Naked roots and mystery nitrate

Poor nodulation, possible rescue applications and strange readings on spring soil nitrate have made this year an interesting one for pea agronomy

Field peas show symptoms of nitrogen deficiency.

[UPDATED: July 26, 2021] Some pea roots across the province are noticeably bare.

Agronomists are pondering a lack of pea nodulation in some acres, as well as suggestions for a possible rescue application of nitrogen.

Why it matters: Agronomists hope challenges this year will help build recommendations for years to come.

Laura Schmidt, agronomist with the Manitoba Pulse and Soybean Growers (MPSG), says she does not think the issue is widespread, nor is there any real geographic cluster of problem cases.

“The fields I’ve seen it on so far are more sandier soil types,” she said. “Kind of the expectation there is that the rhizobia that is native to our soils for peas and fabas and lentils… they’re not surviving as well on sandy soils because they don’t have that clay content or organic matter to hook on to for survivability.”

At the same time, she noted, those are the same soils with less tendency to hold water.

Nodulation issues may be an example of this year’s dry conditions coming back to roost, both Schmidt and provincial crop nutrition specialist John Heard said.

The province had seen little rainfall coming into June, following far-below-average snow cover over the winter and very dry growing conditions the previous year.

Nodule-less pea roots have been a sporadic issue this year. photo: MPSG

Heard speculated that the poor growing conditions may have put some pea inoculant on poor footing coming into the season.

“We do know that, if it was a dry seedbed under dry conditions, those are generally harsh environments for inoculant that we just kind of stick on the seed,” he said. “Those tend to be environments where granular inoculants tend to thrive.”

Schmidt concurred.

“It was pretty challenging conditions for that on-seed inoculant application,” she said. “In those fields, I think a granular would have probably had a little bit more success just because it’s more resilient to those conditions.”

Mystery nitrate

At the same time, agronomists are attempting to tease out the reason behind high nitrate levels in some fields this spring — fields that showed significantly lower levels when soil samples were taken in the fall.

The Manitoba Pulse and Soybean Growers On-Farm Network found high nitrate levels in more than half of their 33 sampled fields this spring. Fifteen fields are being monitored in the top two feet of soil in light of the findings.

There is debate over the cause, Heard noted.

According to the June 23 Bean Report, put out by the MPSG, the grower group is looking into the histories of the field, while other eyes are turning to the Birch effect — a pulse of decomposition and nitrogen mineralization observed after dry soils are rewetted — for a possible explanation.

In any case, Heard said, high nitrogen levels also tend to hold back nodulation.

“If that’s the case, if there’s a lot of nitrogen there, then maybe you’ve got enough to feed the crop anyways, but the problem is when you have just enough to inhibit nodulation but not enough to completely fertilize the plant,” he said. “That’s where our suggestion (of a rescue application) would kick in.”

It may be hard to determine what side of the line the producer falls on, however, he added.

To the rescue

A rescue application of nitrogen may help pod fill in fields where there are low nitrogen symptoms and poor nodulation is a problem, but where yield potential is still there, agronomists have suggested.

Heard cautioned, however, that there is very little research on the subject.

“This is one of those things where sometimes we don’t know the answers and research has not yet provided the answers,” he said. “But if people are out there with naked roots and they’re looking for something, based on what we’ve gleaned from some of the other provinces, some assessments, and from what we have learned from soybeans — where we do have experience, we do have research — we said, ‘rather than have people go all year with naked roots, here is a kind of middle-of-the-road suggestion that they could go with.’ But it’s not a suggestion that comes easy.”

Peas ranging from healthy (left) to increasingly nitrogen deficient are sampled out of a Manitoba field this year. photo: MPSG

First, Heard urged, producers should take a hard look at exactly what kind of nodulation they have in the field.

Heard pointed producers to a nodulation analysis tool, available on the June 23 Bean Report and on the MPSG website. The assessment tool scores fields on the basis of plant vigour, nodule colour and number and nodule position.

“With peas, nodules are kind of like grape clusters. They’re not big like soybeans,” Heard said, noting that producers will want to see those nodules both on the plant’s crown and lateral roots.

“Pea growers, especially if they’re new pea growers, probably want to be doing this assessment until we become familiar with this crop again,” he also said.

About 146,500 acres of field peas were insured by the Manitoba Agricultural Services Corporation last year, up from 104,400 the year before and 78,400 in 2018.

Heard also urged producers to dig, not pull their plants when assessing for nodulation, since dry and hard soil could strip nodules.

If that nodulation assessment comes out short and there are signs of plants being shy of nitrogen, a rescue application may be on the table.

Agronomists suggested 40 to 50 pounds per acre of nitrogen at the 9- to 12-node stage.

Producers who do opt for a rescue application, however, should stay away from UAN. There is significant risk of leaf burn with UAN in this situation, Heard said. Instead, he suggested producers stick to broadcasting granular fertilizer to permeate the crop canopy.

The MPSG also urged producers to add a urea inhibitor if there is no rain in the forecast.

Building knowledge

While no one wants a nodulation problem, Schmidt says they are using the opportunity this year to fill in some knowledge gaps, and hopefully develop more concrete suggestions if the problem shows up again in a future year.

The MPSG is hoping to assess whether high nitrate in some of those mystery fields is enough to carry the crop through, she said, while other fields are showing low nodulation and low nitrogen, and are therefore a good candidate for testing out rescue applications.

It is very similar, both Heard and Schmidt said, of how recommendations for soybeans were developed.

“We had those couple of failures in soybeans and went in for rescues and that kind of gave us some good recommendations to go off of now if we see another field that needs a rescue, so maybe we can get to that with peas this year too,” Schmidt said.

*Update: Photo captions in the article were corrected.

About the author

Reporter

Alexis Stockford

Alexis Stockford is a journalist and photographer with the Manitoba Co-operator. She previously reported with the Morden Times and was news editor of  campus newspaper, The Omega, at Thompson Rivers University in Kamloops, BC. She grew up on a mixed farm near Miami, Man.

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