With summer quickly ticking by many farmers will be digging up their legume plants to see if they have formed proper nodules. In this week’s edition of Growing Possibilities, we look at how root nodules form, how to check for root nodules in your legume crop, and what to do if you don’t find nodules.
How do root nodules form on legume plants? Root nodules are formed thanks to the symbiotic relationships between legume plants and rhizobia. Rhizobia are bacteria which infect the roots of legume plants to supply them with atmospheric nitrogen (1). In return the plant provides the rhizobia with carbon (2). Symbiosis between the plant and the rhizobia begins when a plant releases organic compounds to attract rhizobia to their root zone. The rhizobia will then infect the roots of the plant. The plant can regulate this infection with specialized genes that prevent the infection from becoming parasitic (3). Once nodules form the rhizobia will begin to fix N for the plant in return for carbon.
How do I check for nodules on legume plants? To check for nodules, wait four to six weeks after planting then carefully remove a plant with all its roots and surrounding soil. Gently remove or wash away the soil from the roots to expose the nodules. Be careful as pulling the plant out of the ground or wiping away the dirt without water can cause the nodules to fall off the roots. On-seed inoculation will cause nodules to appear near the crown of the root system, while in-furrow inoculation will cause nodules to appear on the lateral roots. Slice a nodule in half to see the colour within. Pink indicates that N fixation is occurring. Pale white nodules can mean that the plant is still young, and N-fixation has not started. Grey or green coloured nodules can indicate that N-fixation has stopped and is no longer active (4).
What should I do if I have no nodules on my soybean or pulse plants? If you don’t find any nodules you will need to investigate further to determine why the nodules failed to form properly. A lack of nodulation could be caused by any number of reasons such as: the soil pH is below 6.0 or above 8.0, a lack of soil moisture, too high or too low temperatures, high soil N levels, excessive salinity, compatibility issues with other seed treatments, or improper handling and storage methods (of commercial inoculants) prior to application (5). By determining why nodules failed to form you can remedy the issue so that it does not happen again in the future. However, you cannot inoculate mature plants to induce nodulation. Inoculants must be applied before or during seeding to allow nodules to form (6).
Nodules can form naturally in soil where legume crops have been repeatedly grown. But numerous third party research studies conducted on commercial inoculant products like XiteBio® SoyRhizo®, XiteBio® PulseRhizo® and XiteBio® OptiPlus® have shown that applying inoculants on-seed or in-furrow produces higher yielding crops (on average) (7). Commercial inoculants contain specific bacterial strains and other ingredients that promote N fixation and plant health. Using a commercial inoculant is an easy step that soybean and pulse growers can take when planting legume crops to ensure they achieve healthier plants and better yields.
If you have any questions about nodulation in your crop or applied XiteBio inoculants please contact one of our sales agronomists by visiting xitebio.com or call 1-855-XITEBIO (948-3246)
References:
- https://pubs.nmsu.edu/_a/A129/#:~:text=In%20legumes%20and%20a%20few,a%20bacterium%20and%20a%20plant.
- https://www.sciencedirect.com/science/article/pii/S0022519320303994
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7194392/pdf/kpsb-15-03-1733814.pdf
- https://xitebio.ca/root-nodulation-what-do-i-look-for/
- https://xitebio.ca/inoculant-and-biological-faqs/#nodulation
- https://xitebio.ca/inoculant-and-biological-faqs/#n-application-specifications
- https://xitebio.ca/xitebio-soyrhizo-premium-liquid-inoculant/