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Expert concerned about low phos levels

Stu Brandt is concerned about low phosphorus levels in Saskatchewan soils

Stu Brandt is concerned about Saskatchewan phosphorus levels. Growers have been mining phosphorus since breaking the land, and it’s still happening today, he says.

“That’s no longer the most appropriate strategy to be using,” Brandt told delegates at CropSphere in Saskatoon. Brandt is research manager at Northeast Agriculture Research Foundation.

Although there’s typically plenty of phosphorus in the soil, very little is available to plants, Brandt says. “And right now over 80 per cent of the soils in Saskatchewan are testing deficient.”

In 2015, soil tests revealed a median of 14 parts per million (ppm) of phosphorus in Saskatchewan soils. Alberta came in at 21 ppm, and Manitoba at 19, says Brandt. Anything lower than 15 ppm is considered critical for most crops, meaning phosphorus needs to be replenished. Brandt notes that 20 ppm is the critical level for crops such as corn and soybeans. (To figure out what that works out to in lb./acre, Brandt says to double it).

Unavailable phosphorus in the soil converts to available forms each year. But Brandt says it’s typically only enough to support three bushels per acre of wheat. Yield and phosphorus in the soil will come into balance, he says.

“So if we continue to mine this nutrient out of our soils, we can look forward to declining crop yields,” he says.

Brandt says it’s tough to find good data on phosphorus-use efficiency for crops. But the long-term rotations in Swift Current do provide some information.

In 1967 to 1979, those soils had 28 pounds per acre of phosphorus, Brandt says, and phosphorus-use efficiency was 54 per cent. Over the years researchers built up soil phosphorus. Between 1994 and 2005, phosphorus was at 53 lbs./ac., and phosphorus-use efficiency had grown to 88 per cent.

“So what it tells us is that as we deplete this nutrient from our soils, the response that we can expect from fertilizer phosphate is going to go down,” says Brandt. “And the reason is that our crops aren’t the only things in the soil that want phosphate. And as you deplete it, you increase that competition.”

Seed-placed fertilizer is the most efficient way to meet the current crop’s requirements. But there’s a limit to how much phosphate can be seed-placed without injuring the crop.

Building phosphorus in the soil requires different thinking than meeting crop requirements, Brandt says. It’s more of a long-term investment. Les Henry, who was in Brandt’s session, commented that work in Swift Current shows that phosphorus fertilizer is 98 per cent efficient in the long-run.

Previous research shows that a one-time application corrects phosphorus deficiencies more economically than incremental applications. Farmers can band high rates of phosphate for a one-time correction. When it comes to “building soil phosphate levels, I suspect that broadcasting may be every bit as efficient as banding,” says Brandt.

However, incorporating phosphate in a no-till system is a problem. Leaving it on the soil surface risks leaching and environmental issues, Brandt adds.

Other options include:

  •  Side-band rates in excess of removal;
  •  Seed place a low rate of starter, and side-band the rest;
  •  Mid-row band; or,
  •  Apply manure.

Brandt also sees variable-rate technology as a potential tool to correct phosphorus-deficient zones.

The Northeast Agriculture Research Foundation has been measuring the return on applying a high rate of phosphorus once in deficient soils.

When compared to check strips, the high-phosphorus areas did see a yield boost. After two years, the yield bumps in two fields hadn’t yet made up for the extra costs, but they’d more than recovered their costs in a third field.

Asked whether foliar fertilizers are a more efficient way to feed phosphorus to the plant, Brandt was skeptical. The amount of phosphate that can be safely applied wouldn’t offset what the crop removes, Brandt says. It might be a way to correct a phosphorus deficiency in that crop, but he hasn’t seen the data.

“That really becomes a strategy to mine more of the phosphate out of our soils,” says Brandt. “If you’re removing a pound of phosphate, the only way you balance that equation is by applying a pound.”

Something like foliar copper is a different story. “I have no qualms about recommending that on copper-deficient soils,” he says.

About the author

Field Editor

Lisa Guenther

Lisa Guenther is field editor for Grainews based at Livelong, Sask. You can follow her on Twitter @LtoG.

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