identifying and responding to poor nodulation in soybeans

MIKE STATON MSU EXTENSION SOYBEAN EDUCATOR

symptoms. Inoculation failures are most likely to occur in first-year soybeans or when soybeans are planted following forage legumes or manure applications. Stress from saturated soils, dry soils and very high temperatures can also cause poor or failed nodulation.

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. Dysfunctional nodules will most likely be white, green or brown. White nodules are immature and have not begun fixing nitrogen. Green nodules are mature but no longer fixing nitrogen and brown, mushy nodules are usually caused by saturated soil conditions lasting at least five to seven days.

Timely applications of 60 to 80 pounds of actual nitrogen per acre to soybean plants showing nitrogen deficiency symptoms and having few or no nodules can increase yields by up to 10 bushels per acre. 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) if rainfall or irrigation occurs soon after application.

28% urea-ammonium solution (28% UAN) is the preferred nitrogen fertilizer for this application. One reason is half of the nitrogen is immediately available to the plants. Another reason is the fertilizer is applied using sprayers having wide booms and narrow tires, reducing the potential for tire damage to the plants. Broadcast applications of 28% UAN will burn the leaves, so the fertilizer must be applied to the soil using drop tubes or hoses. Urea can be broadcast without burning the leaves if the foliage is dry. However, more tire traffic damage will occur when applying urea due to the narrower spreading pattern and wider width of the tires on the spreaders.

Other downsides of applying urea are that the nitrogen is not immediately available and is in a form subject to volatilization losses under hot, dry conditions.