I’m going to plant some garlic soon.
For that, I figured I’d add sulfur to my garden soil. Everybody says that’s a good idea. And if it says it on the Internet, it has to be true, right?
Yeah, well, there’s a little more nuance to the story. Near as I can tell:
- Sulfur only helps if your soil is truly sulfur-deficient. I.e., you can’t create “super-garlic” by loading your soil with sulfur.
- As a home gardener, you have no good way to tell whether or not your soil is sulfur-deficient.
- So a modest addition of sulfur to your garlic bed is a form of cheap (and mostly harmless) insurance against growing “bland garlic”.
With soil sulfur, as long as you meet the minimums that garlic requires, any excess is wasted. You just have no easy way to know what that minimum is.
Soil sulfur and garlic flavor, an interpretation of the scholarly literature.
Above: The heartbreak of bland garlic.
Garlic is about 0.5% by weight sulfur (reference). Give or take. Allicin is the main chemical that gives garlic pungency when cut or crushed (reference). About 40% of the weight of the allicin in garlic is sulfur (calculated from this reference). So, for sure, you need sulfur to make garlic pungent.
Gardening folklore says that garlic grown in sulfur-deficient soil will be bland. Presumably that’s due to reduced alliin/allicin content. Given the chemical composition of allicin, that makes some sense.
While this is frequently repeated on the internet, it’s hard to demonstrate empirically. Research results appear mixed. Some research shows that garlic allicin content rises strongly with application of sulfur. Some research shows no effect at all.
A plausible explanation offered for these mixed results is that most soil already has adequate sulfur for garlic cultivation, and that additions beyond the level have no effect. Interestingly, the sole study of hydroponically-grown garlic showed a strong positive relationship between sulfur provided to the plant, and allicin content of the finished garlic (contrast Table 1 and Figure 3 of this reference). If I’ve interpreted those findings correctly, then you can’t produce super-garlic by loading your soil with sulfur. But you can produce bland garlic if your soil is truly sulfur deficient.
In this case, then, there’s probably some truth to this bit of gardening folklore. Sulfur soil amendments don’t appear to be a path to growing extra-garlicy garlic. Instead, sulfur soil amendments are more like insurance against a poor crop of garlic.
Fun factoids about garlic and sulfur
Weird fact #1: Once upon a time, rain provided all the sulfur you needed. Eastern U.S. farmers didn’t used to have to worry about having adequate soil sulfur. Acid rain saw to that. Go back a few decades, and rain falling through the ambient air pollution — including sulfur dioxide — deposited more than adequate sulfur annually.
Weird fact #2: Rainfall now removes sulfur from the soil. Plants can absorb sulfur once it forms sulfate, that is, a mineral salt containing sulfur. These sulfates are typically so water-soluble that they wash out of well-watered soils. And now that we’ve cleaned up most of our acid rain problem, rain water leaches sulfur out of the soil, instead of providing it to the soil.
This same phenomenon leads to:
Weird fact #3: Hilltops tend to have sulfur-deficient soil. It gets leached out as rain runs through and out of the hilltop soil. More generally, sulfur levels can vary considerably within a given farm plot, both horizontally (from area to area) and vertically (within the soil profile). All those phenomena are driven by the high water-solubility of sulfates, and so, by the typical flow of water in a field. The upshot of that is that a home gardener, taking a soil sample, and getting a reading on sulfur, may not know much about the level of sulfur in the garden as a whole.
Weird fact #4: Plants can’t use pure sulfur, but you can buy and apply pure sulfur to your soil. Plants can only absorb the sulfur after it’s converted to sulfate, that is, a metallic salt of sulfur. That conversion is done mostly by the action of soil bacteria. In effect, elemental sulfur is slow-release sulfur, with the rate of release being controlled by the rate of bacterial action, which depends on size of the sulfur particles, soil temperature, moisture, and acidity.
Weird fact #5: There’s no easy test for it in your soil. I looked up the for-real tests for sulfur levels in the soil, and it took me right back to college chemistry classes. Use of carefully measured reagents, long periods of agitation, followed by careful (drop-by-drop) titration to get a quantitative estimate, and so on.
Few soil tests available to the home gardener measure sulfur. On Amazon, I found just two options, and both involved mailing a soil sample to a laboratory.For example, this test kit, $30 on Amazon, will show you the nutrient levels — including the sulfur level — in one soil sample. As will this $100 soil test kit. Both are mail-in kits, with the testing provided by some centralized lab.
Here, in Fairfax County, the soil tests provided via Virginia Tech’s extension service do not measure sulfur (per this listing on the Fairfax County website). I don’t know whether that’s because sulfur isn’t an issue in the heavy clay soils of Virginia, or whether it just rarely matters to the home gardener.
In fact, there’s an argument that there’s no one, universal good test for the bio-availability of sulfur in the soil. (That’s my take on this discussion.) It seems plausible that even if you “test your soil for sulfur”, the results will be a poor guide to the amount of sulfur available to plants grown in that soil.
The upshot is that you, as a home gardener, can throw 30 bucks at it, and get a number back, for the amount of sulfur in your soil. Whether or not that number actually tells you anything useful is debatable.
Weird fact #6: Mostly harmless?. Apparently you need little enough of it, for most plants, that it’s often applied pro-actively, without testing, in commercial farming. And it doesn’t lead to (e.g.) algae blooms when it runs off with the storm water, the way nitrogen and phosphorus do.
Weird fact #7: If you add organic matter to your soil on a routine basis (e.g., compost, mulch, etc.), you probably have adequate sulfur for most of the plants you would care to grow. That seemed to be almost regardless of the exact organic matter that you are adding. In any case, that’s the way I interpret the gist of what I’ve been reading.
Bonus fact: The allicin in freshly-cut garlic has such strong antibiotic properties that it earned the nickname “Russian penicillin”, for its use in expedient wound dressings, by the Russian army, in WWII.
My upshot: Gardening via Swedish death cleaning.
When I run all that through the blender, my take on it is that the home gardener is mostly flying blind on this one. You can pay for a one-shot, mail in soil test that will show you the sulfur level in one soil sample. It’s not clear that’s going to tell you what you really need to know about the sulfur in your garden, generally.
Instead, you might add some sulfur to your soil, when growing garlic, just in case. Just in case your soil is so deficient in sulfur that it will reduce the quality of the resulting garlic.
As far as I can tell, it’s pretty hard to over-sulfur you soil, at the levels we’re talking about. For my 8’x4′ bed of garlic, a bumper crop of garlic would take up about 32 grams of sulfur. Replacing that, at 2 grams per cubic centimeter, would require just about one tablespoon of pure powdered sulfur.
My bottom line is that I’m going to rely on Espoma Holly-Tone as the sulfur source for my garlic. (See prior post). That decision is based on the time-tested gardening rule of “I already own a big bag of it.” I can’t buy less than a one-pound bag of elemental sulfur. At the rate I would use it, that’s way more than a lifetime supply of it. I’d end up with yet another bag of gardening stuff, sitting on the shelf. As I am currently in a round of Swedish death cleaning, that’s not the direction I want to take.
So Espoma Holly-Tone it is.
Ancient folklore instructs me to pour an uninterrupted circle of Holly-Tone around the perimeter of the garden bed, moving widdershins. This both ensures a good crop and keeps vampires out. It’s 5% sulfur, and, best guess, a scant three cups (1.5 pound) of it, on a 4’x8′ bed, should provide the 32 grams of sulfur I might plausibly need for complete garlic and vampire insurance.
Photos are from Gencraft.com and Freepik.com AIs.
