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How much N? A soil test isn’t the only answer

Different crops, different varieties, different inoculants, and different soil environments all affect the N-application decision these days

Kristen MacMillan says interactions such as bean type, variety, inoculant type and environment can affect how edible beans interact with the N in the soil.

You need to know two things to calculate your nitrogen application — the crop requirement and the available N in the soil. At least that used to be the case, but it’s not now, not since pulse crops have become such an important part of the rotation.

When it comes to fixing N, not all pulses are created equal, University of Manitoba resident agronomist Kristen MacMillan said at a field day at Portage la Prairie, Man. in July. Fababeans are the most efficient N fixers, able to provide 80 to 90 per cent of their own requirements. Soybeans and peas are somewhere in the middle, fixing 60 to 80 per cent of their N needs.

Dry beans are the “lazy” legumes, typically fixing less than 50 per cent of their N requirement, so it’s assumed they need to be fertilized. But recent research by MacMillan and others is showing that interactions such as bean type, variety, inoculant type and environment can affect how beans interact with the N in the soil.

Research into N fertilization on pinto beans in the 2000s was based on older varieties, yet newer varieties like Windbreaker and T9905 have higher yields and respond to inoculant differently.

“Current recommendations are about 100 pounds of soil and fertilizer N for a yield goal of 2,400 lbs./ac., but we can easily achieve 3,000 and 4,000 lbs./ac. in some years,” said MacMillan. “Production practices have also changed. We have more narrow-row production and less inter-row cultivation so we have even less mineralization happening to provide the beans with N.”

So what’s the right rate?

MacMillan is updating N recommendations through a project with plots in Carman, Man., and Portage la Prairie. And although there are still more questions than answers, she does have some observations about nodulation.

“Similar to soybeans, if there is free nitrate in the soil, edible beans are going to take that up instead of investing energy into nodulation,” said MacMillan. “In the first year of the experiment we had significantly higher nodulation with the zero-N beans compared to the higher rates of N fertilizer, so we’re seeing that hindrance on nodulation with fertilization.”

These plots demonstrate options for managing N for yield and protein in spring wheat. photo: Angela Lovell

Interestingly, there was no yield response to applied N in the Windbreakers and T9905s, which produced a 3,000-lb. crop with no added N. It could just have been an anomaly of that year, which is why MacMillan is repeating the experiments, but it could also have something to do with the difference in varieties.

Another possible explanation could be that as beans have evolved over the past 18 years, strains of inoculant in the soil may have changed and potentially work better at fixing N than they did in the early 2000s.

Newer wheat is more N efficient

For nitrogen on spring wheat, the usual recommendation is 2.5 pounds of N per bushel for target yields, so an 80-bushel yield needs 200 pounds of total N (soil and applied). With new varieties of wheat yielding up to 120 bu./ac. in trials, that would mean 300 pounds of total N.

But newer varieties are more efficient with their N and can still give good profitability at fertilization rates of 2.0 lbs./bu., University of Manitoba soil scientist Don Flaten said in his presentation at Crops-A-Palooza.

A series of his trials with U of M research agronomist Amy Mangin averaged two pounds of N per bushel, with anywhere between 50 and 140 pounds of N applied as fertilizer providing the optimum combination of yield and protein premium. Some work was also done on placement and timing.

“What Amy found was that if she did a split application with some at seeding and some at stem elongation, she got just as good a yield response and a little bit more protein as well,” said Flaten. “If she delayed it all the way to flag leaf, she got just as much yield and even more protein. If she went with a supplemental application of 30 pounds of N after flowering, late in the growing season, she didn’t get any yield increase at all because applying the N that late wasn’t able to influence yield but it did increase protein by an average of almost two per cent. So there are lots of different things you can do with N in spring wheat.”

Why corn needs less N

Why apply less N on corn than wheat? Because corn taps soil reserves more efficiently, said Manitoba Agriculture soil specialist John Heard.

“The mineralization of N from soil or residue decomposition better matches the uptake pattern of corn,” he said. “Corn is slow and steady over a long period. Wheat has taken up all its N by the time corn has only taken up half.”

Heard said corn is far better able to exploit this “summerfallow” N that’s between the corn rows and is probably much better able to exploit in-season mineralized N.

“In fact, in some of our studies, we’re finding upwards of 150 pounds of N being mineralized.”

As a rule of thumb, it takes 1.2 pounds of N to grow one bushel of corn, Heard said.

“At our sites producing 125 bu./ac. or less, it’s taking about 1.2 pounds of N and sites yielding 150 to 230 bu./ac., it’s about one pound. In higher-yield production systems, if the environment in soils and management is appropriate for higher yields the N is used more efficiently,” said Heard.

Corn growers have always been able to apply N at various times in the growing season. A recent survey of Manitoba corn growers found that a third put all their N on in the fall, usually as banded ammonia. Another third fertilize most of their N in the spring before planting as broadcast urea. Of the others, 15 per cent are side banding most of it at seeding and another third are putting the majority on in-season as a side dressing or dribble application. “There is no one system that all farmers are using,” said Heard. “There are many good ways to apply split N.”

Reducing losses

A number of products are on the market that can help reduce denitrification, including ESN, a plastic-coated urea granule that responds to temperature and moisture and releases N slowly over time.

“A producer can apply this all in one operation and get some release early in the season and some release later in the season so it can influence protein and protect against N loss,” said Ray Dowbenko, senior agronomy specialist with Nutrien. “If planting corn further along in the season, this product would go in a blend, usually about 80 per cent of a controlled-released product with regular N. You wouldn’t have to side dress through the season or apply N. It would be there to provide some early-season release but the majority later in the season to provide the yield for corn.”

Other products such as nitrification inhibitors and urea inhibitors also offer options to apply N in season while protecting against losses, but Dowbenko said they are not excuses for bad management.

“We’d rather that you apply the right rate of fertilizer, at the right time, in the right place, using the right source for nutrient stewardship. These products are there if you have to make a choice that’s outside proper agronomic decisions.”

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Angela Lovell

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