plants. There’s a substantial list of indoor plants that really appreciate acidic soil. But today, you’re getting another two-item list, because these two are the ones that I grow: tropical hibiscus, and gardenias. These plants are indoor plants (at least they’re part-time indoor plants) by necessity. They require consistently warmer year-round average temperatures than my comparatively chilly zone can provide them. They come inside – pots and all – in the fall, and stay there until at least the middle of the following May. I do grow a gardenia variety that is extremely cold hardy, but the kind I’m referencing in this article won’t survive the winter in any areas colder than zones 8 or 9. Both tropical hibiscus and warm weather gardenias absolutely need acidic soil to assimilate iron and other nutrients.
How to Tell if It's an Iron Deficiency That's Turning Leaves Yellow
The following two conditions are what you need to check in order to determine if it’s an iron deficiency that’s causing the yellow leaves issue on your plants. You can apply this mini list to both your indoor and outdoor plants.
1. Yellow leaves, green veins. A really good way to determine if your acid-lovers, whether indoor or outdoor plants, are suffering from iron deficiency-induced chlorosis is by giving those yellow leaves a careful once-over. Look closely at the yellow leaves and try to determine if their veins are green and it’s just the expanses between the veins that are yellow. If this is the case, than it’s almost guaranteed that an iron deficiency is the problem. Also, those green-veined, yellow leaves usually first appear as new growth at the top or at the tips of the plant. This is because ferrous iron is a cumbersome ion that moves slowly up and throughout the plant’s nutrient delivery system.⁴ Look for these signs in your identification process. The photos below should be helpful.
2. Check soil acidity levels. This is critical. Test your acid-loving, yellow-leafed chum’s soil for acidity. (For a tutorial on testing soil pH, please have a read of my article “How to Lower Soil pH for Acid-loving Plants”). The results are going to take you in one of two directions.
- Non-acidic soil. If your plant’s soil pH reads as neutral or higher, employing granular iron that doesn’t contain an acidifying agent ain’t gonna help those yellow leaves. An acid-loving plant needs its soil to be acidic in order for it to assimilate nutrients – including iron. If the soil’s not acidic, your plant is going to suffer from an iron deficiency, even if there’s plenty of iron in that soil. Plain and simple. Before you do anything else, check your potted plant’s soil pH. If it’s at 7.0 or higher, you’ll need to acidify your plant’s soil. No ifs, ands, buts, or maybes.
- Acidic soil. If you determine that your yellow-leafed, iron-deficient, chlorotic plant is growing in acidic soil (or you’ve corrected the acidity levels per the above), than the reason that it’s iron deficient is a lack of iron in the soil. Remember, sometimes a potted plant’s soil can be depleted of nutrients over time. Unlike the soil in your outdoor garden, which is part of the Earth’s crust and is therefore typically rich in iron, the soil in your plant’s pot won’t have iron in it unless you add some (particularly if it’s been in that pot for years). In my situation, the outdoor plants growing in my garden have access to not only the iron that exists in the soil, but also to a liquid iron foliar and root drench product I administer, and also to the iron-rich well water that supplies my home and that I use to irrigate my garden. When I water my indoor plants over the coldest months of fall, winter, and early spring, however, they receive water that comes from my kitchen faucet, which first passes through a multi-filtered water softening system and ultimately contains little to no iron. In the case of my own yellow-leafed tropical hibiscus and gardenias, the iron deficiency was caused by a lack of iron in the soil. The acidity levels of the soil in the pots of these plants was in the perfectly acceptable 5.5-6.0 range. I was fortunate in that respect because sulfur, which is the basis for all soil acidification processes, is also one of those nutrients that could have easily been depleted while the soil sat in its pot.
The Fix: Granular Iron for Yellow Leaves on Indoor Plants
You got it. It’s granular iron that’s my fix for yellow leaves on indoor plants. “Why granular iron for yellow leaves on indoor plants?” you might ask, particularly if you’ve read “How to fix yellow Leaves on Rhododendrons (and Azaleas, and Pieris, Too).” In that article, I discuss my extensive use of an awesome liquid seaweed product that delivers iron for immediate assimilation by my outdoor acid-lovers via a root and foliar drench. Why don’t I use this product instead of the granular iron product on my indoor potted acid-lovers? Two reasons. The first concerns the fact that my chlorotic outdoor garden plants are chlorotic and iron-deficient because their dirt’s pH drifts into higher than preferable territory. Their soil needs to be acidified. The liquid iron root and foliar drench bypasses the plant’s need for acidic soil and is quickly metabolized by the plant. This product doesn’t make the existing iron in the soil plant-accessible, instead, it immediately delivers iron to the plant when it’s applied, and addresses the yellow leaves issue quickly. But it’s not a permanent fix because it doesn’t amend the soil and open the door for that soil’s iron assimilation by the plant. It’s an excellent supplemental fix in the overall repair of yellow leaves in iron-deficient for outdoor garden dwellers, but the real root cause fix comes in the form of soil acidification. In the case of my own chlorotic, iron-deficient indoor potted acid-lovers, the soil is already at an acceptable acidic pH level. Iron needs to be added to the soil in those pots that sticks around and effects a real change in the soil itself. The granular iron product that I’m recommending does that exact thing. You’ll read a bit more about this momentarily. The second reason I don’t use the liquid root and foliar drench product is a very practical one: If I tried to use it on my potted indoor plants, it would spill all over the floor and surrounding area. The granular, soil amending iron product is the clear choice. It’s a total no-brainer.
So, what’s the granular iron product I use for repairing yellow leaves on my indoor plants? It’s Espoma Organic Iron-tone. Read all about it, and it’s insane benefits, below.
Espoma Organic Iron-tone. regular RGG readers and subscribers will know that I am a huge fan of Espoma feeding and soil amending products. Organic Iron-tone is just one more that I keep in my personal stable. This product possesses an amazing proprietary blend of ingredients that work together to deliver absorbable iron to chlorotic plants by amending the conditions of the soil in which they grow. In my article, “Acidifying Soil in Winter in the Midwest,” I explained in substantial detail the role that soil microorganisms play in the soil acidification process. They oxidize elemental sulfur, converting it to a sulfate form that drops soil pH levels. Here, in today’s article, I explained that the soil in a particular plant’s flower pot, especially if it’s been there for a long time, can be depleted of nutrients. And although soil acidifying microbes can and do persist in soil for a long time, their levels can be depleted in such a situation, as well. So, it’s Espoma Organic iron-tone to the rescue! This product contains almost everything you’ll ever need to deliver much needed iron to your chlorotic acid-loving indoor potted plant: Elemental iron? Yep. At 7%. Elemental sulfur (for providing the basis for soil acidification in the event that your potted plant’s dirt registers a higher than recommended pH level)? You bet – at a whopping 17%. What about the necessary microorganisms for oxidizing that sulfur into soil-acidifying sulfate? You’d better believe it. This product has in its formulation Espoma’s proprietary Bio-Tone component, which contains enormous populations of beneficial microbes, including such sulfur-oxidizing species as Bacillus amyloliquefaciens. This product is basically a one-stop shop for infusing your anemic potted plants with the absorbable iron they need by fixing the soil in which they grow. I absolutely love this product. Click the #advertisement link to learn more, or to order it right here, directly from Amazon.
A note of caution: When using granular iron for yellow leaves as a fix for potted plants, it’s absolutely critical that you follow the manufacturer’s directions exactly. This goes not only for the granular iron product I’m recommending here, but for the soil acidifying product you’ll read about below. Granular soil amending products like these do not “wash away” or “leach off” in containers, as they do in the ground. Overdosing could harm or kill your potted plants.
The three photos/image collages below illustrate the amazing, beneficial effects that the application of Espoma Organic Iron-tone had on my erstwhile chlorotic, iron-deficient tropical hibiscus and gardenias. Note that if your potted plant’s soil is alkaline, the time that Iron-tone will need to take effect will be longer than if the soil is already sufficiently acidic. Still, even in the case where an iron-deficient plant is already growing in acidic soil, the green-up process doesn’t happen overnight. Elemental iron needs to first be reduced to its ferrous (Fe²⁺) state by the presence of water and the action of soil microbes before it can be easily assimilated by the plant.¹⁻⁴ Give it a bit of time.
Note: This excellent granular iron product is not just a repair for yellow leaves on indoor, potted plants. If your particular garden’s soil is iron deficient – which in some cases is entirely possible – implementing the effective use of this product within the parameters of appropriate soil pH levels (Iron-tone’s included elemental sulfur and microbe colonies will help with those pH levels) would be an excellent way to boost absorbable iron levels for your plants. It’s an excellent product which absolutely can be used in outdoor garden soil.
Although this particular granular iron is a great fix for yellow leaves as a stand-alone product, there are three products I’ll recommend below that will help round out this process for you. As mentioned above, you’ll need to have an idea of your potted plant’s soil pH levels when you start the repair process, and you may want to do some additional soil acidification. Below, you’ll find a relatively reasonably priced and surprisingly reliable and accurate soil analysis meter (the one I regularly use and recommend), and another excellent Espoma product to help you out on your potential acidification quest. And when lighting, and not just granular iron for those yellow leaves is what’s needed, I’ll dial you in to my tried and true grow light product.
Luster Leaf Rapitest 4-way Soil Analyzer. I bought this little soil analysis meter in February of 2022 and have used it successfully time and time again since then to test for soil fertility and pH levels. It also measures sunlight and soil moisture levels. It’s fast, easy to use, convenient, and accurate. By using this device, I’ve been able to determine the necessity of appropriate fertilizer applications and pH amending to the soil of a large number of the plants in my garden. Order it here, directly from Amazon, by clicking the #advertisement link. Note that this unit doesn’t test for iron levels in soil. Also note that I really love this little meter!
Espoma Organic Soil Acidifier is the perfect product for lowering the pH and increasing the acidity of your garden’s soil. It’s a fabulous organic product which I use to lower soil pH for plants like my blue hydrangeas and my rhododendrons, azalea, and pieris. It’s very effective and allows acid-loving plants like these to properly access and utilize nutrients from fertilizing products. Order it here, from Amazon, by clicking the #advertisement link.
The Lxyoug Full Spectrum LED Grow Light. This fully adjustable, full spectrum LED grow light is the perfect indoor grow light for all houseplants, succulents, and tropicals. It’s kept all of my indoor plants healthy, happy, and green. And, it’s proven to be THE miracle cure for all of my tropical plants that suffer from relocation shock when they first come indoors in the fall after spending the warm and sunny spring and summer months outdoors. Its flexibility, capability, convenience, and proven effectiveness make this grow light an absolute must-have. It’s an invaluable tool in any indoor plant care arsenal, and it steps up to the plate and delivers when granular iron alone isn’t a sufficient remedy for yellow leaves. Please read my review of this unit in “Help Your Tropical Plants Recover from Relocation Shock.” I simply can’t say enough about this product, and I am pleased to wholeheartedly recommend it. Please click the #advertisement link to learn more, or to order it here, directly from Amazon.
The Granular Iron for Yellow Leaves Wrap-up
Did I lose anybody? Geez, I hope not. This whole thing is really pretty basic. Here’s the rundown:
1) Outdoor acid-loving plants typically have plenty of iron in their dirt to keep them happy and healthy. They just need acidic pH levels in that dirt to get to it. Acidify their soil, fertilize them, and give them a periodic shot of liquid iron leaf and root drench and they’re good to go.
2) Potted indoor acid-lovers could possibly need acidifying done to their soil, and could need actual iron and sulfur added to that soil, as well. This is because soil sitting in pots for a long time can be depleted of nutrients including iron (which makes plants’ leaves healthy and green) and sulfur (which provides the basis for acidifying soil). And soil sitting in pots could even be depleted of the all-important beneficial microbes that keep the soil acidic by oxidizing that sulfur.
3) The granular iron product that I use as a repair for yellow leaves on my potted indoor plants is Espoma Iron-tone. It’s got it all: iron, sulfur, and beneficial microbes. You can use it on your indoor potted acid-lovers to correct their iron deficiency issues, and you can use it for your acid-lovers growing in the ground out in your garden, as well.
Got all that? I hope so. We’ve gotta lose those yellow leaves!
Bibliography/For Further Reading
Below are the cited works which I referenced in writing this article. They’re listed here in link form (and in my typical lazy format), so you can click on any of them to read more. The list of cited works/links is as follows:
- “How plants breathe.” The Royal Horticultural Society, pp. 1-7.
- “Photosynthesis.” Wikipedia, pp. 1-32.
- Nicholas LePan, Bruno Venditti. “Visualizing the abundance of elements in the Earth’s crust.” World Economic Forum, December 14, 2021, pp. 1-16.
- “Iron in Well Water.” Minnesota Department of Health, January 17, 2024, pp. 1-4.
Thanks for sticking this one out. I hope the info here will be helpful to you in your green-up efforts. It’s definitely good to have a plan, as well as the tools to implement it. As always, I appreciate your incredibly kind interest and readership.
Cheers, and Happy Gardening!
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Very interesting and informative article.
Thank you🌸🌺🌸
Thank you for reading it, Roxxy. I’m so glad that you found it interesting. Sometimes this particular problem is hard to figure out and eliminate. This article will hopefully provide good insight. Thanks again, Roxxy.
Very informative article explaining why indoor plant leaves turn yellow. Thanks for all of your product recommendations to solve the problem.
Thank you for reading and commenting, Kevin. I’m glad you found the article informative, and I really appreciate your reading it. I think that the granular iron product is an effective, long-term fix for iron deficiency-related chlorosis. Particularly in the case of a potted plant that’s in nutrient-poor, microbe-poor soil. It’s worked beautifully for my acid-loving potted plants, and I have no reason to believe that it wouldn’t be just as effective for RGG readers who’d like to give it a try. Thanks once again for reading the article, Kevin, and for sharing your thoughts here.
A great deal of important information. especially tips to recognize iron deficiencies on yellow leaves.
Thank you for reading the article, Rick, and for leaving your thoughts here. I tried to provide enough baseline information to readers so that they could identify an iron deficiency as the cause of their respective plants’ chlorosis issues. In that way, they’d at least be pointed in the right direction in terms of a correction of the problem. Thanks once again, Rick. I appreciate your very kind interest.