Canola School: Closing the hole in the phosphorus cycle

Don Flaten holding a sample of struvite fertilizer.

Phosphorus is a critical nutrient in farming, but it has also received a lot of negative attention for the impact it can have on water quality.

If we consider the path of a single phosphorus molecule, it probably originates in a rock formation in the U.S. or North Africa. From there it becomes fertilizer and nutrition for a crop. Once in the soil, it can be taken up by plants, and there’s a good chance it will be recycled (in crop residue decomposition or in canola meal fed to livestock and returned to the field in manure).

The phosphorus continues to be cycled on the farm, until it leaves to be consumed by humans — that’s where there’s a gaping hole in the phosphorus cycle.

A 50 bu/ac canola crop requires approximately 80 lbs of phosphorous. Around 50 lbs are removed in the seed, with 30 lbs returning to the soil in the remaining plant residue.
“Where the big disconnect occurs is after people consume canola and other agricultural products, because 90 percent of phosphorus that’s consumed by people is not recycled back into the food system, and that creates a big challenge in terms of sustainability and environmental problems,” explains Don Flaten, soil scientist at the University of Manitoba, in this Canola School episode, filmed at the Manitoba Canola Growers’ Community Summit at Kelburn Farm south of Winnipeg.

Instead of being recycled to grow more food, much of the phosphorus that leaves the farm in food ends up where it’s of no value and is often detrimental to the environment — in landfills, sewage systems, and waterways.

Composting leftover food is only slightly better than throwing it out, because the concentration of phosphorus from household waste in a small yard will quickly build to too-high levels, says Flaten. “We have to have systems that take that composted organic material and get the nutrient back to where the food was produced.”

Researchers are looking at how phosphorus fertilizers extracted from consumer waste compare with traditional commercial fertilizers.

At the same time, there’s only a finite, non-renewable supply of rock phosphorus in the world, he notes.

So what does this mean for the production of canola and other phosphorus-loving crops?

Slowing this leak in the overall phosphorus cycle requires recovering these nutrients after they’re consumed in food, opening the possibility of more non-traditional phosphorus sources like struvite and biosolids. Cities like Saskatoon, Edmonton and Portage la Prairie are investing in technology to extract phosphorus from their sewage systems.

(One of the obstacles to biosolid extraction for some cities and towns is that human waste has to meet the same regulations that apply to livestock manure — for example, storage requirements and winter spreading bans.)

“We’re working to evaluate these fertilizer products to make sure they’re at least as good as regular commercial fertilizers, and that’s what we’ve found,” says Flaten. “Everybody’s working together, but we still have a ways to go.”

Dig into the Canola School video library here.

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