As soybeans are introduced and grown more frequently in rotations that already include canola in Western Canada, growers may find their soil bank accounts for phosphorus and potassium running low.
Both canola and soybeans are heavy users/removers of P and K, explains Laryssa Grenkow of Manitoba Pulse and Soybean Growers in this Canola School episode, filmed at CanoLab in Brandon last week.
“Every time we make a withdrawal, we have to replace those nutrients,” she says, noting canola will remove at least a pound (per acre) of phosphorus (P2O5) per bushel of seed produced, while soybeans take up about 1.4 pounds of potassium (K2O) per bushel.
“I think a lot of growers are running into issues in terms of getting that phosphorus and potentially potassium back into the soil,” she continues. “With both canola and soybeans, we’re running into issues with seed row toxicity.”
For canola, the maximum safe rate of phosphorus during seeding is 20 pounds per acre (depending on conditions and spacing), while the max rate for soybeans is even lower — around 10 pounds. That’s nowhere close to how much is being removed with an average yield (the Canadian Prairie average for canola in 2015 was 38 bu/ac, 35 bu/ac for soybeans, according to Statistics Canada.)
So how can a grower replenish their P and K accounts? A separate fall application is one option. Banding fertilizer into the soil is agronomically and environmentally preferred to broadcasting, notes Grenkow.
“Another option is to load up your phosphorus and potassium on other crops where you do have the capacity to put those nutrients down,” she says, suggesting growers check out this (click to download Excel spreadsheet) rotational phosphorus removal rate calculator built by Don Flaten of the University of Manitoba and John Heard of Manitoba Agriculture. Extra P or K can be applied when seeding wheat, for example, to rebuild for future canola or soybean years.
“Soil testing is a very important step to establish where you are and to monitor your trends over time,” stresses Grenkow.
For phosphorus, 10 to 20ppm Olsen is considered a “maintenance” level. Growers who are below 10ppm should find ways to build phosphorus reserves. While farmers in most of the soybean growing region of the eastern prairies have traditionally not applied much potassium due to naturally high levels, Grenkow says growers should start paying attention when K soil test results drop below 150ppm.