Root nodulation occurs when bacteria infect legume plants and provide usable Nitrogen (N) in exchange for food. In this week’s edition of Growing Possibilities, we give you the How, Why, Where, When and What (to do if you have problems) of nodulation.
How: Root nodules are formed thanks to the symbiotic relationships between legume plants and rhizobia. This symbiosis is kept in check by the plants, which possess a gene called “autoregulation of nodulation” (AON) (1). AON ensures the plant can control nodule formation, and therefore control the energy consumed by the rhizobia within the nodules (2). This prevents the symbiosis from becoming parasitic. Symbiosis begins when a plant releases organic compounds called flavonoids. The flavonoids attract a species of rhizobia specific to that crop, which will then infect the root hairs of the plant (3). The rhizobia release their own chemicals, called lipo-chitooligosaccharides (LCO), which cause the root to wrap around the bacteria, forming a nodule.
Why: Once nodules form the rhizobia will begin to fix N for the plant in return for carbon (4).
When: To check for nodules, wait four to six weeks after planting then carefully remove a plant with all its roots and surrounding soil.
Where: Gently remove or wash away the soil from the roots to expose the nodules. 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 (3). Slice a nodule in half to see the colour within; pink indicates that N fixation is occurring, while white, green, grey, or no nodules indicates N-fixation is not active.
What: to do if you have a lack of nodulation. There are many factors as to why nodules are not pink or do not appear at all. 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 Grey or green coloured nodules can indicate that N-fixation has stopped and is no longer active (3). If there is no nodulation whatsoever, this could be due to many reasons: 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 prior to application (5, 6).
Nodules can form naturally in soil where legume crops have been repeatedly grown. But numerous third party research studies conducted on our inoculant products (XiteBio® SoyRhizo® and XiteBio® PulseRhizo®) have, on average, shown that commercial inoculation produces higher yielding crops. Commercial inoculants contain specific bacterial strains and other ingredients that promote N fixation and plant health.
References:
- 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/rhizobia-and-legume-roots-why-do-they-bond/
- https://xitebio.ca/legume-root-nodules-helping-after-harvest/
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8247157/pdf/12870_2021_Article_3102.pdf