Acidifying Soil in Winter in the Midwest
Acidifying soil in winter in the Midwest (or anywhere else) doesn’t need to be complicated. Using the right product is way more than half the battle.
Table of Contents
Acidifying Soil in Winter in the Midwest: An Introduction
Greetings, readers and subscribers. I’ve got a good one for you today that I’ve written in reference to a practice I routinely advocate – and a corresponding product I routinely recommend – here on The Renaissance Garden Guy website but that’s created a minor controversy on The Renaissance Garden Guy YouTube Channel. Specifically, my YT video, “How to Acidify Soil for Acid-loving Plants, “ elicited some comments (some of which I accidentally fat-fingered and deleted) that have necessitated my writing of this article. So, to the RGG YT Channel viewers who left those excellent comments, I not only apolgize for losing them, I want to thank you for bringing up a great point and providing the basis for this particular article.
The upshot is this: I use a product that I routinely recommend on this site to acidify the soil in my hardiness zone 5B/6A Michiana Shores garden for plants that need lower pH levels to effectively assimilate nutrients (evergreens like rhododendrons and hollies are just two examples). In the above-referenced RGG YT vid, and in my RGG article of May, 2022, “How to Lower Soil pH for Acid-loving Plants,” I discuss my use of Espoma Organic Soil Acidifier, and the technique I use to apply it to the soil for my acid-lovers. In the video, which was shot in February of 2023, I further reference the fact that I implement some of my soil acidifying activities during the winter months. The comments on the video – I think there were two in particular – which I inadvertently lost, referenced the use of elemental sulfur alone for acidification purposes rather than the Espoma product I use. One of those commentors even mentioned the phenomenon of leaching (when nutrients, elements, and or minerals seep down beyond the reach of a plant’s roots), probably in reference to my own comments in the video about my garden’s sand substrate “leaching acidity” out of the soil. Another comment, which is thankfully intact, brings up an interesting point about wintertime microbial activity (specifically, an alleged lack thereof) in the soil.
In this article, I’m going to address the points that these excellent comments have raised, and offer 1) my rationale behind the use of the terrific Espoma product for acidifying soil in winter here in the Midwest, and 2) my rationale behind my wintertime soil acidifying methodology. But before I do, I’m going to lay some facts on you regarding the use of elemental sulfur alone (or with a component of bentonite clay as a secondary ingredient), the differences between sulfur and sulfates, the importance of sulfur as a plant nutrient, and the impact of microbes on helping with the soil acidification process. Here you go…
Chemistry and Creatures: Elemental Sulfur, Sulfates, and Microbes
Sulfur and Sulfate
Simply put, sulfur, or one of it’s forms, is the stuff that you need to use to acidify soil. At the chemical level, it’s not the actual sulfur itself (chemical symbol S) that performs the acidifying, it’s a sulfur molecule – an oxidized form of sulfur – known as a sulfate (chemical symbol SO₄) that not only does the acidifying (it’s a specific sulfate molecule – sulfuric acid, H₂SO₄, that performs that actual acidifying part), but packages sulfur in a form that plants are able to assimilate and use as a nutrient.¹⁻⁵ Since sulfates are what you want in your soil in order to acidify it (and to act as a fertilizer for your plants, too) why am I even talking about sulfur (S) at all? Because sulfur (S), otherwise known as elemental sulfur, is where sulfates ultimately come from. Below is a heads-up on elemental sulfur and its properties and characteristics, some info on three popular sulfates found in/as soil acidifying agents, and a few facts about the importance of sulfur as a plant nutrient.
- Elemental sulfur. This is the progenitor – the granddaddy of all sulfate molecules. Elemental sulfur (S) becomes a soil acidifying, plant fertilizing sulfate (SO₄) in the presence of water and soil, and through the action of microbes (microorganisms) that live in the soil which, when active, convert elemental sulfur into sulfate through the process of oxidation.⁶⁻⁷ Elemental sulfur must first be oxidized into sulfate form (into sulfuric acid – H₂SO₄ – for soil acidification, and into various other sulfate molecular combinations for fertilization) by the action of microbes in order to have an acidifying effect on soil and a fertilizing effect on plants.⁶⁻⁹ It can take anywhere from a week or two, to several months for the soil acidifying effects of this process (as well as its plant fertilizing effects) to be realized.¹⁻⁵ Some elemental sulfur products, when offered for sale as soil acidifying agents and/or fertilizers, contain bentonite clay in their formulations. Bentonite clay, in particulate form, is highly moisture-absorptive.¹⁰ When an elemental sulfur soil acidifying/fertilizing product containing bentonite clay is applied to soil and subsequently exposed to water, the bentonite clay particles expand and break the elemental sulfur particles down into physically smaller particulate which is more readily oxidized by the microbes living in the soil.² A key feature of elemental sulfur is the fact that it doesn’t move very much, or very quickly, through the soil. In other words, once it’s applied, it will typically remain within a general area, and at a fairly stable depth, without leaching down into deeper levels of soil beyond the reach of a particular plant’s root system. Since elemental sulfur’s conversion to soil acidifying and plant-accessible sulfate is a relatively slow process, and since it doesn’t move very quickly or very deeply through the soil, its effects on soil acidity and fertility are generally cumulative: the more that’s applied over time, the more effective it becomes.²
- Aluminum sulfate (Al₂(SO₄)₃). This compound is used primarily as a soil acidifying agent.¹¹ Unlike elemental sulfur (which must first be converted to sulfate through microbial oxidation action), this compound’s readily available sulfate component acts immediately to acidify soil. There are, however, some drawbacks associated with its use. If not used judiciously, it can burn and kill plants. It also leaches very quickly to soil depths beyond the reach of a particular plant’s root system, and higher amounts of it are needed to effect long lasting changes in soil acidity.⁹
- Iron sulfate (FeSO₄). Iron sulfate¹² is used not only as a soil acidifying agent, it can also be used as a fertilizer to treat plant chlorosis (yellowing foliage). In terms of its soil acidifying properties, it’s got a couple of drawbacks. The first is that it can be expensive. The second drawback, which amplifies the first, is the fact that you need to use a lot of it to drop soil pH. In fact, if you’re going to use iron sulfate as a soil acidifier, plan on buying about eight times as much to achieve the same level of soil acidifying as you’d get with elemental sulfur.⁹
- Calcium sulfate (CaSO₄). Calcium sulfate is commonly known as gypsum. Although this naturally occurring compound has sulfate as one of its primary molecular components, due to its unique reactive properties when it’s dissolved in water or soil, it does not change soil pH at all. So gypsum does not work as an acidifying agent.¹³⁻¹⁴ It does, however, work beautifully as a fertilizer, supplying both sulfur (a really important plant nutrient) and calcium to plants in readily accessible amounts.¹³
- Sulfur as a plant nutrient. Sulfur is hugely important for plant nutrition and overall plant health, growth and development¹⁵⁻¹⁷ – it is one of 17 nutrients defined as essential for plant development, it’s presence is necessary for chlorophyll formation, and it’s considered “The 4th Major Plant Nutrient” right after nitrogen, phosphorous, and potassium (N-P-K).¹⁵⁻¹⁷ But, as I’ve been saying, sulfur, in its elemental form, can’t be assimilated by plants. It needs to be oxidized into sulfate form first. The good news is, there’s a product available that contains immediately accessible sulfate in the form of gypsum. But I’m getting to that…
Microbes and Their Role in Acidifying Soil in Winter in the Midwest (or Anywhere Else It’s Cold)
Here’s the deal: Microbes in soil can and do continue to oxidize elemental sulfur into sulfates (including H₂SO₄) in cold, wintertime temperatures. Microbes such as Acidithiobacillus thiooxidans oxidize elemental sulfur at colder temperatures, albeit at a diminished rate.¹⁸⁻²⁰ They’ll work more quickly at turning elemental sulfur into a useful soil acidifier or fertilizer in warm summertime temperatures, but they’ll still be active in 5B/6A winters like those here in the Midwest. In fact, the freeze and thaw cycles which occur throughout the winter months and impact the workability of soil here in 5B/6A can actually serve to stimulate soil microbe oxidation activities.²⁰ Does there come a time during the absolute coldest depths of a 5B/6A winter that these microbes take a break and go dormant until things warm up? Probably. But I’m reassured that for at least part of the winter, my microbial buddies are working on making my soil more acidic. Since I’m definitely not a chemist or microbiologist, I’m not going to try to explain how all of this works. I’ll leave that to the scientists, and I’ll invite you to do the deciphering for yourselves if your up for it: Feel free to follow the citation numbers (found throughout this article) to my bibliography/reading list at the end of the article if you want to bone up on the heavy-duty science action. But for now, trust me when I tell you that these organisms do acidify soil in winter here in the Midwest. Maybe not in full-on mode, but definitely in some mode. More on this later.
Acidifying Soil in Winter in the Midwest: Why I Do It, What I Use, Why I Use It, and How I Apply It
Why I apply soil acidifier in winter. One of the RGG soil acidifying YouTube video commentors expressed the opinion in her really excellent comments that I shouldn’t engage in wintertime soil acidifying activities due to her belief that the microbes living in the soil aren’t active in the winter months. That position has been widely held among horticulturists, botanists, and microbiologists for a long time, but has now been re-evaluated.¹⁸⁻²⁰ Not only do more recent studies show that sulfur-oxidizing soil microbes can and do remain at least somewhat active in colder temperatures, I’ve got my own soil testing results to prove it. In any number of the locations where I’ve applied Espoma Organic Soil Acidifier in late fall/early winter, I’ve found substantial decreases in soil pH levels from the time of application (very late fall/early winter to midwinter) to the time of soil testing (typically in early spring). But even more importantly, my regimen of this Espoma product’s application has resulted in incredibly healthy, non-chlorotic rhodoendrons, azaleas, and pieris, and extremely blue, blue-flowered bigleaf hydrangeas. My own field results are enough to convince me. I’m going to keep applying this product throughout the year – including during the winter months. So, ultimately, my response to the excellent YT vid comment is as follows: I’ll be acidifying soil in winter here in the Midwest for as long as I continue to garden.
- The cumulative effects of elemental sulfur. Remember, the soil-acidifying (and fertilizing) effects of elemental sulfur, which is the soil-acidifying ingredient in Espoma Organic Soil Acidifier, are cumulative.² As more elemental sulfur is added to the soil, sulfur oxidizing soil microbes will have more sulfur to convert to acidifying (and nutritious) sulfates. Consequently, soil pH should continue to drop, or at the very least, remain at a constant lower level as more elemental sulfur is added to the soil over time. Acidifying soil in winter in the Midwest with elemental sulfur will only add to the stock of sulfur already in the soil, and will remain on tap for microbial oxidation throughout the year. I believe that a regimen of Espoma Organic Soil Acidifier application throughout the entire year, including the winter months, is critical for increasing and maintaining soil acidity levels.
What I use for acidifying my soil in winter here in the Midwest. You already know this. Espoma Organic Soil Acidifier is what I use to acidify my garden’s soil in winter, spring, summer, and/or fall. This product’s active ingredients are elemental sulfur (S) and calcium sulfate, aka gypsum (Ca SO₄). The elemental sulfur provides the product’s soil acidifying action while the gypsum adds calcium and sulfur in readily plant-accesible form as highly effective fertilizer. Remember, gypsum’s unique chemical properties, in spite of its sulfate molecule, render it absolutely ineffectual at changing soil pH.¹³ The sulfate in gypsum will not acidify soil. It’s the elemental sulfur component of Espoma Organic Soil Acidifier that effects the drop in pH and acidifies the soil.
Why I use Espoma Organic Soil Acidifier. There are a few really excellent reasons.
- It’s a trusted product manufactured by a trusted company. Espoma’s been in business selling organic fertilizers and soil amendment products since 1929. They’re a trusted name in the industry, and i’ve had nothing but success in my garden as a result of using their products. Please click here to learn a bit more about Espoma and its fascinating history.
- It contains an excellent complementary fertilizing component. In addition to elemental sulfur, Espoma Organic Soil Acidifier contains gypsum, or calcium sulfate, which is an excellent source of calcium and all-important sulfur for plants.¹³⁻¹⁶ Once the soil of my acid-loving plants is treated with this product, it’s being amended by a poweful acidifying agent while at the same time immediately delivering essential nutrients to the plants. There are other elemental sulfur fertilizing products available, but the sulfur in them would first need to be oxidized to sulfate before their fertilizing effects could be realized. Espoma Organic Soil Acidifier delivers its calcium and sulfate ingredients, ingeniously package in its gypsum component, immediately to treated plants. No waiting involved.
- It works beautifully in proprietary fashion with another excellent Espoma fertilizer, and with an additional complementary, non-Espoma fertilizing product, as well. One of the benefits to using soil amending and fertilizing products from the same manufacturer together is the assurance of proprietary compatibility and mutual enhancement. In my article of 2022, “How to Fix yellow Rhododendrons (and Azaleas and Pieris, Too,” I explained how the combined use of Espoma Organic Soil Acidifier and Espoma Organic Holly-tone (which also contains elemental sulfur as one of its active ingredients) effectively amends soil and ensures the quick and direct delivery of accessible nutrition to treated plants. In that article, I also detailed my use of another excellent product, Ohrstrom’s Maxicrop Liquid Seaweed Plus Iron, which, when used in conjunction with the two Espoma products, ensures healthy, iron-rich foliage for treated plants. I love all of these products. The Espoma Organic Soil Acidifier is an integral component to my acid-loving plant care and feeding regimen. I wouldn’t use any other soil acidifying agent or product for my own plants.
- I am lazy and cheap. I’m not going to kid you. You can buy gigantic bags (usually in 100 or 50-pound sizes) of pure elemental sulfur (or, for more money, elemental sulfur with bentonite clay mixed in) at a not too unreasonable cost. Big bags of 100% elemental sulfur (or with a percentage of bentonite clay mixed in) are reasonably economical, and, since a gigantic sack is filled with 100% of the stuff (or a slightly lower percentage if bentonite clay particles are involved), you could probably acidify an entire cornfield if you decide to buy it this way. But I’m not lugging 100 or 50-pound bags of anything around with any regularity. And I’m not going to spend more money to get the same stuff in a smaller bag. Espoma Organic Soil Acidifier, with its perfect amounts of elemental sulfur for acidifying, and gypsum content for feeding, all in convenient 6-pound bags (or, if I’m particularly motivated, I’ll opt for the 30-pounder) suits my purposes perfectly when I’m acidifying soil in winter (or any other season) here in the Midwest. I believe that, for a reasonably sized residential ornamental or fruit/vegetable garden, Espoma Organic Soil Acidifier offers a practical, effective soil acidification solution, and fits the bill in ideal fashion.
- It works. I can’t argue with success. I’ve had nothing but great results with this product. My acid-loving plants stay healthy, and my blue-flowered bigleaf hydrangeas bloom with huge, really blue flowers. When I check the pH levels of the soil that I treat with Espoma Organic Soil Acidifier, they remain in the acidic range. I use this product successfully and to a totally satisfactory end. I would not recommend this product if I didn’t believe in it.
How I apply Espoma Organic Soil Acidifier. Elemental sulfur isn’t really too mobile in the soil. It tends to stay put during the oxidation process. But once it’s been fully oxidized to sulfate, it moves much more easily through soil. The existing sulfate molecules in the gypsum (calcium sulfate) component of this product are mobile in the soil as soon as the product is applied. Before I apply any product, I’ll clear any wood chip mulch away from the application area. I personally tend to take some liberty with the manufacturer’s recommended application amount, but I do not recommend doing this. Follow all manufacturer directions. I typically will work this product into only the top inch or two of soil because I don’t want the sulfates (oxidized from the elemental sulfur or already present in the gypsum) to leach down too quickly and too deeply away from the roots of the plants they’re intended to benefit. And many of those plants – namely rhododendrons, azaleas, and pieris – are shallow-rooted. When treating the soil of these plants, I’ll only work the Espoma product down into the first inch or two of soil with a hand cultivator. If I’m applying the product as treatment of plants with deep taproots, I’ll work it a bit deeper into the soil (maybe 2 – 4 inches down, max). If I’m applying the product for purposes of soil pH testing, I’ll work it down to a depth equal to the length of the probes of my soil testing meter. Acidifying soil in winter in the Midwest may necessitate some amendments to this methodology. Obviously, if the soil is warm enough to be workable, you can apply the product exactly as I’ve outlined here. If you’ve had a few days in a row of freezing daytime and nighttime temperatures, you may need to break up the top inch or two of soil with a spade before attempting to apply this product. Ideally, if you do this early enough in winter, or in late fall, the soil will still be workable. Once temperatures drop substantially below freezing, and stay that way for a long time, that dirt’s gonna be like concrete. Wait for protracted periods of warmth when the soil becomes workable again before you try to apply this stuff.
How long does it take for Espoma Organic Soil Acidifier to work? Pretty good question. The answer depends on a few factors.
- Temperature and season. Although relevant soil microbes do remain at least partially active during the winter months in the Midwest (albeit at an admittedly diminished level), the rate at which they oxidize the elemental sulfur in this product and convert it to soil-acidifying sulfate slows. The soil-acidifying action of Espoma Organic Soil Acidifier will be faster in warmer temperatures.
- Microbial presence in soil. Obviously, the more sulfur-oxidizing microbes you have in your soil, the faster the conversion of sulfur to sulfate will happen.
- My own observations and stats. In my article of May, 2022, “How to Lower Soil pH for Acid-loving Plants,” I recorded a drop in pH from 7.0 to about 6.8 within 24 hours of applying the Espoma product to a sample of organic garden soil in a pot. After a week, the pH had dropped down to nearly 6.4. This was pretty fast. I’m assuming that the garden soil in this sample contained a decent population of oxidizing microbes. In the soil in my garden, I typically record drops of 1.0-1.5 in pH in one to two months after application, but sometimes it takes as long as 3 or 4 months to get that kind of drop. And in some cases, I’ve seen the soil around some of my planted rhododendrons drop by nearly a full point in a week’s time after application.
Product Recommendations to Assist in Acidifying Soil in Winter in the Midwest
No RGG gardening article would be complete without a few relevant product recommendations. As always, I use and believe in every single product I recommend here. In addition to the obvious Espoma Organic Soil Acidifier rec that you know is coming, I’ve included three other related products here that will assist you in your efforts to provide the perfect soil for your own garden’s inhabitants. Whether you’re acidifying soil in winter in the Midwest, or in summertime in San Juan, these products are the real deal.
I bought this Luster Leaf Rapitest 4-way 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’ve used 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.
Espoma Organic Holly-tone is the almost perfect fertilizer for acid-loving plants and evergreens. It’s an organic formulation with an N-P-K of 4-3-4, a multitude of beneficial microbes, and a respectable amount of sulfur (5%, in elemental sulfur form) on tap for performing its own bit of soil acidifying. Like I said, it’s almost perfect. And when it’s used in conjunction with the following recommended fertilizing product, it is perfect. Click the #advertisement link to learn more, or to order it here, directly from Amazon.
Ohrstrom’s Maxicrop Liquid Seaweed Plus Iron is a remarkable source of iron for acid-loving plants. When used in conjunction with Espoma Soil Acidifier and Espoma Holly-tone, it literally works miracles. It’s got an N-P-K of 0-0-1 and it contains 2% iron. It’s sourced from Ascophyllum nodosum seaweed harvested from the waters along Norway’s coastline. The beneficial effects of this product are noticeable almost immediately after its application. Click the #advertisement link to learn more and to order it here, from Amazon.
Bibliography/For Further Reading
Below are the cited works (some in relaxed format) that aided me in writing this article. Each of them is included here as a link to the actual work, which you can click if you’d like to read it. The list of cited works/links is as follows:
2. “How Long Does It Take for Elemental Sulfur to Break Down?” Keg River Blog, August 9, 2022, p.1.
3. “Sulfate vs. Elemental Sulfur Part 1: There’s a Difference.” Mosaic, p. 1.
4. Jim Ippolito and Troy Bauder. “Changing Soil PH – 0.315.” Colorado State University Extension, p.1.
5. Allen Pyle. “Understanding Sulfur.” Earth Science Blog, p.1.
7. “Microbial oxidation of sulfur.” Wikipedia, p.1.
9. “How to Decrease PH in Soil?,” Atlas Scientific Blog, October 19, 2022, p.1.
10. “Bentonite.” Wikipedia, p.1.
11. “Aluminum sulfate.” Wikipedia, p.1.
12. “Iron (II) sulfate.” Wikipedia, p.1.
14. “Calcium sulfate.” Wikipedia, p.1.
15. “Sulfur – The 4th Major Nutrient.” Nutrient Stewardship, p.1.
My dear readers and subscribers, thank you for hanging around until the bitter end. This one was a bit lengthy, but I felt it was necessary to address the excellent questions my YouTube video prompted. I’m entirely satisfied with my methodology, and with the excellent product I use when acidifying my 5B/6A garden’s soil in winter here in the Midwest. I heartily endorse both my plan and the product. If any of you follow my recommendations in your own gardens, I’d love to hear how things turn out. Leave a comment right here, or submit a “Contact Me” form. I’ll respond to you right away. Thanks once again. As always, I appreciate your readership and your interest.
Cheers, and Happy Gardening!
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Wow – great article. Everything I always wanted to know about sulphur but was too afraid to ask. Thank you so much for this great article.
Blessings
A
Thanks for giving this one a read, Annie. I thought the chemistry and microbiology (at least the parts I could understand) behind this strategy was pretty interesting. And the results, for me, are definitely good. Glad you liked it, Annie, and thanks again!
Amazing article, everything you need to know in perfect detail, and more 🙏❤️🌸
Thank you so much for reading it, Roxxy. I’m glad you found it comprehensible! It’s sort of a tricky subject – the microbiology/chemistry is pretty intense. I’m glad there are so many great resources available for helping to make sense of it, and for helping gardening enthusiasts make good decisions regarding the care of their plants. Thank you again, Roxxy!
The science of soil acidification is very interesting. Thanks for recommending products that you have used in your own garden. The results of your research and hard work are evident when I see how healthy and beautiful your garden is.
Thanks for reading this one, Kevin, and thank you for the kind words. I felt it was imperative to explain my rationale behind not only my decision to apply acidifier at this particular time of year, but also the product I use. For me, it’s an equation that’s been yielding the right answer for several years in a row now. I’m glad you’ve found the article interesting and informative, and I appreciate your excellent thoughts. Many thanks, Kevin!
Another great informative article. You always listen to your readers and respond to our comments.
Thanks for giving it a read, Rick – it’s much appreciated. My apologies for its lengthy nature, but I wanted to try to cover as many bases as possible (while trying to write succinctly and intelligibly – some of the science is tricky if you’re not a chemist or microbiologist) in addressing the really great comments from those RGG YT Channel audience members. I’m glad you enjoyed it, and I hope it reasonably answers those great questions. Thanks once again.