A new approach to phosphorus applications

With variable-rate application at soil-building rates, Scott Keller’s investment in phosphate fertility is paying dividends in higher yields, improved nutrient use efficiency and fewer days to maturity

Scott Keller.
photo: Supplied

Scott Keller didn’t fully appreciate the benefit of phosphorus fertilizer until he ran out a few times at seeding. If the drill tank petered out of prills part-way along a pass, he’d continue to the end of the field before filling up. It was phosphorus after all, not nitrogen. “But these missed strips stood out like a sore thumb,” he says. The delayed crop development was so obvious. “I was floored.”

After observing this a few times, Keller, who farms at New Norway, Alta., took his fertilizing to a new level. His focus is now to build soil phosphorus.

“I had been using 30-32 lbs./ac. of P2O5, or around 60-65 pounds of monoammonium phosphate, across the farm, but this rate was still only three-quarters of crop removal across the whole rotation.” Now he includes phosphorus in his variable rate (VR) fertilizer program, which allows him to “apply phosphorus where the soil is depleted and mine the areas with higher amounts.”

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Phosphorus doesn’t leach like sulphur or nitrogen, but levels can still be surprisingly variable across a field. Doon Pauly, an agronomy research scientist with Alberta Agriculture in Lethbridge, has researched phosphorus periodically for 30 years. For one project, Pauly and a few colleagues did some intensive grid sampling at five locations across Alberta over three years. Results showed that soil test phosphorus levels could range from very low (less than 10 lbs./ac., or five ppm) to very high (over 100 lbs./ac., or 50 ppm) along one strip through a field. Usually, the high rates were at lower slope positions and low rates at upper slope positions.

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This would tend to support Keller’s VR practice, since soils with 100 lbs./ac. probably don’t need any phosphorus fertilizer and could be mined a bit to reduce the runoff risk.

However, when Pauly and his colleagues tested different rates across these strips, the yield differences at the end of the day were minimal. “Over my career, I keep getting sucked in by these phosphorus experiments.” They show promise at the beginning, he says, but in the end leave him with little on which to form a recommendation.

Pauly notes that for these particular trials the fields were all in conservation tillage, so the soils were well managed and fairly healthy. With healthy crops and higher organic matter, the phosphate will be there and the crops will be able to find it eventually. These factors reduce the benefit of VR phosphorus, in his mind.

The difference for Keller is that he’s already set up with VR for nitrogen, so it’s not much of a stretch to add phosphorus. And he knows from his strips without phosphorus that the crop is responding to the fertilizer.

Keller’s rates and placement

Keller kicked off his plan to build soil phosphorus reserves in 2015. Through his VR and zone-sampling program, any areas with less than 20 lbs./ac. (10 ppm) of soil test phosphorus get a rate of 80 lbs./ac. of P2O5 (equal to 160 lbs./ac. or so, of monoammonium phosphate). “My aim is to have everything at 25 ppm on the bicarb test,” he says, working off the notion that yields are better if soils are maintained with a moderate baseline amount of phosphorus.

He says the return on investment didn’t take long. “Yields are up across the board and days to maturity are reduced.”

But Keller emphasizes that the higher rates only make economic sense with applied VR. “I would never do the program I’m in if I had to do it over the whole field,” he says. His overall farm average is 45 lbs./ac. of P2O5 (90 lbs./ac. of MAP).

Placement of phosphorus at rates high enough to build soil reserves or even to match crop removal does require some thought. These rates are often well above the recommended safe seed-placed rate — which, for canola, is 20 lbs./ac. of P2O5 per acre. Keller avoids this risk entirely and puts his phosphorus in a side band with the nitrogen. “Seed safety is an important factor and shouldn’t be ignored,” he says.

While phosphorus fertilizer close to the seed helps young plants access the phosphorus they need early in the season, especially in cold soils, this proximity may not be entirely necessary — especially in fields with a history of adequate phosphorus fertility, which Keller’s have. New research by Jessica Pratchler and Stewart Brandt at the Northeast Agriculture Research Foundation at Scott, Sask., compared side-band and seed-row placement of phosphorus for canola. They found that even at low rates, seed-placed and side-band phosphorus fertilizer showed similar results. At high rates, results for side-banded phosphorus were always equal to or greater than seed-placed. These results are helpful if farmers want to apply a replacement rate, which would be 50 lbs./ac. of P2O5 for a 50 bu./ac. canola crop. The replacement ratio is about one to one.

One risk with side banding all phosphate along with the nitrogen is that the hot band of nitrogen inhibits root growth into the area until the nitrogen dissipates into the soil. That means plants may not have access to this phosphate fertilizer really early in the season. Seeding into warmer soils improves the availability of phosphate reserves in the soil and speeds up plant and root growth, which can help canola plants get by until the fertilizer band has cooled off. Keller applies about two-thirds of his nitrogen as polymer-coated ESN, which is another helpful option to make sure the “band isn’t as hot,” he says.

Crops tax phosphorus reserves

Farmers have perhaps been able to get away with phosphorus fertilizer rates well below crop removal because western Canadian soils, which are relatively young, still have large pools of phosphorus. Plant-available phosphorus accounts for only about 0.1 per cent of phosphorus in the soil, as noted in “4R Management of Phosphorus Fertilizer in the Northern Great Plains: A Review of the Scientific Literature,” a new report co-authored by retired AAFC research scientist Cindy Grant and University of Manitoba soil scientist Don Flaten. As plants take up this available phosphorus, the pools release more into the soil water solution to maintain the balance.

But the situation for many fields now is that decades of lower-than-removal rates are reducing the size of the pools and the available phosphorus is going down. Another factor is that canola, soybeans and corn are big phosphorus users, so as acres of these crops increase, the draw-down of soil reserves is that much faster.

The simple practice of setting rates to match crop removal stops this soil degradation and ensures that available phosphorus is not limiting for yield.

Keller’s experience with a few application misses helped him see the hidden hunger in his crops. “Everyone has increased their nitrogen rates, but we were still putting on Dad’s phosphorus rate,” Keller says. Not any more.

Jay Whetter is communications manager for the Canola Council of Canada. Email him at [email protected].

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Jay Whetter is communications manager for the Canola Council of Canada.

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