NPK is gospel when it comes to corn fertility, but when raising soybeans, farmers tend to leave off a couple of those letters, according to Ross Bender, senior agronomist with The Mosaic Company. In fact, recent surveys indicate as many as 80% of farmers do not fertilize phosphorus and potassium on their soybeans.
“In a lot of cases, they’ll fertilize P and K the fall before they plant their corn crop, but save a fertilizer pass and let the soybean crop scavenge whatever’s left over the following year,” he says.
Bender says with higher-yielding germplasm on the market today, the fundamentals of nutrient management have changed for the soybean crop. His concern in particular is phosphorus – because in a corn-soybean rotation, 80% of P goes to the grain, with the stover holding the last 20%. That’s just not a lot of residual P left over to scavenge, he says.
On the other hand, it can be easy to miss a P deficiency altogether, notes Ron Gelderman, South Dakota Extension soils specialist at his state’s 2013 Northeast Farm Tour.
“One thing about soybean phosphorous deficiency, it’s pretty difficult to see,” he says. “The soybeans may be a little less green, but if you’ve got a whole field that’s deficient, you really can’t pick it out that easily.”
Bender recommends a three-step approach to better understanding soybean fertility in your fields.
First, a soil test is a great way to get started – even if it’s only every three or four years, he says. Second, take tissue tests in-season. Finally, ground truth the results.
“Does that data make sense, or does something look off?” Bender says. “That’s the importance of verifying the results.”
Timing may be more critical than rate in some instances, he adds. Higher application rates aren’t always the answer, either, he says.
“Depending on a farmer’s nutrient management program, we may not recommend higher application rates,” Bender says. “Instead, splitting your applications for each crop will save a little for the soybeans.”
University of Minnesota trials indicated that when soil tests show low or very low phosphorous levels, a moderate yield bump is possible. In these trials, the control checks netted yields of 23.0 bu. per acre. By adding 23 lb. of P205, yield jumped to 37.0 bu. per acre. And when that rate was doubled to 46 lb., yields climbed an additional 1.5 bu. per acre.
It’s worth investigating if farmers can capture extra soybean yields through fertility management changes, Bender says.
“When prices are high, yield is a luxury,” he says. “When prices are low, yield is a necessity. Farmers need every single possible bushel.”