Soybeans obtain up to 70% of their total nitrogen requirement from biological nitrogen fixation conducted by Rhizobia bacteria colonies (nodules) living on soybean roots. If the nodules fail to form, the plants will become deficient in nitrogen and significant yield reductions can occur.
I received more calls about poor nodulation in 2013 than I have in the previous 24 years combined. In some cases, the poor nodulation was discovered soon enough for supplemental nitrogen fertilizer to be utilized by the plants and increase yields. In other cases, the plants were too mature to respond to the supplemental nitrogen.
The soybean seed was inoculated in all cases, but nodules either failed to form or formed and died. The conditions that are most likely to cause inoculation failures are listed below:
- Planting soybeans into fields for the first time
- Soil pH levels below 5.5 and above 8.0
- Residual soil nitrogen levels greater than 40 pounds per acre
- Improper fungicide and inoculant application to the seed
- Poor quality inoculant due to improper storage
Most of the inoculation failures and poor nodulation situations are the result of conditions found in first-year soybeans or fields having high levels of residual soil nitrogen. The potential for poor nodulation in first-year soybeans can be reduced by applying the inoculant to the seed and to the soil as an in-furrow treatment.
It is also a good idea to plant soybeans for two consecutive years and apply inoculation both years when planting soybeans into fields for the first time. This is the only situation where planting soybeans back-to-back is recommended.
Inoculation failures due to high residual soil nitrogen levels typically occur when soybeans follow manure applications or a legume such as alfalfa or clover. Yield losses due to poor nodulation in these situations are difficult to predict as the high nitrogen levels that suppressed nodulation may or may not be available to meet the plant’s nitrogen demand later in the season. If the plants are light green and short at R1, yields will be reduced and supplemental nitrogen is recommended.
I’ve also seen fields where the nodules formed, but died due to flooding. In these cases, the nodules turned brown and mushy. Soils must be saturated for at least three days for nodules to rot and die.
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 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 2 millimeters or larger and pink or red when split open should be counted.
Applying 60 to 70 pounds of actual nitrogen per acre will correct the 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, or one open flower on 50% of the plants, and the R2 growth stage, or one open flower on one of the upper two nodes on the main stem on 50% of the plants.
However, positive results have been reported from nitrogen applications made at R3, or one pod 3/16 of an inch long on one of the upper four nodes on the main stem on 50% of the plants.
Broadcast applications of Urea-Ammonium Nitrate (UAN) solutions such as 28% UAN will damage the foliage, so these materials should be applied to the soil. Urea can be broadcast without burning the foliage as long as the foliage is dry. Consider adding a urease inhibitor to urea and 28% UAN to reduce volatilization losses.