Profitable Pulses: Don’t hesitate — inoculate, says pulse expert

Applying inoculant is an inexpensive — and essential — way to make sure pulse crops fix nitrogen

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Thinking of saving a few bucks by not inoculating your pulse crops? Don’t, says a pulse research agrologist.

Most growers recognize the benefits of rhizobium inoculant (the bacteria present in both soil and commercial inoculant) in fixing all-important nitrogen in the soil. However, when it comes to cost cutting, some may be tempted to forgo the supplemental peat, granular, or liquid inoculant “just this one year” — relying instead on native, soil-based rhizobia or carry-over from previous seasons.

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But that would be shooting yourself in the foot, said Mark Olson, a pulse specialist with Alberta Agriculture and Forestry.

At $3 to $10 per acre for a single inoculation, inoculant is one of the least expensive crop inputs available, easily beating the cost of nitrogen fertilizer. That’s “cheap insurance,” he said.

“If you don’t have good nodulation — which is your biological nitrogen factory — then you’re looking at applying a bunch of manufactured nitrogen fertilizer, which is a whole lot more costly,” said Olson. “For example, a 50-bushel field pea crop will take 138 to 168 pounds per acre of actual nitrogen — which is substantial. That would have to be added to the crop without biological fixation.

“If you’re not inoculating and using your natural biological factory to fix nitrogen, you’re forgoing the whole idea around growing pulse crops in rotation.”

Inoculation is good for the environment as well, he said.

“You’re lowering your entire carbon footprint — not just in your year of growing pulses but in your entire crop rotation — by including a crop that fixes its own nitrogen.”

The process of ‘fixing’ nitrogen in the soil (called nodulation) begins when pulse crops send out a signal two weeks after germination. This signal attracts rhizobia which infect the root hair, breaking down its cell wall and sending out an infection thread.

This infection eventually creates spherical nodules on the roots in which rhizobia live and convert nitrogen from the air (which is 78 per cent nitrogen) into plant-usable forms. In exchange, the plant provides energy in the form of carbohydrates and sugars.

“It is through a fascinating symbiotic relationship with rhizobium that pulses and other legume crops are able to fix and utilize atmospheric nitrogen,” said Olson. “This symbiosis allows the pulse to obtain fixed nitrogen from the nodules in exchange for the energy that the bacterium needs to carry out its metabolic processes.”

The process of nodulation, which allows pulses such as fababeans to ‘fix’ nitrogen — creates spherical nodules on the roots of the plant. Rhizobia live in these nodules, converting nitrogen from the air into plant-usable forms.
photo: Alberta Pulse Growers

This process is a significant reason why most pulses are relatively economical crops to grow.

“Nitrogen at the levels required by the crop is expensive. Properly inoculated pulse and legume crops do not need the addition of nitrogen fertilizer for optimal growth.”

The right stuff

But not all rhizobium normally present in soil fixes nitrogen. There are many different kinds of native rhizobia and almost every one works exclusively with a specific pulse. Although it is possible for carry-over rhizobia to form nodules, the genetic material that codes for nitrogen fixing can be lost over time.

“New, fresh, and compatible rhizobia will always be your best bet,” said Olson.

Although all pulse crops require inoculation, not all pulses fix nitrogen in equal amounts.

“The best example of this is dry bean, which is not known to be a really efficient nitrogen fixer. In that instance farmers may be inoculating plus adding nitrogen fertilizer just to get more consistency out of the crop,” he said.

Growers can tell if a plant is fixing nitrogen by cutting open a nodule to see if the inside is dark red in colour. This colour is created by hemoglobin, the protein which creates the same colour in mammal blood. Nodules that are not fixing nitrogen will look very different.

“We’ve studied pulses planted on virgin ground where no inoculant has been applied. You’ll find nodules but they’re not the right colour; they might be white or even florescent green.”

Double inoculation, or adding a second treatment of inoculant, has become a common practice in some quarters, but the jury’s still out on whether or not it’s an effective practice, said Olson.

“I don’t know if there’s a whole lot of research to back double inoculation. I’m not sure there’s any advantage from the practice,” he said. “I do hear of some growers doing it but I don’t know if they have ever left a check strip to examine nodulation or have measured yield or if anybody’s researched it to that extent.

“We seem to hear more about double inoculation in soybeans, which is a legume but not a pulse crop per se. I’ve talked to some research agrologists who are scratching their head as to why some farmers are doing it.”

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