Soybeans do not normally respond to applications of nitrogen (N) fertilizer, as long as they are well nodulated with rhizobia bacteria. 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, adequate rhizobia may not be present for successful nodulation and N fixation. This is usually overcome by inoculating the seed with rhizobia. These inoculations are not always successful, however. Poorly nodulated, N-deficient soybean can result.

In both 2009 and 2010, a number of fields planted into “virgin” soybean ground or into returned Conservation Reserve Program ground in north central Kansas were observed to be poorly nodulated and N-deficient even though the seed was commercially inocu­lated. A field study was conducted in 2009 and continued at a different location in 2010 to determine whether these poorly nodulated, N-deficient soybean would respond to applied N fertilizers, and, if so, how much N could successfully be used.

Methods

The 2009 study was conducted on a farmer’s field near Solomon that had noticeably N-deficient soybean. Soybean variety NK S39-A3 was planted no-till into sorghum residue from the previous year on May 20, 2009, at 140,000 seeds/acre. A liquid inoculant was sprayed on the soybean seeds as they were loaded into the planter. This field had no history of soybean production. Nitrogen fertilizer was applied on July 20, 2009, to soybeans displaying N-deficiency symptoms at the R1 to R2 growth stages. The N was applied as urea co-granulated with a urease inhibitor and nitrification inhibitor (Super-U) by surface banding the material between the soybean rows. Rainfall occurred within a few hours of N application.

The 2010 study was conducted on a farmer’s field near Gypsum that had poorly nodulated, N-deficient soybean. The soybean variety P93Y70 was planted into conven­tional tilled soil at 130,000 seeds/acre on June 19, 2010. Soybean seed was treated with Optimize Inoculant prior to planting. This field also had no history of soybean production. The N was again broadcast-applied as urea co-granulated with a urease inhibitor and nitrification inhibi­tor (Super U) on July 22, 2010. Rainfall did not occur until 14 days after treatments were applied.

Results

The results from both studies for 2009 and 2010 are summarized in the table below. In 2009, response to the highest rate, 120 lb N/acre, was near-linear and highly significant, with a 21 bu/acre advantage over the control.

Yields at Gypsum in 2010 were lower due to dry weather; however, similar results were obtained, with an 11-bu response to the first 120 lb of N/acre compared to the control. No additional response was obtained to the 150-lb rate applied in 2010 compared to 120 lb N/acre in 2009. When averaged across years, the data show a clear linear response to N, with highest yields obtained at 120 lb N/acre.

The data from these studies show that applying N fertilizer to poorly nodulated, N-defi­cient soybean enhances yield. Applying up to 120 lb N/a has been effective in each of the past two years. At current fertilizer and commodity prices these responses would provide a good return on investment, even on the modest yields obtained in 2010. Additional research will be conducted to further refine appropriate N rates if opportu­nities develop in the future.

Effect of nitrogen fertilization on yield of N-deficient soybeans: 2009-2010

 

Solomon 2009

Gypsum 2010

Average

N rate (lb/acre)

Yield (bu/acre)

0

28d

18c

23d

30

37c

23b

30c

60

42b

26b

33cb

90

43b

26b

34b

120

49a

29a

39a

150

N/A

29a

N/A

This study was conducted in cooperation with Tom Maxwell, Central Kansas District Extension Agent, along with Antonio Asebedo and Andrew Tucker, graduate students in agronomy.

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

* Leaf burn. It would be much safer to apply urea than UAN solution.

* 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 Agrotain.

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

* Plant damage during the application process. At this time of year, making a fertilizer application with ground equipment could damage some of the plants. Whether the benefits would outweigh the amount of plant damage is a judgment call.

Irrigated soybeans with high yield potential may respond to N applications, even if they are not N deficient. There was some K-State research several years ago on late-season applications of N to soybeans, conducted by Ray Lamond, former K-State soil fertility specialist, and colleagues. This research was on irrigated soybeans with high yield potential, and the plants were not showing N deficiency at the time of application. Lamond applied 20 and 40 pounds of N 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 N 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 lbs N/acre) of UAN caused severe leaf burn. Lamond concluded that late-season supplemental N at a rate of 20 lbs/acre should be applied to irrigation soybeans with high yield potential at the R3 growth stage.

For complete details of the study at Solomon and Gypsum, see 2010 Kansas Fertilizer Research, K-State publication SRP 1049.