BRANDON – The 38 million acre Assiniboine River watershed puts 2.6 million pounds of phosphorus into the Manitoba river every year, which pencils out to less than .1 pound per acre.
Is that really significant?
“It is less than a half percent of the total amount of phosphate removed by crops each year,” said Don Flaten, a soil scientist at the University of Manitoba and chair of the National Centre for Livestock and the Environment.
“The numbers mean that nearly all the phosphate exported from this land is in the form of crop taken from the field and sent to the city for human consumption. Some people like to think our farmland is just gushing phosphorus into the waterways, but that simply is not true. Nearly all our phosphorus leaves the farm in the form of food exports to city people.”
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Following a presentation at a zero till workshop in Brandon, Flaten said farmers in the Assiniboine watershed typically export about 20 lb. of phosphorus per acre each year in the form of crops, which means they need to put 20 lb. back into the soil to make their soil account balance out.
“When a farmer applies phosphate from synthetic fertilizers or livestock manure, he is replacing the phosphorus he sent to the city folks who consume our products. Now, if we want to get philosophical about it, what happens to all that phosphorus after it leaves our farms? In most cases, it’s never recovered or recycled. Phosphorus that goes into the human food chain ultimately goes into the sewage systems of the cities and towns.”
Flaten said some cities on the northern Great Plains have made significant advances in phosphorus recovery systems, while others, such as Winnipeg, lag behind.
Flaten joked that city people could keep phosphorus out of the province’s waterways by not eating food that contain the element.
“That might solve one problem, but it would be very unhealthy for those folks. We all require phosphorus to live, just like our crops and livestock require phosphorus to live. We can’t change that. But when you compare the city to the farm, we’re doing a much better job on the farm of recycling phosphorus with our crop and livestock than the city is with the human food systems. In the prairie provinces, our farms are remarkably efficient in retaining phosphorus and preventing it from getting into the waterways.”
He attributed this to a level landscape and minor soil erosions caused by rain. As well, spring runoff generally occurs on frozen soil, which lessens erosion.
Flaten said prairie farmers have learned to do a good job keeping water on the farm because they need it to grow crops and livestock.
Flaten said looking at the larger prairie drainage basin as a whole, Lake Winnipeg watershed gathers water from one million sq. kilometres.
“Of all the great lakes on earth, Lake Winnipeg is the tenth largest. But it has the highest ratio of watershed area to water area of any of the world’s great lakes.
“It’s a very large area of farmland losing just a little bit of water and a little bit of phosphorus per acre. But the total number of acres makes the total phosphorus volume quite high.”
“As a result, any small amount of water that does escape can have a high concentration of phosphorus. So we can still end up with high concentrations from farmland, high enough to cause water quality problems.
“It may seem like a small amount (of phosphorus) in terms of the absolute mass, but that’s not an excuse for us to do nothing. Farmers, along with everyone else, need to reduce the amount of phosphorus going into the waterways.”
For more information, contact Don Flaten at Don_Flaten@umanitoba.ca.
