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Sulphur: Neglect it at your cost

Sulphur levels are falling in our soils due to reduced air pollution. It’s time to start monitoring yours

For years, something that has been a scourge to the rest of society has been a serendipitous benefit to growers across much of Eastern Canada. But with industry and governments combining their efforts to “clear the air,” farmers are faced with a situation that’s slowly becoming an issue: sulphur deficiency in soils.

It’s not that all soils in all regions of Ontario and Quebec are suffering from sulphur deficiencies. But this is another soil nutrient that growers will need to monitor — and maintain — in years to come.

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The U.S. Fertilizer Institute dubs sulphur “the 4th Major Nutrient,” and says it is an essential part of crop production. Yes, it is grouped with magnesium and calcium as a secondary nutrient, but sulphur is needed for the production of amino acids, proteins and oils. It’s also necessary in the formation of chlorophyll, as well as for promoting nodulation in legumes and for helping to activate certain enzymes and vitamins.

According to Dr. John Lauzon from the University of Guelph, sulphur was applied in the form of gypsum in Ontario prior to 1950, as far back as the 1800s. That practice ended when single super phosphate (0-20-0) was developed, containing about 12 per cent sulphur.

By the 1950s, mono-ammonium phosphate (MAP), at 11-52-0 became the leading formulation for phosphate, although it contains no sulphur. However, at that time, research indicated that sulphur wasn’t required due to high rates of deposition via acid rain, a product of pollution caused by high-intensity industrialization across much of the U.S. Midwest as well as southern Ontario.

In fact, southern Ontario became a so-called “hot spot” for deposition, with between 30 and 50 kg/ha (roughly 28 to 47 lbs./ac.) of sulphate, which was more than enough to meet crop demands.

Times have changed

That particular pollution problem began easing in the 1980s, when environmental concerns sparked a campaign to cut emissions. As a result, acid rain and sulphur deposition have decreased dramatically, to only about eight to 10 kg/ha per year (seven to nine lbs./ac.).

And that amount, says Lauzon, is still decreasing.

“This is a good thing environmentally, but it’s still elevated compared to most of North America,” he says. Now, however, studies on alfalfa and canola have seen responses on roughly half of the sites tested. Some alfalfa plots have had very large responses with two to four times the biomass on some cuts. “The main problem in Ontario is that because we did not need sulphur in the past, we never developed any recommendations — other than we didn’t need it. And we never developed a sulphur soil test that works on our soils.”

Sulphur is like any plant nutrient, whether they’re classed as macros or micros, they’re all equally important: you can’t shortchange one without having an impact on crop production or performance. The challenges with sulphur are complex and multi-layered. They can include everything from the form of sulphur to the timing of applications, or from understanding that there are sources other than those applied by humans to the amounts needed. They can even vary from region to region.

“The level (i.e. approx. 10 kg/ha.) is still elevated,” says Lauzon, an assistant professor in the school of environmental science at Guelph. He adds that Western Canada typically gets about three kg/ha (2.7 lbs./ac.) of sulphate (SO4) per year “Further reductions in Ontario are possible, and a side effect is that we’re starting to see a need for S here.”

The same deficiency is evident in other regions of North America. Lauzon says New York state is also seeing similar drops in sulphur — which again, is a positive for the environment — yet adds to the costs for farmers.

Form and timing

The other keys with sulphur are the forms and timing of application, as well as understanding that some of the sulphur needed by plants can come from existing soil organic matter. Peter Johnson, agronomist for Real Agriculture, agrees that there are many different sources of sulphur, yet it’s the form — and the timing of application — that determines its efficacy.

“If you apply elemental sulphur, you can build up your sulphur content in the soil so that you should have enough, most of the time, at least,” says Johnson. He adds that elemental S comes in flakes or pellets, and since it’s neither positively nor negatively charged, it can remain in the soil during winter.

The drawback with elemental sulphur is that it takes time for it to oxidize as a result of microbial activity in the soil. There are several micro-organisms involved in sulphur oxidation, with thiobacillus considered the most important. It’s a bacterium that converts sulphur to sulphate, which is the more readily available form for plant uptake. That process can take three to 12 months depending on soil types and crop rotations, among other factors. Other sources include ammonium sulphate, Sul-Po-Mag (potassium, magnesium and sulphate together — also sold as K-Mag), potassium sulphate and liquid ammonium thiosulphate. There’s also Micro Essentials Sulphur Zinc (MESZ) from Mosaic Company, developed four or five years ago. It contains 12 per cent nitrogen, 40 per cent phosphate, 10 per cent sulphur — divided into five per cent elemental sulphur and five per cent sulphate — and about one per cent zinc.

“The trick, of course, is what are the costs of all of these different forms?” poses Johnson, adding that elemental sulphur is by far the least expensive. “Last spring, we actually shorted the market on Sul-Po-Mag — and you essentially couldn’t afford it. It was cost-prohibitive to use it as your primary sulphur source.”

There were growers who were frustrated by the lack of Sulfa Mag, but as Johnson reiterates, there are many different forms of sulphur, and in affordable amounts. For instance, the price of Sulphur 90 (elemental S) is roughly 25 cents per pound. That’s not an onerous cost, adds Johnson, citing wheat as an example. Early data in 2016 called for 10 pounds of sulphur, and depending on the source, it might cost between $6 and $10 per acre.

“In the plant, sulphur to nitrogen is roughly at a ratio of 10 pounds of N to one pound of S,” says Johnson. “So if you typically spend $100 per acre on nitrogen (and if there was no sulphur in the soil), you’d spend $10 on sulphur, just the way it works in a plant. But, we get nitrogen out of the soil from organic matter, and we also get sulphur out of the atmosphere. We’re greatly reduced but we’re still getting nearly 10 pounds, so for a crop that has a 15- or 20-pound demand, you don’t need to apply that much.”

Johnson echoes Lauzon in stating that all plants need sulphur, but there’s also a difference in timing. Brassicas, including canola, as well as alfalfa are large consumers of sulphur. Red clover, if grown as a crop, also is a high sulphur user.

“We’ve had a number of trials when the red clover did not establish in our zero sulphur strips, but did establish in the strips where we applied sulphur to the wheat crop,” he says. “We weren’t pushing for the red clover but the outcome was that it did establish where there was sulphur.”

Soybeans are also a high protein crop, yet they don’t require large amounts of sulphur, and that directly relates to timing. A soybean undergoes its highest demand for sulphur in August, yet this provides more time for mineralization to occur, releasing more applied sulphur as sulphate. As a result, there’s little sulphur available when the crop needs it.

The 2016 wheat crop in Ontario ran 140 to 150 bu./ac. for some, leaving them low on sulphur, based on demand for a 90- to 100-bushel crop.
photo: Submitted

“A wheat crop doesn’t have that high a demand, but I’ll go out in the fall and broadcast 50 pounds of Sulphur 90 — elemental sulphur —and someone might say, ‘There’s lots of sulphur there now — that wheat crop shouldn’t need it,’” says Johnson. “The problem is that any of that elemental sulphur that gets transformed into sulphate in the fall will leach, so it’s not there in the spring.”

In colder soil conditions, the thiobacillus that’s part of the oxidation process slows in its rate for turning sulphur into sulphate. Under the same conditions, however, wheat grows at its usual rate, thus driving up its sulphur demand. In the end, Johnson states it’s better to be applying sulphur in the spring shortly after the crop comes out of dormancy.

He also believes most farmers understand the need for sulphur in wheat, and although the knowledge base might be a bit weaker on alfalfa’s demand, it is catching on among dairy producers. Yet there is a wrinkle to this familiarity: in 2016, the wheat crop across much of Ontario did so well that Johnson’s standard recommendation of 10 pounds per acre was actually low for the sulphur demand of what would turn out to be such big yields. The standard 10 pounds, he notes, is sufficient for a 90- to 100-bushel crop, but at 140 to 150 bu./ac., that same rate was insufficient.

Soil test for S?

That plays into one of Lauzon’s points on sulphur demand and application, that there are no recommendations from the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) to guide growers.

“Some have not got the information they need, so some may not be applying on crops that are likely to respond, and some are applying on crops that have not yet been found to respond,” says Lauzon. As for a suggestion that it’s time to develop a soil test that includes sulphur, he concedes that no work has been done on such a test. Currently, there are two that are out for consideration. “We’ve put in several proposals to do so, but none have been accepted.”

As well, adds Lauzon, no work has been done in Ontario for a soil test. Other regions of the world have access to a variety of tests, but due to changes in soils and climate, the lab method may or may not relate well to plant uptake, and there hasn’t been any local work to calibrate the test according to Ontario or even eastern Canadian conditions.

It’s similar to the challenges encountered with the N test, except that a sulphur test would be complicated by deposition from the atmosphere. Even if it’s decreasing, there is still a significant amount that’s available from the atmosphere and deposited into the soil. Lauzon adds that the pattern spatially and temporally from the deposition may affect an S test. Yet he still believes it’s worth developing, to see if a soil test can help sort out which fields may show a response and avoid added costs on sites that do not.

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CG Production Editor

Ralph Pearce

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