Interested in starting a debate within agricultural circles? Just mention sulphur. “There are lots of opinions on whether it needs to be added to soil now or in the future, where, how much and in what form,” says Keith Reid, soil fertility specialist with Agriculture and Agri-Food Canada (AAFC).
One thing, however, is for certain: plants need significant amounts of the nutrient to grow properly and fend off disease. Canola is the most responsive crop to sulphur, notes Reid, followed by corn, hard winter wheats and alfalfa, with soybeans unlikely to respond to added sulphur in most Ontario soils.
Ever since heavy industry started to boom in southern Ontario and the neighbouring US states, sulphur has been deposited on soils through acid rain. In fact, the practice of adding supplemental sulphur to fields was stopped during the 1850s. Farms in northwestern Ontario which are upwind of the province’s main industrial regions, have never seen as much sulphur from acid rain as their southern counterparts.
However, Reid says that due to manufacturers using new air quality technologies, the passage of legislation restricting how much sulphur can be emitted into the atmosphere from factories, and the recent slowdown in industrial output due to the recession in 2008-2009, the amount of sulphur coming down from the skies has declined considerably. “I would argue that the recession had more impact than anything,” observes Reid.
Adding to lower sulphur soil levels is the fact that fertilizers, insecticides and fungicides are “cleaner” nowadays, and contain less incidental sulphur. There have also been some long-term decreases in cattle, sheep and pig livestock farming in Ontario, which means fewer fields are receiving sulphur through manure applications. Lastly, ever-increasing crop yields mean more sulphur is being removed from the soil than in previous decades.
Need to add sulphur?
Whether sulphur needs to be added to soils is dependent on a few factors. Farmers should first be reminded that soil tests, critical for other major nutrients like N, P and K, are not terribly helpful when it comes to sulphur. “Soil testing for sulphur can result in more confusion than enlightenment,” says Reid. “Sulphur is difficult to get a handle on because it becomes ‘tied up.’ It cycles from soil organic matter into the soil solution and back again.”
In soils with a lot of calcium, it combines with the calcium atoms, goes back into solution and back to binding with calcium again, depending on how much calcium is present, the type of soil, and whether the sulphur is exposed to open air. However, Steve Redmond, a certified crop advisor and new environmental specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA), sees value for cash crop farmers who do not have livestock, and thus have not been applying manure, in testing the soil for sulphur at the same time every year to get a sense of any long-term trend.
Soil pH is not a factor that affects sulphur availability to crops, but soil type can seem to matter in many instances. In trials carried out while Reid was still with OMAFRA, he observed that adding sulphate resulted in a crop response in the “occasional” field, generally those with sandy soils and low organic matter content, but not always. “The results have been inconsistent,” Reid concludes. “We use sulphate because that’s the form which plants can absorb. Elemental sulphur is not available until bacteria in the soil convert it to sulphate, which can take a year or more.”
Redmond, who has worked extensively in the area north of London as a certified crop advisor, has seen cash crop clients get improved yield from using sulphur in corn starter mix on a regular basis. “Generally, this is not the case south of Palmerston, but north of that, especially the canola and corn growers, are using a lot of sulphur and it’s making a difference.”
Redmond took soil samples in the fall of 2010 from five fields in the Lucan area, with four of the five having a history of manure application and all five containing a similar average organic matter level. “Soil test results showed that these four contained 18 pounds of sulphur per acre (with somewhat less available in the spring due to some loss over the winter), but the one that hadn’t had manure in the top six inches had half that,” says Redmond.
Dr. Hugh Earl, an associate professor of oilseeds physiology in the department of plant agriculture at the University of Guelph, recently analyzed data from a fertility trial at Elora. “Of course, we already know that N and S interact in canola, but these data show that with no added sulphur, our nitrogen additions actually reduced yields, while we got a nice positive response to N fertilizer when we added enough ammonium sulphate to provide 20 kg of S per hectare (about 18 lbs per acre),” he says. “At the highest N rate, the S almost doubled the yield.
Earl says that like many growers, he is in the habit of applying ammonium sulphate to canola just as cheap insurance. “We won’t be changing that practice any time soon,” he concludes.
Reid says that overall, with all the sulphur that has been added to soil through acid rain, “the excess isn’t what it was, but we still have adequate sulphur. From everything I’ve learned, we still have more than enough at this point in time, with some occasional fields being the exception.”
He advises those farmers who are concerned to look into it. “Especially if you have low organic matter, sandy soils, or you’re in northwestern Ontario,” Reid says. “Switching from urea to ammonium sulphate, if the nitrogen price stays the same, won’t cost you anything. If you want to know for sure, alternate strips of urea and ammonium sulphate in the field to see if there is any yield difference.”
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