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Published on Jul 10, 2015
This video discusses the damage of roots and nitrogen-fixing nodules to varying degrees of water stressed soybean plants. Saturated soils for extended periods will cause soybean yellowing in portions of or whole fields. Applying supplemental nitrogen in late vegetative stages (V4 onward) as soybeans transition to early reproductive stages (flowering) to these yellowish fields may help with nutrient needs while nodules are recovering from water-logged (oxygen-deprived) soils.
Under normal growing conditions, soybeans accumulate ~10 lb N/acre by V4 then accumulate another ~3 lb N/acre daily until R2 (full bloom). Thus, soybeans normally accumulate another 60 lb N/acre by R2 (full bloom). Nitrogen stress during this period will impact yield.
Forty to sixty pounds of N per acre has been beneficial in these situations (6+ bushels/ac and double digit yield benefits under severe situations). Urea treated with a urease inhibitor is the preferred N source since straight UAN will damage foliage at these rates. You may also consider a blend of polymer-coated urea to provide a slow release of N until the nodules become active. Soybeans will take up the N forms of urea and ammonium without compromising N fixation. High levels of nitrate in the soil can temporarily inhibit or pause nodule activity, but 100 lb of urea (46 lb of N) per acre should not be a problem. Other N sources to consider would be ammonium sulfate (100 lb of product spread dry to supply 21 lb N/acre). Many foliar fertilizers will not supply enough N to be warranted, but diluting UAN in a foliar application may be beneficial since more N could be supplied. One option would be ~3 gallons of UAN/ac to supply 10 lb N/ac with water making up the difference in broadcast rates of 10 to 15 GPA.