It has been debated for many years, the need for supplemental nitrogen (N) on soybeans. Things have changed recently in soybean management to the point that some additional nitrogen can be beneficial to yields. However without good soil fertility, including macro, secondary macro and micro nutrients, added N won’t make much difference.
Nitrogen management starts with making sure your soybeans nodulate properly; free N is always the best N. Most southern Illinois soils have enough residual rhizobia bacteria that a commercial inoculant may not be needed. However, today’s commercial rhizobia are much more effective than natural strains that reside in the soil. They can provide more viable nodules, but they come with more management needs. Rhizobia are living organisms that die very quickly in hot conditions, like a seed tender left in the sun. Talc, seed box micro nutrients (especially molybdenum) and some in-furrow row starters can kill rhizobia in seconds. Consider those factors when making your management plans.
Next, adding too much supplemental nitrogen early in the season will make the beans “lazy” nodulaters and inhibit proper node production. This can happen even if you use a commercial inoculant, since bean plants want some nitrogen early before fixation kicks in. A start of up to 50 lbs. per acre of N is ok. So applications of DAP (diammonium phosphate) or MAP (monoammonium phosphate) will provide both phosphate and some nitrogen. Just be careful if you are following a corn crop not to overdo it if there is any residual N from the previous crop.
As I see it, the best time for a real dose of nitrogen on the soybeans is at R2 – R3. There are a couple of ways to accomplish this. First, you can look at an application of 60 – 70 lbs. of N as urea broadcast over the beans. This application should be made no later than R3 to give the product time to be absorbed by the roots as the plant is in full pod fill mode. The urea should also be treated with a urease inhibitor like NutriSphere-N®, N-Fixx® or Agrotain® to prevent N loss. The second option is a slow release foliar nitrogen product like CoRoN®, SRN 25™ or Flex-N that will not burn the foliage.
We have had good luck with both options, but I like to run a tissue test just before R1 to see if we need to add any other micros to alleviate any other stresses in the bean crop. If the plant is hurting for Fe, Mn or Cu, it could keep the plant from fully utilizing the supplement N. I also like add Utilize at R3, Utilize is a stress reduction agent that also increases nitrogen uptake in the plant. There are other similar products available, but these are the ones I am familiar with. It has been my experience that this approach increases pod count by 3 – 4 pods per plant, and if the plant can feed them that can have a significant impact on yield.
As always, nothing is easy in high management soybeans. Like in-furrow row starter, seed treatments and soil fertility, nitrogen management can be tricky. Remember, more is not always better. But for those willing to take the plunge the rewards can be impressive. We managed to hit 86 bushels per acre (bpa) in 2013 and 92 bpa in 2014 using these foliar programs, so I know they can work if weather conditions cooperate.