Will farmers see a yield bump from sulphur, given that most soils aren’t severely depleted? How tolerant are crops to sulphur fertilizer in the seed-row? How long does it take for plants to use sulphur?
Those were a few of the questions Dr. Jeff Schoenau, University of Saskatchewan soil scientist, set out to answer at CropSphere in Saskatoon in January.
Schoenau and his colleagues have completed two years of a three-year project looking at sulphur fertilization. They ran trials in different soils — grey luvisol soil near Star City, black chernozem near Melfort and at Schoenau’s farm in the brown chernozem zone near Central Butte.
They tested five different sulphur fertilizers on wheat, canola, and yellow peas. Treatments had 20 kg per hectare of sulphur fertilizer. Some treatments also had 20 kg per hectare of monoammomium phosphate (MAP). All treatments and controls had 100 kilograms per hectare of nitrogen.
Soils generally had good phosphorus stores, Schoenau said. “It’s really hard to find a highly sulphur-deficient soil out there these days, folks, because most growers out there have been putting lots of sulphur on for their canola and their rotation.”
Canola yield response varied. Researchers saw significant responses in the grey luvisol soils of Star City both years, especially with the potassium sulphate, Schoenau said. They also saw a significant response to treatments in 2014 at Central Butte, especially with gypsum fertilizer. This may be because since gypsum is only slightly soluble, it wasn’t leached away during the wet year.
The Melfort site wasn’t very responsive either year, which Schoenau attributed to plenty of sulphate available through mineralization.
As farmers might expect, researchers didn’t see a yield response in wheat or peas. In fact, they saw some pea injury with liquid ammonium thiosulphate. “And that’s because we didn’t get very good separation with the liquid ATS band and the seed.”
Researchers also collected soil samples from seed-rows one week, four weeks and eight weeks after seeding. They measured sulphate and phosphate, and took samples to the synchrotron to examine sulphur compounds.
Based on those samples, they found crops took up nearly all the sulphate between days seven and 28. Gypsum treatments had some sulphate left after 28 days, as it’s only slightly soluble. That made it an effective fertilizer for wet soils prone to leaching, Schoenau said.
Researchers also applied elemental sulphur fertilizer, which requires time to oxidize before it’s plant-available. Schoenau said the elemental sulphur wasn’t effective in the short term, but could work if applied well before crop demand.
Elemental sulphur tends to work best when broadcast “and allowed to weather,” Schoenau said. But even broadcasting it in the fall doesn’t give it a lot of time to oxidize, he warned. He suggested soil tests in the spring to see how much sulphate is in the soil.
There was no evidence that the MAP interacted with the sulphate to affect sulphur availability, he added.
The study also examined crop tolerance to seed-row placed sulphur. Researchers used controlled environment chambers at the U of S, with optimum moisture conditions on a loamy textured soil. They looked at several different brassicas, including hybrid canola, open-pollinated canola, high erucic acid rapeseed, polish canola, juncea canola and camelina.
Researchers found that rates exceeding 20 lbs. per acre of ammonium sulphate reduced emergence for most crops. But Schoenau cautioned that Manitoba researchers have seen crop injury with rates as low as 10 lbs. per acre in dry, sandy, high pH soils.
If farmers have MAP in the row, they should cut sulphur rates as well, Schoenau said. That’s because the MAP has a salt effect, Schoenau said.
Ammonium sulphate is mobile, so it’s probably best to band it.
“And I guess, thinking about it, if you’re forced to choose between sulphur and phosphorus in the seed row, I certainly would tend to be going with the phosphorus in the seed row rather than the sulphur.”
Researchers also looked at MES-15, an NPS specialty fertilizer. The napus cultivars could tolerate up to 30 kg of sulphur per hectare as the MES-15 product. Schoenau explained half the sulphur is in the sulphate form, making it plant available. The other half is elemental, insoluble, and has no salt effect. But it does need to oxidize to become plant available.
Brassicas varied in their sensitivity to seed-placed sulphur. Rapeseed was quite tolerant to rates as high as 30 lbs. per acre. Argentine varieties were generally more.
Camelina was “really sensitive to the seed-placed sulphur and phosphorus,” Schoenau said. “Once we got up above 10, we started to see some significant reductions in germination and emergence, especially when we had some phosphorus in there.”