The ratio of nitrogen to sulfur needed to produce maximum canola yields may not be the prescription the doctor ordered.
Rigas Karamanos of Westco Fertilizer in Calgary says that despite “sulfur (soil) tests not always being the most accurate,” a soil test is still better than a ratio when judging how much sulfur is necessary to grow a successful crop.
“Alberta Agriculture tells producers that 7:1 is an ideal ratio,” he said.
“Sask Ag tells us that it is 5:1. In Manitoba their ag department says it is between 8:1 and 5:1. Somebody has to be right. Or does it matter? I think we need to consider what a deficient soil is and decide if our soil is deficient to start with. Then plan our sulfur needs from there.”
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He said ratios have been overinterpreted and vary too much to be useful.
“The application of the ratio has resulted in producers thinking they must add the sulfate in a ratio with their nitrogen despite what a soil test says,” Karamanos said.
Sulfur deficiency in canola results in disease or pest vulnerability and yield loss. It can also lower seed quality because canola requires sulfur for protein and oil production.
Karamanos said test results show that canola responds to increased nitrogen without increased sulfur, beyond a level necessary to grow the crop.
“We see levels of sulfur all over the map in some fields when we grid soil sample,” he said.
“Yet provided the soil is not deficient, we do not see wildly variable yields across the same fields.”
As long as the soil has 20 pounds per acre of sulfur in the top 30 centimetres, Karamanos said, it will likely have more than enough to maximize the crop’s yield potential.
He said that even in the most extreme sulfur deficient soil, adding 36 lb. of sulfate addresses the problem. However, tests have shown that 18 lb. is generally enough for even the most aggressively growing hybrid canola.
“The ratio theory doesn’t hold water any more, especially for the new hybrid varieties, which are drawing on a much larger area of the soil profile,” Karamanos said.
“They appear to be much better scavengers than open pollinated varieties.”
Stewart Brandt of Agriculture Canada’s research farm in Scott, Sask., agreed that hybrids are best able to take advantage of soil nutrients.
“Hybrid vigour seems to be having an effect on these plants’ ability to source their nitrogen, sulfur and any other needs they have,” he said.
“That vigorous growth we see above ground is also happening beneath the surface. But we don’t see as much need to provide a ratio in these crops either.”
Brandt said applying more sulfur than needed to grow a crop is an unnecessary cost.
Sukhdev Malhi, who has studied sulfur’s effects on canola development at Agriculture Canada’s research centre in Melfort, Sask., said ratios are only rough guides.
“You need to begin with a soil test to get a starting point. Then you should add that amount to your ratio, if you want to know. There is no rule. We like to see an amount present that is five or seven to one. But your soil test will tell you if you are deficient in sulfur.”
He said sulfate should be added only if the soil test finds a deficiency.
“If there is no deficiency, then adding sulfur will not yield dividends, even if it is in a ratio with nitrogen.”
