The basics of pulse nodulation

Nodulation 101: how pulse crops work with bacteria to fix their own nitrogen

At Saskatchewan Agriculture’s Crop Diagnostic School at Swift Current in July, a lot of the in-field real estate was devoted to plots of lentils and peas. Organizers had seeded plots of both crops with and without nitrogen, and with and without inoculant. These plots gave Garry Hnatowich, research director at Saskatchewan’s Irrigation Crop Diversification Corporation at Outlook, a great backdrop for his talk about nitrogen fixation.

These are legume crops, Hnatowich said, “and they are capable of biological fixation.” Prairie farmers know what this means: these crops fix their their own nitrogen.

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When pulse crops are inoculated, and “if we get effective nitrogen fixation occurring,” Hnatowich told Crop Diagnostic School participants, lentils and peas will supply about 80 per cent of the nitrogen they need. This is the textbook rule, Hnatowich explained, but “in Saskatchewan, those numbers are widely variable.” A University of Saskatchewan study looking at multiple research results found that these pulse crops will supply anywhere from 55 to 85 per cent of the nitrogen they need.

“Chickpeas,” Hnatowich said, “are similar, but maybe a little lower.” They’ll provide about 70 per cent of their own nitrogen.

Soybeans and dry beans can provide themselves with about half of their own nitrogen. “Faba beans are the king. They’ll be 90 per cent plus.”

The basics of fixation

While their nitrogen fixation capabilities are different, all pulse crops have the same basic relationship with rhizobia bacteria.

“We apply that inoculant onto the seed or put it in the seedrow. By the time that seed starts emerging and we get the first true leaves forming on the plant, that plant is sending signal molecules, flavonoids, out into the soil environment.”

When the flavonoid is in the soil, “the bacteria take it into their cellular system and are triggered. They read it. And what it tells them is that there is a host plant in the immediate area. They develop a flagella, a swimmer, and they move up a water film.”

That flagella moves toward stronger and stronger concentrations of flavonoids in the soil, until it reaches the plant.

“As soon as it gets to the plant, it finds a developing root hair and adheres to just behind the root tip, and it starts colonizing.”

Once the bacteria stick to the root hairs, the bacteria send off a signal molecule called an LCO. When the plant receives the LCO, it takes action.

First, the plant starts curling its root hair, wrapping it around the newly-formed colony of bacteria.

The LCO signal molecules also trigger the plant to “stop crystallizing any cellular structure at that point of infection, so that the bacteria can degrade the wall and move into the root hair.”

Once the bacteria has moved into the root hair, Hnatowich said, “that bacteria goes wild. It starts multiplying like mad.”

The bacteria multiplies at the growing point, heading towards the root system. “It gets to the cortex of the root and forms nodules, which are just a tumour-like growth structure that we see on the roots of pulse crops when we dig them up.”

At the end of the season, after the bacteria has multiplied, “and you can count a billion bacteria” in those tumours. That’s important, Hnatowich said, because “this is a numbers game.”

At manufacturer’s recommended rates, inoculant on pulse crops should provide you with a hundred thousand bacteria on every seed you plant in the ground. And, Hnatowich said, “They’ve selected a strain that grows very quickly.” Once they’ve multiplied, you’ll eventually have millions of bacteria on each seed.

Modern inoculant does more than just infect plants with bacteria. It also helps to speed the process along. For example, Tag Team, from Novozymes, comes with LCO promoter technology to help the rhizobia send signal to the plant.

The relationship between plants and the rhizobium bacteria is symbiotic, Hnatowich said, “meaning it’s beneficial to both the plant and the bacteria. What the bacteria get is photosynthesis. This is not cheap for the plant. We often talk about fixation as being cheap nitrogen, and it is, in a sense.” It’s cheap for farmers who can save on nitrogen costs. But it’s not cheap for the plant. “Twenty-five per cent of the total photosynthetic capacity is dedicated to feeding nodules. That’s 25 per cent of all the energy in that plant.” Pulse plants fix nitrogen at the expense of their own flowers and leaves. However, plants get a fair trade off. “Nitrogen is the driving factor for yield.”

The right infection

While the process for nitrogen fixation is the same, different pulse crops need different strains of rhizobium to fix their own nitrogen.

In the case of peas and lentils, the necessary strain of rhizobioum happens to be native to North America. Pea and lentil plants that aren’t inoculated can still develop some nodules.

Originally, Hnatowich said, the strains of rhizobium we’re using to inoculate lentils and peas today “came from vetch plants that were dug up in ditches in Saskatchewan.” Companies selected bacteria strains that worked well could be re-produced at a reasonable cost.

Chickpeas and soybeans need rhizobium that is not native to North America. Soybeans planted in soil where they haven’t been grown before will not develop nodules or fix nitrogen. “You’re going to have to rely on an inoculant for those two crops,” Hnatowich said.

“The very first time you inoculate your seed, you’re also infecting your field.” Once introduced, the rhizobium will overwinter. However, the strains will mutate over time. Our typical four-year rotations give the bacteria four years to mutate and diminish in the soil before they find another host crop, making it worthwhile to re-inoculate the next time you seed a pulse crop.

Hnatowich estimates that in the U.S., only about 25 per cent of the soybean crop is inoculated in a given year. The difference is mainly in the rotation. U.S. farmers tend to plant soybeans every second year. That shorter rotation, and the warmer temperatures, keeps the rhizobium alive and less prone to mutation.

Because the rhizobium soybeans need is not native here, and soybeans are still relatively new, Prairie soybean growers’ current strategy is to use dual inoculant — giving the seed a double shot of bacteria. Now that they’ve been growing soybeans for a while, some Manitoba farmers are beginning to think that dual inoculant isn’t providing the “bang for their buck anymore.”

Adding nitrogen fertilizer

When you add fertilize pulse crops with nitrogen, the plants can access nitrogen through water uptake. “It’s an easy process for the plant. It’s a free nitrogen drink.” When free nitrogen is available, the plant isn’t going to spend energy feeding bacteria, and won’t develop as many nodules.

The problem comes later in the season. If you start with 30 or 60 pounds of nitrogen, “at some point in the season, that nitrogen’s going to be gone.”

And, that’s generally going to occur in mid-June, when plants are flowering. “You do not want a pulse crop suffering from nitrogen stress at flowering. You eliminate your yield by half.”

If you inoculated later in the season, nodules would form and fixation could occur. However, there will be a two week delay.

Because most farmers don’t plan to apply nitrogen with pulse crops, many don’t soil test fields before seeding pulse crops. “That’s fine,” Hnatowich said, “as long as you’re not sitting on a high nitrogen level.”

If you see inoculant failure, he suggested, ask yourself how much nitrogen you had to begin with.

Peas with no inoculant, with applied nitrogen.
Peas with no inoculant, with applied nitrogen. photo: Leeann Minogue
Peas with inoculant, no nitrogen.
Peas with inoculant, no nitrogen. photo: Leeann Minogue
Peas with inoculant, with applied nitrogen.
Peas with inoculant, with applied nitrogen. photo: Leeann Minogue
Peas with inoculant, with applied nitrogen, with applied phosphorus.
Peas with inoculant, with applied nitrogen, with applied phosphorus. photo: Leeann Minogue

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