Soybeans do not normally respond to applications of nitrogen fertilizer, as long as they are well nodulated with rhizobia bacteria, said Dave Mengel, Kansas State Research and Extension soil fertility specialist. But if soybeans are poorly nodulated, they can become nitrogen deficient. This is most likely to occur when soybeans are planted into ground that has no history of soybean production, or when it’s been a long time since soybeans were last planted, he said.

When that happens, Mengel said producers would like to know if nitrogen-deficient soybeans will respond to applied nitrogen fertilizer. Recent Kansas State University research has shown that it can.

”Data from two recent studies in north central Kansas show that applying nitrogen fertilizer to poorly nodulated, nitrogen-deficient soybean enhances yield. Applying up to 120 pounds of nitrogen per acre has been effective in each of the past two years, increasing yields by 21 bushels per acre in 2009 and 9 bushels per acre in 2010,” Mengel said.

“At current fertilizer and commodity prices these responses would provide a good return on investment, even on the modest yields obtained in 2010,” he added.

For complete details on this research, see 2010 Kansas Fertilizer Research, Kansas State Research and Extension publication, SRP 1049. 

While nitrogen applied to nitrogen-deficient soybeans at the pod development or early pod fill stages of growth can increase yields, Mengel noted there are risks:

* Leaf burn. It would be much safer to apply granular urea than liquid nitrogen solution, unless the liquid fertilizer is applied directly to the soil.

* Volatilization. Urea applied to the soil surface under warm, damp, windy conditions may volatilize if it is not worked into the soil by rainfall. This risk can be minimized by having the urea treated with a urease-inhibitor product such as Agrotain.

* Dry weather after application. If it doesn’t rain after the nitrogen application, the nitrogen may not get down into the soil in time to benefit the plants.

* Plant damage during the application process. Making a fertilizer application with ground equipment could damage some of the plants when they are in the pod development stage. Whether the benefits would outweigh the amount of plant damage is a judgment call.

Irrigated soybeans with high yield potential may respond to nitrogen applications, even if they are not nitrogen deficient, Mengel added.

“There was some research several years ago on late-season applications of nitrogen to soybeans, conducted by Ray Lamond, former Kansas State soil fertility specialist, and colleagues. This research was on irrigated soybeans with high yield potential, and the plants were not showing nitrogen deficiency at the time of application,” Mengel explained.

The agronomists applied 20 and 40 pounds of nitrogen per acre to the beans at the R3 stage, using UAN, ammonium nitrate, urea, and urea + the urease inhibitor NBPT, the active ingredient in Agrotain. The nitrogen increased yields at most locations. The yield increases ranged from about 6 to 10 bushels per acre – or about 5 to 10 percent. The high rate (40 pounds nitrogen per acre) of UAN caused severe leaf burn.

The researchers concluded that late-season supplemental nitrogen at a rate of 20 pounds per acre should be applied to irrigation soybeans with high yield potential at the R3 growth stage, according to Mengel.

Source: Kansas State University