With phosphorus, less might be plenty

A long-term trial shows repeated phosphorus applications don't correlate with higher yield

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You could be over-indulging your crops — and it might be costing you.

New Agriculture and Agri-Food Canada (AAFC) research indicates crops growing in soils consistently treated with applied phosphorus (P) take more than their necessary share.

With a steady abundance of the nutrient, soil microbes that work to make P more available to the plant also have less work to do — which makes them lazy and less effective.

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Why it matters: Opportunities to reduce applied phosphorus without losing yield are both economically and environmentally beneficial.

Tandra Fraser, an AAFC research scientist based in Prince Edward Island, was one of the main researchers involved in the culmination of a 20-year study analyzing how long-term P-management strategies influence bioavailability, and how that availability influences crop response.

Set in Manitoba’s Red River Valley, researchers looked at an organic system with no P inputs, an organic system with a single manure application in 2007, a conventional system and restored prairie grassland. Further research with these systems was also conducted in a greenhouse setting using ryegrass as the plant measure to better control other influencing factors such as the presence of weeds.

Luxury nutrients, lazy bacteria

Results indicated the soil under organic management had lower concentrations of plant-available P compared to the conventional and grassland soils. Greenhouse trials showed the biomass uptake from soils receiving no P was significantly lower for the organic management group.

However, it was also observed that P use efficiency increased in the groups with a lower rate of P applications.

Fraser says that in the conventional system the rate of plant P absorption was higher than the same application of the nutrient, but biomass didn’t increase. This indicates a “luxury uptake,” where the plants absorbed more nutrients than they could use.

“The plants were basically just being greedy. It wasn’t translating to yield at all,” she says.

It was also observed that bacteria in the P-starved systems were doing a lot more work to make what P was present available to the crop. Fraser says bacteria and fungi evolved to find P when they needed it and made up for deficiencies. Enzyme activity was therefore a lot higher in the groups where P is not applied in abundance.

“It’s fairly widespread in bacteria to turn on a gene that produces enzymes that alter phosphorus more effectively when needed… basically things become lazy when they are in an environment with a lot of bioavailable phosphorus.”

Don’t overdo it

Fraser says these revelations indicate less can sometimes be more when it comes to applying P. Limiting inputs can promote natural nutrient processes within the field while saving farmers money and reducing the risk of P loading in waterways — something which is of particular concern for farmers in the Great Lakes region.

Those interested in reducing how much P they apply can try test strips to determine any yield differences, Fraser says. Following 4R nutrient stewardship and precision nutrient application strategies also make a significant difference.

“You don’t always need a blanket application,” she says, adding alternative nutrient amendments in organic and conventional systems — such as recycled waste products — should continue to be investigated.

“I do think there should be more focus on the actual alternatives.”

About the author

Contributor

Matt is a freelance writer based between Essex County and Chatham-Kent. He is interested in all things scientific, as well as rock n' roll, hunting and history. He also works with his parents on their sixth-generation family farm.

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