Small phosphorus applications help wheat yields

Largest yield bump occurred with 17 pounds per acre, followed by a smaller but still significant increase at 36 lb. per acre


A Saskatchewan study has found that soil severely deficient in phosphorus produced better spring wheat yields with even small phosphorus applications, but where and when it was placed weren’t factors.

Erin Karppinen, co-research director at the Irrigation Crop Diversification Centre in Outlook, Sask., said the first year of a two-year study done in 2020 showed that the largest yield bump occurred with 17 pounds per acre of phosphorus, followed by a smaller but still significant increase at 36 lb. per acre.

The rate of increase continued to drop as more was applied, but nevertheless yields improved.

“Rate was the most important factor driving yield and there was really no effect of timing or placement in this particular study,” Karppinen said.

The land used for the study, which was sponsored by the province’s Agriculture Demonstration of Practices and Technologies program and Fertilizer Canada, had just four lb. per acre of phosphorus available in the top six inches of soil.

Soil tests recommended an application of 43 lb. per acre.

The study included 11 treatments, all with side-banded urea at 107 lb. per acre. Monoammonium phosphate was applied at rates of zero, 17, 36 and 54 lb. per acre, either in fall or spring, side-banded or seed-placed.

As the fertilizer increased, there was a corresponding increase in the early season biomass, which makes sense because while phosphorus is required throughout the growing period, it is especially critical at the start.

“We didn’t see those classic phosphorus deficiency symptoms in the zero phosphorus plot, but we also didn’t see any evidence of fertilizer damage in the 60 (kilograms per hectare) plot, which actually slightly exceeded the recommended safe rate of seed-placed phosphorus for cereals,” she said.

Karppinen said the rate of phosphorus drove the yield differences, not the timing or placement of the nitrogen fertilizer.

“Specifically the 20-kilogram (17 lb. per acre) application rate, within that application rate there’s no difference between fall side-band, spring side-band, spring seed-placed,” she said. “That observation really held true with all of the other application rates.”

The average yield increase from zero to 17 lb. per acre was 78 percent, she told the December irrigation conference. Between 17 and 36 lb. per acre, there was a “substantially smaller increase, but it was still significant” and then in the next increment it was 11 percent.

She reminded farmers that the original recommendation was to apply 45 lb. per acre.

“As yield increased, we saw the protein content decreased,” Karppinen added. “This can occur when you’re increasing nutrients such as phosphorus without a corresponding increase in nitrogen.”

The plots that received phosphorus matured a day or two earlier but not enough to change harvest management decisions, she said.

The project concluded that the greatest benefit occurred from the first increment of phosphorus.

“If you already have 15 to 20 lb. of phosphorus in your soil there still will be additional benefits of adding phosphorus fertilizer, it just will not be as substantial as what we’re seeing here in our severely deficient soil,” she said.

Saskatchewan soils generally have low levels of available phosphorus, but consistently not applying enough fertilizer can create a deficit because the inputs are less than the phosphorus exported to the grain.

The current best management practice is to seed-place phosphorus near the developing root.

“However, this may not always be possible depending on someone’s specific on-farm logistics or if the amount of phosphorus you need to seed-place exceeds the recommended safe rate,” she added.

A second study on the same piece of land looked at nitrogen in spring wheat using the 4R nutrient management principles of right source, rate, time and place.

Soil samples collected in the fall of 2019 found seven lb. per acre in the top 24 inches of soil. The recommendation was to apply 142 lb. per acre.

Karppinen said the best practice is to side-band granular nitrogen at seeding, if possible.

The study included two separate trials managed as one project.

The first looked at the right rate, which was established at 134 lb. per acre or slightly lower than the recommendation. The urea was side-banded from zero to 1.7 times the rate.

The second trial looked at the right source, time and placement of the nitrogen.

Sources included conventional urea and three enhanced efficiency products: ESN, or a polymer-coated, slow-release product; Agrotain, which is urea with a volatilization inhibitor; and Super U, which contains both volatilization and nitrification inhibitors.

All were broadcasted or side-banded in either fall or spring.

Karppinen said the yield increased as the rate increased up to .75 of the established rate.

“Then there was no difference between 0.75X to 1.5X,” she said. “We ended up seeing a yield decrease at 1.75X, which is likely due to fertilizer toxicity at that rate.”

Toxicity levels will change depending on the year, she said.

The different sources produced little yield differences at the established rate. The ESN produced a five bu. per acre increase over urea, but there was no difference when using Agrotain or Super U.

“The main thing to note… is that the spring applications yielded higher than the fall applications. There was no difference between the nitrogen that was applied as a broadcast or as a side-band in the spring.”

Karppinen said the optimal rate turned out to be 100 lb./ac. or 0.75X the established rate.

Full results of research done at ICDC are expected to be released later this winter in the annual report. In 2021 the plots will be seeded to canola.

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