When you pull up a legume and find nodules on the roots, what should you be hoping for? Big nodules or little? White ones or red?
If you want to know if a legume is fixing nitrogen, you have to pull it out of the ground, look at the roots, and check for red nodules.
That was one of the many bits of wisdom shared by University of Saskatchewan soil scientist Diane Knight at the Western Canadian Grazing Conference.
The province’s ag ministry funds research into the often-mysterious biological processes in soil and Knight’s work is aimed at better understanding the relationship between plants and rhizobial bacteria. This type of bacteria ‘infects’ a plant via the tiny hairs that sprout from a root — but in this symbiotic relationship, infection is a good thing because rhizobial bacteria fix nitrogen.
“The infection process is like sticking a finger into a balloon and making an indentation,” said Knight. “That’s what happens in the root hair. The bacteria indent into the plant, and then they start to enter into the interior of the root.”
The bacteria then create nodules, which are enveloped by plant cells. The nodules are made up of bacteria that can move nitrogen from the soil into the plant. But rhizobium bacteria are selective, and generally only form nodules with certain legume plants.
“There’s no one rhizobium species that can nodulate more than one per cent of the known legume plants,” said Knight. “Some rhizobia will nodulate a lot of different plants and some are very specific.”
Faba beans need a specific rhizobia for nodulation, while sainfoin is less selective.
Nodules come in different sizes and shapes. Some are determinate, which means they grow to a certain size. Others, like those found on red clover alfalfa, are indeterminate and keep growing, although their nitrogen-fixing activity changes during the growing season.
If you pull up your plant, you can have some idea of whether or not it is fixing nitrogen.
“We can usually tell whether or not the rhizobia have been able to initiate nodules within five or six days,” said Knight.
Rhizobia initially form small white nodules on the plant. Nodules don’t form evenly, and can continue to grow during a plant’s lifetime.
Colour is everything with nodules. White indicates they’re not able to fix nitrogen yet, but red or pink ones are busy fixing nitrogen. (Hemoglobin, the same substance found in human blood, makes the nodules red.)
Active nitrogen fixation starts about two weeks after plants emerge, with most pulse crops experiencing maximum fixation around flowering.
But it’s not a one-way street — the rhizobia want something in return.
“We have to remember that nodules are using energy from the plant and drawing away energy that could be used for carbon development and growing leaves,” said Knight. “There’s a balance between how many nodules the plant needs and the energy (they are) using.”
Some people may think big nodules are desirable, but that’s not the case. Small ones are actually more effective.
Knight recommends using a commercial inoculant on all legumes.
“We recommend that all legume crops be inoculated because we want to introduce strains that we know to be effective. Bacteria have very short life cycles, so they multiply and change over in a few days to a few weeks.”
Every time a bacteria goes through a life cycle, there’s an opportunity for a mutation, and so rhizobia can lose their effectiveness over the long term.
Environmental factors also play a role. Rhizobia thrive in soils with good organic matter, and need a little bit of moisture to be effective. Too much water hampers the nitrogen-fixing process while a deficit dries out both plants and rhizobia. Root hairs also don’t form when the soil is dry. As well, nodules won’t form below 10 C, and will die off at 37 C.