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Measuring the N boost from pulses

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Pulse crops can give you a boost of free nitrogen, but how much depends on how much work you give them, says Sheri Strydhorst, executive director of the Alberta Pulse Growers Commission.

“If we grow our pulse crops on a soil that has relatively high background levels of nitrogen, we’re not going to fix nitrogen because it’s easier for the pulse crop to take up the nitrogen rather than fix it. We need to create conditions to fix high amounts of nitrogen,” Strydhorst told a meeting of the Alberta Pulse Growers Commission held in Westlock last month.

She said that to provide a net nitrogen benefit, the pulse crop must fix more than you harvest in the seed.

Strydhorst said a pea crop that yields 50 bushels per acre at 22 per cent protein means you’re harvesting 106 pounds of nitrogen per acre. To be able to get a nitrogen credit, your pea crop has to fix more than 106 pounds of nitrogen per acre. A 60-bushel fababean crop at 27 per cent protein represents 156 pounds of nitrogen per acre.

“So again, your fababean crop is going to have to fix more than that 156 pounds of nitrogen to leave more nitrogen for next year’s crop,” she said.

As part of her Ph. D research, Strydhorst conducted a project in the Barrhead area which examined how much nitrogen is emitted by a pulse crop and when that nitrogen is made available to a subsequent crop. Measuring straw yields was the first step of the experiment.

“We went out to the field and cut quadrants of the pulse crop out, hand-harvested that and were able to measure the straw yields,” she said. The straw was then cut up to mimic straw going through a chopper. “We were trying to recreate the characteristics of what is happening on your farm,” said Strydhorst.

The straw was stored in mesh bags, which were placed in the field on the soil surface to mimic decomposition of pulse straw in a no-till system, or they were buried beneath the soil surface to mimic the decomposition of pulse straw in a conventional tillage system. Fifty nine pounds of nitrogen was obtained from the initial harvest of pea straw used in this experiment.

The researchers followed the nitrogen release from the decomposing pea straw over a 22-month period. “By the time you’d be seeding into that pea stubble, 33 pounds of nitrogen have been released from that decomposing pea straw that is made available to that crop that you’re seeding into that stubble,” Strydhorst said. “By the middle of the growing season, we see that the pea straw, as it’s decomposing, is able to supply 45 per cent of your subsequent crop’s nitrogen.”

The pea straw continues to decompose and release nitrogen even after the next winter and next growing season. Nitrogen released from pulse crops continues to benefit the soil for several years.

The same experiment was also conducted using fababean straw, which releases nitrogen a little bit faster than pea straw, since it has a faster rate of decomposition.

The fababean crop was found to supply 71 per cent of nitrogen needed for the next year’s crop. Strydhorst cautioned all of the nitrogen released might not go directly into the next crop, due to other environmental factors.

About the author

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Alexis Kienlen

Alexis Kienlen lives in Edmonton and has been writing for Alberta Farmer since 2008. Originally from Saskatoon, Alexis is also the author of two collections of poetry, a biography, and a novel called "Mad Cow."

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