Richard Taylor, Extension Agronomist;rtaylor@udel.eduand Shawn Tingle, Extension Agent; tingle@udel.edu
At a recent soybean diagnostic field day, our Delaware Extension staff wanted to show photos of active and inactive nitrogen fixing nodules of soybeans but found a lack of web-available photos of inactive nodules. We were able to find both active and inactive nodules on some plants after visiting a number of soybean production fields and would like to share some of the photos we took of the inactive nodules so growers and consultants can more easily determine the status of the nodules on the soybean crop. As soybeans move to the R5 or beginning seed stage of growth, nitrogen fixation is critical since the demand for nitrogen (N) to build proteins to be stored in the developing soybean seed is approaching maximum.
Photo 1. Soybean plant at the R3 growth stage from which active and inactive nitrogen fixing nodules were obtained.
Photo 2. A small soybean nitrogen fixing nodule shows pink interior color after being sliced open when it is active.
Photo 3. A small soybean nitrogen fixing nodule which shows a green interior color after being sliced open is a non-active nodule.
Photo 4. Of these four soybean nodules, the two on the left showing pink interior color after being sliced open are active nitrogen fixing nodules but the nodule on the right with a green interior is inactive and the nodule on the lower right side which has a white interior is also an inactive nodule. White interior can also indicate an immature nodule or one that has not begun fixing nitrogen. Not shown but sometimes seen is a nodule with a brown or black interior which also indicates an inactive nodule.
Photo 5. The root system of the R3 growth stage soybean plant with an inactive (green interior) nodule is in the center of the photo just to the right of the thumb. A small nodule on another root is appears to be sitting on top of the sliced open inactive nodule.
All these photos are great, but what can you as a producer do about ensuring good active nodulation for your soybean crop. There are a number of equally important management decisions that can affect nodulation and nodule activity. The following list is given in no specific order of importance as any of the points listed can significantly impact nodulation or nodule activity:
• Manage your soil pH to keep it in the optimum range for your soil type and native fertility levels (especially with respect to manganese availability).
• If a field has been out of soybean production for longer than about three years, use an improved inoculum source when returning to soybean production.
• If a field has never been planted to a soybean crop beforem such as newly cleared forest land, it is imperative that seed be inoculated just prior to seeding as well as for soil pH to be brought to optimum well before planting soybean.
• Avoid preinoculated, lime-coated soybean seed if possible and apply fresh inoculum (both liquid and dry, graphite- or peat-based, inoculum is available). Although preinoculated and lime-coated forage legume seed is becoming standard in the forage industry, there have been enough inoculation failures to suggest that producers might be rewarded by going to the extra trouble to apply the inoculum just before planting.
• Avoid manure and commercial N fertilizer applied prior to or shortly after planting since high soil N levels can either delay nodulation or reduce the activity of the nodules so that, when the demand for N peaks during pod development, the nodules are unable to fix enough N to support maximum growth (See Photos 6 and 7).
• Consider using one of the new high efficiency strains of Bradyrhizobia as your inoculum source and apply inoculum to the seed every second or third time you plant soybeans. Many soybean yield trial winners report that they apply fresh inoculum to every soybean crop planted and with the new liquid inoculants the time and expense of applying soybean inoculant is much less than that experienced in the past. Many of the soybean fields in Delaware were found to contain strains of Bradyrhizobia that were either very inefficient at fixing N or actually produced toxins that could reduce soybean yield according to a Delaware Soybean Board project many years ago.
Photo 6. Soybean plants from plots that received no nitrogen fertilizer show large active nodules.
Photo 7. Soybean plants from plots that received 100 lb N/acre at planting show many fewer nodules than on plants from the control plots that received no N fertilizer.