Canola School: Top-dressing sulphur to rescue yield potential

Sulphur is a critical nutrient for maximizing yield potential in canola, and since peak sulphur uptake for canola continues later in the growing season than peak nitrogen uptake, applying sulphate as late as early flowering can rescue yield that's at risk to being lost to a sulphur deficiency.
Heavy rains can result in canola not accessing the sulphur it needs as it flowers and sets pods, explains Marla Riekman, soil management specialist with Manitoba Agriculture, in this Canola School episode, filmed at the 2024 Manitoba Crop Diagnostic School at Carman, MB.
"We're talking about it here this year, because of the fact we've had a lot of rain," she notes. "And especially if we're dealing with sandier or lighter soils that have the ability for water to move through them a little easier, we're seeing more of that sulphur leaching or sulphate leaching. In those cases, we might be in a condition where if that sulfate has moved away from the root zone, we're starting to maybe see a need for some rescue treatments."
Canola yield can also suffer if there's not enough sulphur relative to nitrogen, which is why it's historically been recommended to fertilizer at a ratio of seven or eight parts nitrogen for every part sulphur, notes Riekman. "You can have huge, beautiful crops that end up with very little yield in those cases."
Leaf cupping and yellowing on new growth are signs of a sulphur deficiency, explains Riekman. In more severe situations, the leaf edges will appear purple and new leaves will appear to clasp upwards toward the stem. Pale flowers are also a symptom as a deficient canola plant moves into flowering.
A rescue top-dress or foliar treatment of 10 to 20 pounds per acre of actual S - either ammonium sulphate (dry) or ammonium thiosulphate (liquid) - can make a significant difference in deficient areas, she says. Leaf burn has the potential to limit yield, especially if there isn't rain to wash the liquid off and down into the root zone, but Riekman says leaf damage will not hurt yield to the same extent as a sulphur deficiency.
As for timing, earlier is better. "You can go as late as bolting, early flowering, but the potential for the benefit of it gets less and less at that point in time," she says.
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