Applying nitrogen to N-stressed soybeans

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Although soybeans can fix good part of their own nitrogen (N) if they are well nodulated, this is no guarantee that soybeans will not suffer from nitrogen deficiency at a crucial time in their development. Soybeans are heavy users of N, removing about 3-4 lbs of N per bushel of seed. They need all the N they can fix plus N from the pool of N available in the soil.

Planting soybean without inoculation into soils where soybean has never been grown can result in very poor nodulation and N deficiency. Similar problems can occur when inoculation fails, or if soybeans are planted on severely acid soils that limit nodulation. In these situations, it is logical to ask if soybeans will respond to N fertilizers.

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

In 2009, this study was conducted on a farmer’s field near Solomon that had noticeably N-deficient soybean. This field was planted no-till into sorghum residue from the previous year on May 20 at 140,000 seeds/a. A liquid inoculant was sprayed on the soybean seeds as they were loaded into the planter. This field had no history of soybean production. Examination of the root system showed few or no nodules present. Nitrogen fertilizer was applied on July 20, 2009, to soybean displaying N-deficiency symptoms at the R1 to R2 (flowering) 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.

This study was repeated in 2010 on a farmer’s field near Gypsum that had very poorly nodulated, N-deficient soybean. In this case, the soybean was planted into conven­tional tilled soil at 130,000 seeds/a on June 19, 2010. Soybean seed was treated with an inoculant prior to planting. This field 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 N rate, 120 lbs/a, was highly significant, with a 21 bu/a 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 lbs of N/a compared to the control. No additional response was obtained to the 150-lb rate applied in 2010. When pooled across years, the data show a response to 120 pounds of N per acre.

The data from these studies show that applying N fertilizer to poorly nodulated, N-defi­cient soybean enhances yield. Applying 120 lbs N/a was effective in each of these two years. At current fertilizer and commodity prices these responses would provide a good return on investment, though limiting the application to 60 pounds of N would have been more economical with the modest yields, and N response obtained in 2010, with current N and soybean prices. Additional research will be conducted to further refine appropriate N rates if opportu­nities develop in the future.

Effect of nitrogen fertilization at R1/R2 stage on yield of N-deficient soybean, 2009-2010

 

Yield (bu/acre)

N Rate (lbs/acre)

2009

2010

2-year average

0

28

18

23

30

37

23

30

60

42

26

33

90

43

26

34

120

49

29

39

150

N/A

29

N/A

LSD (0.05)

4.4

4.0

3.9

This study was conducted in cooperation with Tom Maxwell, Central Kansas District Extension Agent, and Andrew Tucker and A.R. Asebedo, graduate students in Agronomy.

click image to zoomDave Mengel, K-State Research and ExtensionCheck plot, with no N applied, on August 20, 2009. While N applied to N-deficient soybeans at the pod development or early pod fill stages of growth can increase yields (as long as the apparent N deficiency is not simply a response to soil compaction), 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, a urease inhibitor.

* 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. Also under moisture stress, as in the 2010 study, the yield may be limited by more than a lack of N.

* 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.

Dave Mengel, K-State Research and ExtensionPlots receiving 90 lbs/acre N on July 20. If producers are willing to take those risks to get a possible yield increase, how should they proceed?  First look closely at the root system and determine if nodules are present. If no or very few nodules are present 4-6 weeks after planting, and the plants are yellow and N deficient, what rate of N should be used? The best advice would be to apply 40 to 60 lbs N per acre as urea, treated with Agrotain, to N-deficient dryland soybeans, preferably at about the R1 stage, or beginning flowering. Higher rates should be considered for high yielding irrigated soybeans. UAN should only be used if it can be applied directly to the soil surface. UAN applied to the foliage will cause leaf damage. Nitrogen can be applied as late as early pod fill and still be effective, provided rainfall or irrigation occurs soon after application. 

There is no guarantee this will help yields enough to pay off, but beans that are chlorotic, poorly nodulated and stunted due to N deficiency will almost surely have very low yields and N application would be beneficial.

Irrigated soybeans with high yield potential may also 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. 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, early pod development, using UAN and urea + 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.


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