French bean could use excess residual nitrogen

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Prairie farmers who had planned pulses for 2022 might be faced with a conundrum: Last year’s drought meant the crop left a lot of nitrogen fertilizer in the soil.

When it comes to nitrogen, legumes “bring their own lunch” with symbiotic rhizobia bacteria that form nodules on their roots to capture fertilizer from the air. Farmers can help this with inoculants that add the right type of rhizobia for the legumes they are planting.

A problem with high nitrate soils is that they provide the legume crop with an easy supply of nitrogen, which interferes with nodulation and can lead to poor performance.

Dry beans could be an exception to this rule, suggests work at the Indian Head Agricultural Research Foundation in Saskatchewan.

Chris Holzapfel, research director and agronomist at IHARF, said one variety in particular showed promise.

“CDC Blackstrap is, to my knowledge, the only variety that is, or so you might call it, relatively well suited to dryland production,” he said.

Blackstrap can be sown with regular equipment with a little extra care to avoid seed damage. It is also suitable for the straight suit.

CDC Blackstrap is a black bean launched by the University of Saskatchewan Crop Development Center in 2015. It was released widely in 2018.

“Beans aren’t something we’ve grown a lot on the prairies in dryland production, but if there’s one variety that will work, it’s basically this one,” Holzapfel said.

He and his colleagues have been working on CDC Blackstrap since 2019. He said growing dry beans, unlike other legumes, means adding nitrogen fertilizer because there is a lack of effective inoculants.

“Inoculants in dried beans are a bit tricky. They’re not widely available and what’s out there tends to be class and variety specific and with inconsistent answers, so most guys will rely on nitrogen fertilizers.

Researchers looked at a wide selection of inoculant treatments – liquid, peat-based, granular – with and without nitrogen fertilizer. Trials were conducted at Indian Head, Redvers, Yorkton, Melfort, Scott and Outlook, the latter on irrigated land.

“We had several different inoculant treatments, with and without nitrogen. I think it was 80 pounds of nitrogen, adjusted for soil residue,” Holzapfel said.

Inoculants were ineffective, but bean crops at all sites showed a strong response to supplemental nitrogen. Unsurprisingly, irrigated land saw the highest yields, and while elsewhere it was “all over the map”, Holzapfel said all fields had “pretty good yields”.

The next step was to determine the optimal rate of fertilizer and see if the beans could provide consistent performance with relatively low risk.

The researchers ran tests ranging from just residual soil nitrogen and some phosphorus to just under 150 pounds per acre of nitrogen (including residual).

Under good conditions, crop response increased with the amount of fertilizer. However, under less than ideal conditions, there was a clear optimal rate.

Holzapfel said in 2020 at Indian Head they harvested a poor crop of just over 600 pounds of beans per acre, with the best yields coming from fields fertilized at 75 pounds per acre of nitrogen.

“So fairly modest (nitrogen) rates were enough to optimize yields,” he said, adding that this was a typical response curve.

“You get to a point where you just get less for your money at these very high rates.”

Now, with 11 years of field data, researchers have seen yields ranging from 630 lbs to just under 1,700 lbs per acre, the latter representing “a really good, profitable yield.”

One thing is clear: nitrogen fertilizer for dry beans can be critical. Holzapfel gives 2019 as an example.

“There was hardly any yield without (nitrogen) fertilizer. We could barely fit it into the combine,” he said. “But we were just under 900 pounds per acre with the nitrogen, so maybe making some money, but not a lot.”

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