The phosphorus conundrum: low soil levels meet Lake Winnipeg pressures

Experts weigh in on managing low phosphorus levels in soil, while minimizing water health impact

Manitoba is in a difficult position of simultaneously having too much phosphorus and not enough.

Manitoba Agriculture crop nutrition specialist John Heard highlighted this contradiction recently during a recent presentation at a nutrient stewardship workshop, noting phosphorus buildup in the Lake Winnipeg watershed has been a source of long-standing tension between regulators and agriculture.

A significant portion of land in Manitoba reports as phosphorus deficient.

From 2010-15, Heard told the latest cohort of agronomists to pass through Fertilizer Canada’s 4R accreditation, the percentage of soil samples testing below critical levels of phosphorus rose from 57 to 64 per cent.

“The disconnect is that we can have phosphorus in the soil and, if it’s there, it’s not going to be adverse. It’s going to, in fact, be beneficial,” he said.

Producers may be underapplying, driven by negative public attention around phosphorus or industry advice, Heard said, while acres of phosphorus-intensive crops such as canola or soybeans have jumped. Canola accounted for 3.1 million acres in Manitoba in 2016, according to Statistics Canada, while soybeans rose to 1.6 million acres. According to data presented by Heard, canola may remove a pound of phosphorus from the soil for every bushel produced, while soybeans may remove 0.85 pound per bushel.

Still high

At the same time, phosphorus levels in Lake Winnipeg remain high, a fact at least partially pegged to agriculture. According to a 2013 Nutrients in Lake Winnipeg report by Environment and Climate Change Canada, the lake’s North Basin tested at 0.033 milligram of phosphorus per litre, over the 0.025-milligram-per-litre guideline. The Southern Basin and Narrows likewise exceeded water quality standards, testing at 0.108 milligram per litre.

Despite the seeming dichotomy, provincial agri-ecosystems specialist Mitch Timmerman says the two issues are not mutually exclusive, nor do the solutions need to be at total odds with each other.

“On the agronomic side, it’s valid to say that phosphorus doesn’t move very well. That’s why it needs to be, for instance, applied very close to the seed, because the young crop establishing a minimal root system can’t find the phosphorus very easily,” he said.

The issue, according to Timmerman, is when that phosphorus, rather than replenishing soil, runs off and builds up in water systems.

“We need to remind farmers to maintain levels or even build within a reasonable agronomic range, or else they may find that they are in an emergency situation down the line when fertilizer’s more expensive and certain crops, like soybeans, don’t respond so well to fertilizer either. They respond more to what’s in the soil,” Timmerman said. “So all of that paints a picture of phosphorus being a valuable resource… that’s all in the soil environment, and farmers are managing it parts per million. In the water environment, algae are responding to parts per billion to what is also a vital nutrient for life, but algae are a nuisance kind of organism.”

Keeping it

Both Heard and Timmerman pointed to banding, the process of injecting nutrients in bands under the soil as opposed to surface broadcast, to potentially mitigate run-off loss while building nutrient levels in the soil. Heard also pointed to application at seeding for higher yields and minimal phosphorus losses.

In instances where broadcast application is necessary, such as forage fields, Heard urged producers to pick their application time wisely.

“A safer time to fertilize forage fields would be after the first cut, which would be in June or July, and then it’s got all year to react with the soil, so it’s not going to be vulnerable to losses in the fall,” he said.

Timmerman further suggested producers use any on-farm manure before other product, supplement nutrients only as needed, and consider if products such as enhanced efficiency fertilizers may fit into their economic and environmental plans.

“Also take into account that too much phosphorus is applied with manure typically when the application’s based on nitrogen, so then give the field a rest. Let the subsequent crops have a chance to utilize it,” he said.

Heard advocated use of crop rotations, with extra phosphorus added to higher-tolerance crops and separation between phosphorus-intensive crops when building up low-phosphorus soil.

About the author

Reporter

Alexis Stockford is a journalist and photographer with the Manitoba Co-operator. She previously reported with the Morden Times and was news editor of  campus newspaper, The Omega, at Thompson Rivers University in Kamloops, BC. She grew up on a mixed farm near Miami, Man.

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