The recipe has to be just right when fertilizing your crops

Phosphorus and potassium work well when placed in the seed row — 
but nitrogen and sulphur are mobile

Nitrogen may be the first thing producers think of when they want to bump up their yield — but don’t neglect the other nutrients.

“In order to get the maximum benefit out of all of our fertilizer nutrients, we need to have a balanced supply,” said University of Saskatchewan soil scientist Jeff Schoenau, who drew big crowds for his FarmTech presentations.

“That’s where phosphorus, sulphur, and potassium come in.”

man standing in farm field

“Placement, especially of immobile nutrients, means you need to have it close to where that root is in order for it to be available.” – Jeff Schoenau
photo: File

Every year, Schoenau drives home the point about balanced nutrition to his soil fertility students by conducting a little experiment. Working with brown and grey soils, the students look at how canola responds to nitrogen alone, nitrogen plus phosphorus, and nitrogen plus phosphorus and sulphur.

In the brown soil, the “main limitation” in crop yield is usually phosphorus.

“Nitrogen alone didn’t give us much of a yield response, but with phosphorus present, we got a big yield gain, and a little bit of sulphur added on top of that gave us the highest yield,” said Schoenau.

Phosphorus “wasn’t really an issue” in the grey soil, but sulphur was.

“When we added nitrogen, we got a little bit of a yield gain, and nitrogen plus phosphorus wasn’t much higher than just nitrogen,” he said.

“But where we had the nitrogen, the phosphorus, and the sulphur together, that’s where we got the highest yield. That’s the importance of balanced nutrition for fertility.”

Placement is tricky

So where’s the best place to put that fertilizer? That depends, said Schoenau.

“Optimum placement of these nutrients is very much related to the mobility of the nutrient,” he said.

Phosphorus and potassium are immobile nutrients, while nitrogen and sulphur move about.

“Phosphorus and potassium will only move a few millimetres or centimetres in the soil from where they’re placed,” he said. “Placement, especially of immobile nutrients, means you need to have it close to where that root is in order for it to be available.”

Sulphur is a different story.

“Sulphate will move long distances — literally metres — in the soil to that root,” he said.

“You don’t need all the sulphate there available for uptake right away.”

Producers also need to consider “how much nutrient fertilizer can we safely place in that seed row before we run into injury issues?”

“Nearly all fertilizers are salt, so one of the negative impacts of too much fertilizer in the seed row is that, because of the salt, it holds back water from the germinating seed and seedling,” said Schoenau.

Tolerance varies

Using a controlled environment with “typical Prairie soil,” Schoenau tested the effects of seed-row-added phosphorus, potassium, and sulphur on cereals, oilseeds, and pulses.

Cereals were the most tolerant to the added nutrition in the seed row.

“Cereals, like wheat and barley, can tolerate upwards of 40 pounds of P2O5 (phosphorus) per acre,” he said. “Above that, you start to see some significant drops in emergence. You throw some potash in there, there’s somewhat of a negative effect, but it’s not huge.”

Oilseeds such as canola had moderate tolerance to seed-row-placed fertilizer.

“For canola, around 25 pounds of P2O5 per acre seems to be about the maximum safe rate,” he said. “And when you throw some potash in there, you have to reduce the amount of phosphorus accordingly in order to avoid burn.”

And while some pulses, including pinto beans and chickpeas, were quite tolerant to the added nutrition, peas weren’t.

“Of the crops that we evaluated, pea was one of the most sensitive to high rates of phosphorus placed in the seed row,” said Schoenau. “When you put some potash in there, it significantly reduced the germination and the emergence.”

Test for sulphur

High rates of sulphur can also cause reductions in emergence, he said, so producers should test their soils for residual sulphur before adding any more.

And even then, there’s really no advantage to placing sulphur in the seed row, said Schoenau.

“Because sulphate is mobile, I’d say the best place for ammonium sulphate is not in the seed row but somewhere else — a mid-row band or a side-row band away from the seed,” he said.

“Putting it away from the seed row would be my preference to avoid that potential for injury. If you’re forced to choose between phosphorus or sulphur in the seed row, I’d go with the phosphorus.”

About the author

Reporter

Jennifer Blair is a Red Deer-based reporter with a post-secondary education in professional writing and nearly 10 years of experience in corporate communications, policy development, and journalism. She's spent half of her career telling stories about an industry she loves for an audience she admires--the farmers who work every day to build a better agriculture industry in Alberta.

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