Despite good fertility programs, nitrogen deficiencies showed up in a number of Kansas no-till soybean fields last summer.

With lots of telephone calls about yellow soybeans, Kansas State University agronomist Dale Fjell says weather extremes were the likely culprit.

“Under normal conditions, soybeans don’t need nitrogen because of a symbiotic relationship with rhizobium, a microorganisms already present in the soil,” he says. “The rhizobium fixes nitrogen from the air which can then be used by the soybean plant.”

Despite the presence of nodules on soybean roots (a sign that inoculation has occurred), plants in some ares still suffered from nitrogen deficiency last summer.

“There were probably several things going on,” says Fjell. “Rhizobium is a living organism and like any organism, it will respond to the weather. It was initially too dry in much of Kansas for rhizobium activity. Then we got too much rain which also curbed the process.”

Crop Rotation Changes

With a greater freedom to grow what you like, more soybeans are being no-tilled on land previously seeded to other crops. As a result, there’s little if any buildup of rhizobium to help get the soybean crop started.

For fields where soybeans were turning yellow, Kansas soil scientist Ray Lamond recommended top dressing supplemental nitrogen. Yet many liquid and dry nitrogen sources contain urea which can volatilize when applied under mid-summer weather conditions.

“Ammonium nitrate is actually a better choice for top dressing because it won’t volatilize,” says Lamond. but regardless of what nitrogen source is used…