Sulphur solutions

Monday, September 27, 2021

Industry has made great strides in reducing sulphur emissions, but as depositions in rainfall decrease, how will field crops be impacted?

By Jackie Clark

As agronomists and growers work to dial in the intricacies of nitrogen, phosphorus and potassium fertility, increased attention is focusing on micronutrients like sulphur.

More researchers and producers are hoping to better understand field-crop sulphur requirements, as environmental factors decrease the natural deposition of sulphur onto soil. The nutrient was historically applied in the form of gypsum, a soft solid mineral.

"We used to apply sulphur as gypsum commonly in Ontario back as early as the late 1700s," Dr. John Lauzon, an associate professor in the school of environmental sciences at the University of Guelph, tells Better Farming. He specializes in soil fertility and nutrient management.

"In the late 1800s, we started to use single superphosphate as our phosphorus fertilizer and it contains significant amounts of sulphur, so we continued to apply sulphur up until about the 1950s," he explains. "That's when we first started to use monoammonium phosphate as our phosphorus source. It contains no sulphur. So, from that time until recently, we applied no sulphur here in Ontario."

Farmers in the province didn't need to apply sulphur at that time, "because the level of coal-fired power plants, vehicles on the road and other industry was causing enough emissions that we got more than enough deposition to meet the demand of any crops we were growing," he adds.

Those sulphur emissions led to deposition of sulphur back onto the soil through acid rain.

"In the late '80s, there was a large program between Canada and the U.S. to reduce acid rain," explains Lauzon. The efforts "reduced deposition from somewhere in the order of 30 to 50 kilograms of sulphur per hectare per year, down to where we are now, maybe five to 10 (kilograms)."

Better Farming connects with Lauzon, other researchers and a farmer from Ontario to gauge our current level of knowledge of sulphur fertility in Ontario, and how sulphur requirements may change in the future.

Time to fertilize

Because of the reduced atmospheric deposition, researchers are starting to observe crop response from sulphur fertilization, says Lauzon.

Scientists in the U.S. are also looking into the trend.

"Research in New York on sulphur needs for alfalfa and wheat showed no response to sulphur additions in the mid-1980s," explains Jodi Letham, an extension field crops specialist at Cornell University.

"However, environmental legislation to limit sulphur additions from industry had a major impact on sulphur deposition rates in the last few decades. In New York, deposition is now currently around three kg of sulphate per hectare, compared to back in 1991 when it was around 10 kg/ha," she adds.

More recent research "found a response in alfalfa, which prompted the next questions: Do other field crops require sulphur? Is it limiting or impacting yield?" asks Letham.

Understand sulphur

In the soil, sulphur behaves a bit like nitrogen, Lauzon explains.

"The only type of sulphur plants can take up is sulphate, SO4, and traditionally that came from the atmosphere and through mineralization of organic matter. Now, organic matter mineralization and the lower level of deposition isn't keeping up with the crop demand for some crops."

Sulphate is easily lost through leaching.

"Most of the sulphur in the soil exists in the soil organic matter. Probably 95 per cent or more of the sulphur is tied up in the organic matter, not available (to plants) until mineralization happens," Lauzon adds. "Elemental sulphur needs to be oxidized into sulphate before it can become plant available, and that can take a significant amount of time depending on how warm it is, how moist the soil is, and how finely ground the material happens to be."

However, elemental sulphur tends to be cheap because it's a byproduct of the oil and gas industry, he explains. Because of the unpredictable nature of when elemental sulphur will mineralize, most farmers use ammonium sulphate or potassium sulphate, which are immediately plant available.

"Sulphur is used to help form the essential crude proteins and amino acids in plants," explains Letham.

Field trials

In Ontario, researchers with the University of Guelph and OMAFRA began investigating sulphur fertility in alfalfa and canola in 2012, says Lauzon.

"Alfalfa has a very high uptake of sulphur, as does canola and most brassicas," he explains. "The first year that we did those trials, we got a massive increase in yield with alfalfa from sulphur application."

In subsequent trials, about half of the site-years had a response to sulphur, and when there was a response with alfalfa, it was typically a large response, he adds.

That work led the researchers to pursue field trials in wheat, corn and soybeans at sites across Ontario beginning in 2018. That research is ongoing, but the results and economic analysis from those experiments was published as part of Alex Sanders' master of science thesis. Sanders was one of Lauzon's students.

"Corn and winter wheat displayed significant sulphur-induced yield increases at two of five and three of eight site-years, respectively, with soybean providing no definitive response across 15 experiments. Significant profitable response was realized at one corn and one winter wheat trial," writes Sanders in his thesis.

Gerard Grubb's farm in Bruce County was one of the field sites for this research. Grubb grows cash crops in a corn-soybean-wheat rotation with red clover and some other cover crops, started no-tilling in 2000, and transitioned to strip tillage in 2008.

"We've been using sulphur on our wheat for many years," Grubb tells Better Farming. He became involved with the experimental trials in 2019 to measure if he was getting a yield benefit.

First, the researchers tested replicated wheat trials with a control versus 20 pounds per acre of sulphur in the form of ammonium thiosulphate, he explains. That sulphur improved wheat yield by about seven bushels per acre.

The following year, Grubb tested another corn field that was formerly in organic production. The wheat field was split into treatments with no sulphur, 20 pounds per acre of sulphur, and 20 pounds per acre of sulphur with additional nitrogen.

"Where we put the sulphur versus no sulphur, we got a 10-bushel increase, and where we put sulphur with extra nitrogen, we got a 15-bushel increase," he says.

The researchers also wanted to investigate the impact of sulphur on corn and soybean yields.

In "2019 we did an extensive corn plot," Grubb says. The researchers again tested 20 pounds per acre of sulphur compared to a zero-sulphur control.

The scientists measured no statistically significant difference between corn yields in sulphur and no-sulphur treatments, he explains. They observed a similar lack of results when testing ammonium thiosulphate on soybeans.

However, the results in corn and soybeans may have been impacted by an application of three tons of biosolids before the previous crop, Grubb adds.

"There is elemental sulphur within the biosolids," he says. Because the sulphur in the biosolids is being mineralized, it's difficult to quantify benefit in the experiments conducted on those fields.

Overall, at all Ontario sites, "economic analyses averaged across sites did not show significant profit from sulphur for any of the three crops. Ontario-specific diagnostic tools are needed to predict corn, soybean and winter wheat field sites that will deliver profitable response to sulphur fertilization within a given season," Sanders writes in his thesis.

Lauzon agrees.

"If we don't have a way of targeting the sites that will respond and we had to put sulphur on every field, then it's probably not economical."

Those results indicate that there is potential for sulphur fertilizer to improve wheat and corn yields in Ontario. However, in 21 soybean field sites only one had a positive response to sulphur. Research conducted in New York found similar results.

"Soybean grain yields did not increase with the addition of gypsum or ammonium sulphate," explains Letham. "In addition to that, crude protein and sulphur content of the grain was not impacted by the sulphur addition."

Sulphur in practice

Overall, sulphur fertilization has become an economically viable management activity in certain situations, says Lauzon.

"On wheat, it's a definite no-brainer; you need to put sulphur on it," Grubb says. "On the other crops, it's hard to justify a benefit through exper imental results, but I'm not convinced that we can go without it."

On his farm, Grubb says he hasn't been able to prove a response in corn yield.

"For at least 10 years now, we've still put about 20 pounds in our starter blend," he adds.

Last year, Grubb didn't put sulphur on corn to save costs, except for another larger fertility trial. However, this year, "we've brought it back in because we've priced it out to be a minimal cost," he says. In corn, "maybe one year we really need it and maybe we'd really suffer if we didn't include it."

Applying sulphur fertilizer on wheat is "a little bit more work on the farm since it has to be brought in during the springtime because the cold weather affects it," Grubb says. However, the extra labour is minimal.

"It's a few minutes more to move your hose from one tank to another," he explains. In soybeans, applying sulphur would require an extra pass across the field.

Sulphur could also help with Grubb's cover crops, he says. "We have seen sulphur deficiency in alfalfa, so I would hazard a guess that the residual sulphur could help in the establishment or growth of the red clover," but this theory hasn't been tested experimentally.

Future potential

Researchers in Ontario want to collect additional data from more sites across the province to better understand which fields are likely to respond to sulphur fertilizer, says Lauzon.

"We're looking at developing a soil test method for sulphur here in Ontario," he explains. "There are sulphur soil tests in other jurisdictions, but it's not as simple as just using their results. (The method) has to be tested and calibrated within the environment that you're working in."

Soil type and weather are going to impact sulphur availability and mineralization, as well as potential for loss or addition after the test is taken, he adds. An Ontario-based test would allow farmers to predict which fields have a high probability of yield response with fertilization.

One challenge is that sulphur tests tend to have the same issues with temporal variability that soil nitrate tests have.

"But, what's nice about sulphur is that because it's required in small amounts, I'm thinking we can use the sulphur test as just an on-off switch," explains Lauzon. Instead of trying to dial in a specific rate, farmers can just use the test to determine if they will fertilize or not.

The amount of sulphur deposited in soils is still changing, which also has implications for the future of sulphur fertility for field crops.

The Ontario "deposition level appears to be continuing to go down," says Lauzon. "We could still see more reductions before we bottom out. The problem with that is we're looking at a moving target in terms of response. What we see today may not necessarily be what we see five years from now."

Letham agrees.

In the New York study, scientists looked at partial nutrient balances and found that sulphur fertilization may be required for more crops in the future. Researchers calculate partial nutrient balances by amalgamating estimations of all the inputs of a nutrient to the field.

"We found that soil organic matter mineralization supplied sufficient sulphur to the crop," Letham explains. "However, soil organic matter was not a good quantifier of soil sulphur supply. By doing the partial balances we also found that in future years, sulphur may become limiting for soybeans."

The timeline for when sulphur may become limiting is "going to depend on farming practices, soil conditions and types of soil," she adds.

Environmental concerns

Another unknown related to sulphur is the environmental impact of sulphur fertilization. Researchers from the University of Colorado Boulder identified fertilizer and pesticide applications as the largest source of sulphur in the environment in a paper published in Nature Geoscience in August 2020.

Comprehensive review of the impact of those sulphur inputs on the health of ecosystems and human health hasn't been conducted, says Dr. Eve-Lyn Hinckley, lead author on the paper, in a press release.

"We've gone from widespread atmospheric deposition over remote forests to targeted additions of reactive sulphur to regional croplands. These amounts are much higher than what we saw at the peak of acid rain."

Collaborative monitoring and research on the impact of agricultural sulphur should take place, she adds.

In Ontario, in "soils that have higher pH, sulphate can leach about the same rate as nitrate can … and so it can potentially get into groundwater," Lauzon says. "The other potential loss is to the atmosphere, similar to denitrification. When oxygen becomes depleted in the soil, some forms of soil life can use other electron acceptors the same way they use oxygen. One of the first ones they'll use is nitrate."

This leads to denitrification, which produces nitrous oxide (a greenhouse gas) and nitrogen gas.

"If the system stays wet for a really long period of time, there's a whole host of other electron acceptors that will come into play, but eventually sulphur will as well. When it does, it's lost to the atmosphere as hydrogen sulphide," he explains. Hydrogen sulphide isn't a greenhouse gas, but environmental scientists are still concerned about atmospheric levels of the gas.

"In a farm field, in most conditions that isn't likely to happen, because it would have to be wet for a long period of time," says Lauzon. More often, hydrogen sulphide emissions come from silage and manure pits on the farm, or wetlands and swamps off the farm.

Ongoing research is investigating how farmers in Ontario and beyond can use sulphur fertilizer in an economically viable and environmentally responsible way. BF