Today’s farmers aren’t responsible for most of the phosphorus that is slowly killing Lake Winnipeg, concludes a draft study examining the issue.
However, heavy phosphorus use in the past is haunting the vast watershed that feeds the rivers that lead into the world’s 10th largest body of fresh water.
“The phosphorus that’s in the system from decades past is still being mobilized and is still loading up Lake Winnipeg,” Hank Venema of the International Institute For Sustainable Development said as he released the study at the recent Keeping Water On The Land conference.
Read Also
Ag In Motion 2025 draws strong attendance, more international visitors
Three clear days meant Ag in Motion 2025 saw strong attendance and a significant increase in international visitors at the large Saskatchewan outdoor farm show.
“Despite the fact that we’re efficient and getting more efficient, there’s still a lot out there.”
In his remarks and in the paper, Venema and his co-researchers repeatedly refer to present-day farmers’ efficient use of fertilizer application rates and methods.
Only two percent of the phosphorus fertilizer that is now applied escapes into waterways, and many parts of Manitoba have declining phosphorus content in their soil as farmers “mine down” surplus amounts.
Yet the nutrient flow to Lake Winnipeg continues as water picks up soil laden with old phosphorus and carries it out of fields. It means farmers can only marginally improve the situation by application and farming methods.
“We can do a little bit on the input side, but probably not a lot, because we’re efficient … but there’s so much in the system from decades past that … the only way to remove them from the system will be by interception,” said Venema.
Interception occurs when water ends up in wetlands rather than rushing through drains off fields and into ditches and streams that quickly lead to lakes.
It’s why preserving wetlands is important, speakers at the conference said.
However, building “distributed storage” is also essential to block the flow of “legacy phosphorus” off the land during major water events, Venema said.
Big water outflows, as happen in floods or major rainstorms, could be held back either by flooding large areas with excess water or using numerous dugouts that could recirculate the water in agricultural operations.
The key is to stop water collecting on the surface of farmland, absorb phosphorus and then pour off through drains into the river system.
Venema said researchers hope water storage can slowly reduce the phosphorus surplus by either using it in agriculture or harvesting marsh plants in which phosphorus is absorbed.
He said there is still room for Manitoba farmers to improve their fertilizer application.
The two percent phosphorus loss that is now occurring seems small, he added, but this small percentage can still add up to a lot of phosphorus because Lake Winnipeg gets its water from such a large watershed of farmland.
However, Venema said Manitoba’s farmers are some of the best when it comes to using fertilizer properly.
“Manitoba Agriculture is actually a very efficient user of fertilizers in total, compared to other jurisdictions in the world, and they’re getting better,” said Venema.
