Mind Your Ps and Ks
By Ron Friesen
About a decade ago, when soybeans were being introduced into Manitoba, one of the main selling points was that soybeans were low-input because they fixed their own nitrogen.
Many Manitoba growers took that to mean soybeans didn’t need fertilizer at all.
It seemed too good to be true. And it was.
Although soybeans do fix their own nitrogen, they are still heavy users of phosphorus (P) and potassium (K).
When soybean crops consistently don’t get enough P and K, soils can become depleted of these two essential nutrients. In some cases, high levels of fertilizer applications may be needed to restore soils to productive levels.
To set your soybean crops up for the highest yield potential and to minimize the risk of nutrient depletion, industry officials are urging growers to use a fertility package that includes adequate amounts of P and K. Otherwise, farmers risk losing yields and money, says Michel Poiron, a fertilizer marketing representative with Nutrien.
“I believe (in certain geographies) we’ve hit a point where it’s hurting growers in producing better yields and quality for soybeans,” Poiron says.
It’s hard to say how much yield potential soybean growers are losing from inadequate levels of P and K. But Poiron says without enough of these soil nutrients, growers may get only average yields when they should be seeing more.
Poiron says a rule of thumb is that a soybean crop removes about 0.8 pounds of P and K for every bushel produced. That means a 40-bushel crop removes 32 pounds of P and K from the soil. That’s substantial over time if it’s not replaced.
“Land is expensive, and we can’t afford to lose productivity.”
Growers should be proactive when developing a fertility program for soybeans, Poiron says. He recommends doing so two to three years before actually seeding the crop.
Poiron suggests first applying P and K on other crops in the rotation and then putting the fertilizer down in fall before planting soybeans the following spring. Soybeans respond better to residual fertilizer than applied fertilizer because their taproots act as scavengers which poke around in the soil and bathe themselves in nutrients.
Poiron also encourages growers not to ignore other fertilizers, as well. One of them is sulfur, which rhizobia bacteria require for nitrogen fixation. Sulfur is also important for protein.
The take home message is that soybean growers need to examine fertility options in order to push yields and maximize returns, says Poiron.
“Fertility is the foundation of a good crop. If we don’t have a good foundation, how do we expect to have great results?
“Growers should start considering fertility as the most important decision to make on the farm in conjunction with genetics.”
Growers shouldn’t consider P and K in isolation with soybeans, says Brian Elliot, Manitoba Sales Manager with NorthStar Genetics. Wheat and canola are also heavy users of those nutrients. Therefore, tending to the fertility needs of soybeans also benefits other crops in the rotation.
“It’s very important to apply fertilizer not only for soybeans but for everything in your rotation,” Elliot says. “If we start depleting our phosphate and potash, it’s not only going to affect how your soybeans perform, it will affect how your canola and wheat and everything else in that rotation will perform.”
Elliot says soybean growers are starting to realize the importance of P and K, but the word still needs to get out there.
“We still have some work to do on educating growers on the importance of applying P and K.”