If Tony Vyn could offer advice to soybean farmers, it would be to monitor exchangeable potassium levels in their soil and manage them appropriately.

Vyn, a Purdue University agronomist, discovered that properly applying potassium on fields can improve seed quality and reduce in-field variability of oil content without sacrificing yield.

More than five years ago he began studying the effects of nutrient management on soybean seed quality at the University of Guelph in Ontario with graduate student Xinhua Yin.

“What I've found is that increased potassium levels in soybean leaves and seeds are associated with higher oil content, but somewhat lower protein,” Vyn says.

Furthermore, he says the total oil harvested per acre consistently increases when potassium application increases soybean yield. Total oil yield is more dependent on soybean yield than on differences in oil concentration.

For example, a soybean seed might have 20% oil content, on average. Increasing the potassium level in the seed can boost the oil content to 20.4% or 20.7%, but may increase the oil yield by up to 10%, which is an added advantage.

Vyn found isoflavone concentrations increased significantly — and proportionately more than oil itself — when seed potassium went up. It's possible, Vyn says, that farmers could see financial incentives for raising high-isoflavone and high-oil soybeans.

For now, he is looking at how and when potassium is applied to fields in corn-soybean rotations. And how that affects soybean seed quality. He's found that banding potassium at a 6-in. depth, either before the rotation's preceding corn crop or just before the soybean crop, marginally increases oil content and seed potassium in soybeans.

Early results of banding at 6- to 12-in. depths into the profile for increased yield are encouraging, says Harold Reetz, Midwest director of the Potash & Phosphate Institute and vice president of the Foundation for Agronomic Research. If it's dry late in the season, he says, plant roots will draw nutrients from lower in the soil, so nutrients at the 6- to 12-in. depth should be more available.

Vyn is now looking at whether there may be an additional response to broadcasting more potassium before no-till soybeans when potassium has been band-applied to corn the year before. Reetz says it's a general practice across the Midwest to apply fertilizer before corn. He says it's a matter of labor and economics.

Vyn's research looks at alternate fertility approaches for a better-quality crop and increased profits. Last year he planted two corn hybrids in 30-in. rows, with each hybrid divided into five different treatments: no fertilizer, broadcast phosphorus and potassium, banded phosphorus and potassium at a depth of 6 in. with strip tillage, banded phosphorus alone and banded potassium alone.

This year, the plots have no-till soybeans in 15-in. rows, half with broadcast potassium and half without.

When the plots are ready, Vyn will look at the harvested beans to determine quality and yield.

He's not looking at different varieties or tillage systems in this experiment. “Tillage has little effect on oil and protein content,” he says, “and breeders continue to look at high-yielding variety effects. It's probably fair to say that year-to-year environment and variety are bigger factors in determining oil content than fertilizer management. But it's important to manage fertilizer well.”

Proper nutrient management can help soybean varieties with potentially higher seed quality obtain the genetic advantages gained by breeders' research, and potassium is a key nutrient, says Vyn.