How efficient is your nitrogen fertilizer — really?
Farmers invest heavily in nitrogen fertilizer, but not all of it makes it into the crop. Some is lost to the environment, tied up in the soil, or simply underutilized. Understanding how much nitrogen actually gets taken up by plants is key to improving efficiency — both for profitability and sustainability.
Measuring efficiencies
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The most common method for measuring nitrogen efficiency is the “difference method,” in which researchers compare two plots — one fertilized, one not — and calculate the difference in nitrogen uptake. This approach is simple, inexpensive and practical.
However, it assumes fertilizer doesn’t change soil nitrogen dynamics — an assumption which isn’t entirely accurate. Since soil nitrogen interacts with applied fertilizer in complex ways, this method can give a rough estimate, but lacks precision.
A far more accurate method involves tracking nitrogen itself. Using a stable isotope of nitrogen called N-15, researchers enrich fertilizer and trace its exact movement in the soil and plants. This “isotopic method” provides a direct measurement of how much applied nitrogen reaches the crop, rather than relying on assumptions.
To that, though, there’s also a downside: it’s incredibly expensive. A single acre’s worth of isotope-labelled fertilizer can cost over $170,000, limiting these studies to small research plots.
Despite the cost, isotopic research is uncovering valuable insights into nitrogen efficiency. At the CropConnect Conference in Winnipeg in February, Kelsey Griesheim, a North Dakota State University (NDSU) assistant professor of soil fertility, shared what her isotopic studies have revealed about nitrogen uptake and efficiency in cropping systems.
The bulk of Griesheim’s presentation centred on several studies that she conducted in central Illinois, prior to her tenure at NDSU.
Subsurface versus surface dribble
In one of those studies, Griesheim looked at nitrogen placement during sidedressing, comparing a traditional subsurface knife application to a Y-Drop surface dribble system.
The study tested four treatments. The first was broadcasting all nitrogen up front; the second was a 50/50 split with subsurface sidedressing; the third was a 50/50 split using Y-Drop; and the last treatment was a repeat of the third treatment.
“That’s not a typo,” Griesheim says, referring to that repeated treatment. “We wanted to know whether that first application more efficient than the second. By taking the difference between those two treatments, you can prise that apart and look into it.”
The results showed more nitrogen was taken up by the crop at the sidedress stage, meaning late-season applications were generally more efficient. However, under certain conditions, the subsurface application was more efficient than Y-Drop.
“If you have conditions conducive to ammonium volatilization, the subsurface application will be higher in efficiency than the Y-drop application,” she says.
Banded placement outperforms broadcasting
Griesheim also tested nitrogen placement at planting using Precision Planting’s Conceal system. The study compared four treatments: a single-band and a dual-band UAN application straddling the seed row, a surface dribble with a drag chain, and a broadcast application. All treatments applied 80 lbs. of N per acre.
“What we saw was that overwhelmingly, a banded placement was consistently more efficient than broadcasting,” Griesheim said. This result wasn’t surprising, since the banded application places the fertilizer closer to the roots.
However, neither the Conceal system, nor the drag chain application, nor the number of bands appeared to have any impact.
“There was really no difference there,” she says.
Nitrogen form matters
Griesheim’s final project examined different nitrogen fertilizer forms, all applied using Y-Drop placement. The study compared potassium nitrate, UAN and liquid urea, along with an untreated check. To ensure accurate results, potassium levels were balanced across treatments to prevent any yield response from added potassium.
“What we found for this project was that potassium nitrate was highest in efficiency, followed by UAN and then by urea,” Griesheim says, adding the trend showed fertilizers with more ammonium had lower efficiency.
Overall findings and future research
Her research in Illinois revealed interesting findings about nitrogen efficiency that could impact future research.
First, in a large number of the studies, Griesheim notes, treatment differences weren’t detectable by yield, but they were detectable by efficiency.
That’s encouraging, she says. “It means that we can have differences in our management practices that aren’t reducing yield but are increasing efficiency, which is where we want to be.”
Second, nitrogen loss was a major factor influencing efficiency. Whether through immobilization (where nitrogen gets tied up in organic matter and becomes unavailable) or losses such as through volatilization, her research shows more research is needed to understand how nitrogen moves in the system.
Another big takeaway was timing. Fall-applied anhydrous ammonia led to significant nitrogen loss before crops could use it. She noted, however, this is likely a factor of the warmer ground temperatures in central Illinois, and farmers in Canada probably wouldn’t see the same kind of losses. Nevertheless, it does indicate synchronizing fertilizer application with crop uptake has the potential to improve efficiency.
Finally, her results showed the soil — not the inputs — was providing the bulk of the nutrients for the crops.
“There was not a single time where the fertilizer was the major source of nitrogen for the crop,” Griesheim says. “It was always the soil. I find that really interesting.”
What that suggests is that nitrogen movement isn’t only driven by the placement of the fertilizer. It also suggests more attention should be paid to management techniques that look beyond 4R practices that focus on placement, to practices that influence how nitrogen is stored, released and lost in the soil.
That insight led Griesheim to her latest ongoing study at NDSU that is looking at the isotopic efficiency of nitrogen applications for conventional till and no-till cropping systems.
“We know from very extensive literature that tillage will impact carbon quality and quantity, as well as soil moisture and temperature,” she says. “Those are things that would definitely impact nitrogen.”
One year of that multi-year study has been completed.
