Soybeans use a lot of nitrogen, up to 5 pounds of nitrogen per bushel, but nitrogen rarely limits soybean yields and nitrogen fertilizer applications are not recommended. This is because most of the required nitrogen is provided by biological nitrogen fixation occurring in bacterial colonies (nodules) attached to soybean roots. In some cases, the nodules will not form or function properly and the plants will show visible nitrogen deficiency symptoms. Inoculation failures are most likely to occur in first-year soybeans or when soybeans are planted after forage legumes or manure applications.
If you see short and light green soybean plants in late June or early July, you should suspect poor nodulation. The best way to identify the problem is to dig up plants in the affected areas, wash the roots carefully and count the root nodules. If fewer than seven nodules are found per plant, the plants will probably be nitrogen deficient. Only nodules that are two millimeters or larger and pink or red when split open should be counted.
Research has shown that applying 60 to 70 pounds of actual nitrogen per acre will correct the nitrogen deficiency and provide an economic return, up to 10 bushels per acre, if applied timely. Ideally, the supplemental nitrogen should be applied between the R1 growth stage (one open flower on 50% of the plants) and the R2 growth stage (one open flower on one of the upper two nodes on the main stem on 50% of the plants). However, Kansas State University has reported positive results when nitrogen was applied to nitrogen-deficient plants at R3 (one pod 3/16 of an inch long on one of the upper four nodes on the main stem on 50% of the plants) provided that rainfall or irrigation occurs soon after application.
Broadcast applications of urea-ammonium nitrate solutions such as 28% UAN will damage the foliage, so apply these materials to the soil. Urea can be broadcast without burning the foliage as long as the foliage is dry. However, more tire traffic damage will occur when applying urea due to the narrower distribution pattern and width of the tires on the spreaders. Adding a urease inhibitor to urea and 28% UAN solutions will reduce the potential for volatilization losses.