The right amount of nitrogen for soybeans

Between hearing of U.S. farmers adding extra nitrogen, and 
warnings about choosing fields with low nitrogen for soybeans, it’s easy to get confused

Some western Canadian farmers new to soybeans are understandably a little confused about the issue of whether or not they need to add nitrogen fertilizer to achieve better yields, especially if they are sourcing production information from the U.S.

Differences in soils types, conditions and crop rotations mean that U.S. information isn’t necessarily applicable to most Canadian Prairie soybean production. While some U.S. farmers are adding nitrogen to soybeans, researchers don’t see benefits to adding nitrogen on the Canadian Prairies.

“They may be reading that some U.S. producers are putting nitrogen on their soybeans in the hopes of higher yields,” says John Heard, soil specialist with Manitoba, Agriculture, Food and Rural Development. “Those are for ultra high yields and it’s definitely not a sure thing.”

Prairie testing

Heard, in association with John Lee of AGVISE Laboratories and Ron Tone of Tone Ag Consulting, recently fertilized demonstration plots in 13 farmers’ fields in North Dakota and Manitoba.

“We called them ‘controlled observations’ to assess whether there is any detrimental affect or benefit to nitrogen on soybeans,” says Heard. “We started with a field and caused high nitrogen levels by putting on some fertilizer. So the objective really wasn’t to fertilize soybeans with nitrogen, it was to study the impact of having soybeans grown in a soil that has high nitrogen.”

nitrogen on soybeans

These pictures show the impact of nitrogen on soybean root development. This check plant grew with no added nitrogen.
photo: John Heard, MAFRD

What’s causing a lot of new soybean growers some distress, says Heard, is the rule of thumb that excessive carryover nitrogen from a previous crop or cropping system may impair the ability of the rhizobium to develop nodules on a subsequent soybean crop. “It’s a common recommendation that as you select candidate fields for soybeans you should select fields with a low chance of iron chlorosis and that are low in nitrogen,” says Heard. “In fact the two may go hand-in-hand since in addition to impairing nodulation, high soil nitrate tends to worsen iron chlorosis of soybeans.”

The study observations showed, however, that it’s important to differentiate between new and established soybean fields. “Having high nitrogen in soil always reduced the amount of nodules,” says Heard. “But there’s no gold star for having lots of nodules. You just need enough nodules to express full yield. If previous soybean crops were properly nodulated, they appeared to have enough nodules. We found only a couple of our field plots, that were typically first year soybeans, where the excessive nitrogen in the soil impaired nodulation enough to lower yield and also the protein level.”

So the take home message is that the rule of thumb for selecting soybean fields with lower nitrogen soil levels may not be as critical where there have been previously successful soybean crops. “We didn’t see any advantage in adding more nitrogen,” says Heard. “We doubt it’s beneficial to put nitrogen on. There were good yields this year and there was no advantage to putting more nitrogen on. I think this notion will go away quickly.” Also, adds Heard, this year the high nitrate levels did not contribute to iron chlorosis in the demonstrations.

Nitrogen carry-over?

Another misconception that may also come from more southerly climates is that there will be a nitrogen carryover from soybeans that farmers can take advantage of, reducing the amount of nitrogen fertilizer for next year’s crop.

nitrogen on soybeans

These pictures show the impact of nitrogen on soybean root development. This plant had 100 pounds of nitrogen, as urea.
photo: John Heard, MAFRD

“That comes from the idea that soybeans are fixing their own nitrogen and they are leaving some nitrogen behind so when farmers are in a corn and soybean rotation, as they often are in Iowa and other parts of the U.S. corn belt, they may be applying 50 pounds less nitrogen on their corn. So there is a misconception that there is some kind of nitrogen credit but that’s not really correct,” says John Schmidt, a research scientist with DuPont Pioneer.

“It’s correct that they apply less nitrogen to their corn when they are following soybeans but the primary reason for that is because the soybean residue breaks down much faster than corn residue and so the nitrogen is not tied up.”

Soybeans always use whatever nitrogen is available, whether it’s residual nitrogen from previous crops or nitrogen that mineralizes from organic matter. “Once people have grown soybeans in northern areas they learn that there’s very little carryover of nitrogen, the lowest carryover of nitrogen of any crop according to the soil tests,” says Heard. “It’s because soybeans are so efficient at cleaning the system. If they don’t need to make nitrogen they won’t, they’ll use what’s there. What we found was there was always a lot less nitrogen left at the end of the year than what we put on because the plant used it.”

Salvage situation

Additional nitrogen is beneficial, says Heard, in a salvage situation, where a soybean crop has not been properly inoculated, which is crucial for nodulation. “Because we have so many virgin soybean fields we will have mistakes happen where a farmer doesn’t get the crop properly inoculated because the machinery plugs or something,” says Heard. “Farmers should regularly scout their fields and assess nodulation so they know whether they need to make a salvage operation, because if they do not have nodules the crop it will need to be fed with nitrogen.”

In another experiment a few years ago, Heard tested nitrogen application in a salvage situation and found that timing was important to achieve good yields. “In Ontario they say get the nitrogen on early during flowering and our observations were that it’s of more benefit to put it on a little later when the pods are filling,” says Heard. “That might simply be because we have better organic matter levels in our soil that will carry the plant through that early flowering stage, but the additional nitrogen is greatly needed for seed filling.”

It doesn’t appear that there are any economic or production advantages in Western Canada to additional application of nitrogen on soybeans, but farmers should soil test to find out how or if nitrogen levels are likely to affect a soybean crop. “If nitrogen levels are very high I don’t think we need to tell farmers don’t plant soybeans there, they just need to understand that if they’re going to plant soybeans there it’s likely to reduce nodule numbers,” says Heard. “Inoculation is still very important and now that crop margins are a little tighter maybe a crop that really needs nitrogen like canola or wheat would benefit better by being planted on that soil.”

This article first appeared in the Feb. 25 issue of Grainews.

About the author

Contributor

Angela Lovell

Angela Lovell is a freelance writer based in Manitou, Manitoba. Visit her website at http://alovell.ca or follow her on Twitter @angelalovell10.

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