Overapplying Nitrogen: When More Isn’t Better

Thursday, July 25, 2024

‘The relationship between N and yield is not simply a straight line.’

By Richard Kamchen

More isn’t always better when it comes to applying nitrogen fertilizer – agronomically or financially – certainly not in the face of federal goals to slash nitrous oxide emissions.

“In some cases, a little less N may be more,” says Colin Elgie, OMAFRA soil fertility specialist.

He explains nitrogen (N) fertilizer plays a critical role in farmers’ cropping systems, particularly in grain crop production.

“When used at the proper rate, nitrogen fertilizers promote increased growth and yield, and improved profitability,” Elgie says. “However, overapplication of N fertilizers can and does occur, as the relationship between N and yield is not simply a straight line, where more is always better.”

Different factors illustrate why adding more N to crops may not necessarily be in a farmer’s best interests.

Agronomic optimization

Elgie says grain crops in Ontario respond well to N fertilizer, but that the relationship has its limits, as increasing rates of N applied to a crop will eventually reach a yield plateau.

“At this rate, additional N fertilizer will not increase yield, and if applied at exceptionally high rates, can decrease yield,” he says.

Issues such as delayed maturity and stalk lodging can reduce quality and harvestable yield. The effects are easily seen in winter wheat, for which high rates of N, coupled with early summer storms, can lead to flattened fields prior to harvest. Impacts are also evident in corn yields.

Alfons Weersink, an agricultural economist at the University of Guelph, says in terms of corn’s response to N, yields increase at a decreasing rate until a maximum yield is hit, and then it essentially plateaus.

“It does not tend to fall within a reasonable range. While yield may not be lowered with rates beyond that associated maximum yield, it is not profitable to do so,” Weersink says.

Josh Nasielski, an agronomist at the University of Guelph, adds that for corn, taking up N costs carbon (or sugar). The plant needs to expend photosynthate to assimilate it from the soil to a usable form. Corn won’t expend energy taking up much more N than it needs: It normally has a small reservoir of N, but the amount is small, he says.

“So, from the perspective of more is better, the corn plant itself may not even be using the extra N that was applied,” Nasielski says.

This leaves excess nitrate in the soil, which is vulnerable to losses to the environment, notes Saskatchewan agronomist Edgar Hammermeister at Western Ag Professional Agronomy.

He adds that what’s underappreciated and largely not understood is the back side of the N yield response curve, where each additional pound of N starts taking away from yield. How steep that decline is relates to a growing season’s weather conditions.

For instance, in a wet season, crops could experience lodging if they’ve been overfertilized.

“The implication of the kinked stems is moisture and nutrients can’t be delivered to fill the kernels, resulting in a poorer quality crop,” says Hammermeister, who also notes the greater harvesting challenge.

In a dry season, however, overfertilizing could result in “haying off,” particularly in the brown soil zones of Saskatchewan and Alberta.

Haying off refers to a crop growing too aggressively at the start of the season, and thereby depleting its water reserves before it can set yield. Having provided such a crop more N than it needed could amplify those effects.

Economic optimization

“From a farmer perspective, there’s a loss of profitability, because he’s spending money on inputs that aren’t necessarily going to give him a return on investment,” Hammermeister says.

Elgie explains that the ideal scenario is to choose the rate of N where $1 of input equals $1 of output, at a point known as the Most Economic Rate of Nitrogen (MERN). Applying N at a rate higher than MERN means spending more in fertilizer input than you will make up for in yield, he says.

“While maximum yield may be the focus of yield contests, it may not be in the best interest of the farm business to target that maximum yield with relation to their N rate decision,” Elgie says. “Nitrogen fertilizers are expensive inputs, and excessive use can reduce the return on investment of these products.”

Weersink notes that one dilemma for farmers trying to determine optimal rates at the beginning of the cropping season is that MERN varies significantly across years, depending on weather conditions, which are unknown beforehand.

Nasielski says that some farmers will apply more than they need to in order to compensate for environmental N losses, with the logic being that rainfall could leach some N away.

“I understand this thought process, but it doesn’t always work that way. Generally, losses increase the more N that is applied,” Nasielski says.

Environmental optimization

Avoiding N losses can not only help improve profitability, but also reduce environmental impacts, Elgie says.

He warns that overapplication of N poses significant environmental risks. At rates higher than MERN, uptake of additional N fertilizer decreases, and every additional kilogram of N has a higher potential for loss.

“Excess N is prone to losses via volatilization, leaching, and denitrification,” Elgie says.

“In Western Canada, the leaching is less of a concern,” adds Hammermeister, “but there can be denitrification losses, and depending on the moisture circumstances, it might be a small loss (or) quite a significant loss.”

Denitrification leads to losses of nitrous oxide (N2O), a powerful greenhouse gas, approximately 298 times more potent by weight than CO2, Elgie says.

For its part, the federal government wants to see N2O emissions from farms reduced 30 per cent by 2030.

Although Ottawa’s goal isn’t mandatory, Hammermeister worries that that might change if the sector faces years when farmers are powerless to stop Mother Nature from forcing N losses.

One way to mitigate that or avoid overapplication in typical years is to determine the right rate.

Soil testing is key, but they’re not all equal or the same, and the wrong test could miss N mineralization, causing overapplication of N, which would then get released into the environment.

Elgie explains that as microbes break down soil organic matter, N is released in a process called mineralization.

Measuring this mineralization can be done by collecting a Pre-Sidedress Nitrate Test (PSNT), the results of which can assist in finetuning N application rates, Elgie says.

Hammermeister points out PSNT hasn’t been used in grains and oilseeds in Western Canada because that region’s “100-day” crops grow too aggressively early on.

Elgie also recommends 4R practices, such as split-application, injection or incorporation of N fertilizers, and use of N stabilizers such as urease or nitrification inhibitors, to reduce N losses.

Explains Weersink: “Splitting application allows one to assess growing conditions and adjust rates accordingly. Inhibitors slow the release of N so its availability better matches the needs of the crop and less is lost.”

Birch Effect

Complicating N management decisions in Western Canada have been recent droughts and/or very dry falls and their impact on how soil biology mineralizes organic matter.

In scientific literature, the process is beginning to be nicknamed the “Birch Effect” after a researcher who started publishing his observations back in the 1950s.

Hammermeister explains that when soils go through an extended period of drying, there are some physical and biological changes that occur that can result in a N mineralization burst. Triggering the burst is warm soil and moisture.

“The degree of nitrate release relates to the duration of the drought and the organic carbon (OC) content of the soil.

“The greater the drought duration and the greater the OC content of the soil, the greater the mineralization potential,” Hammermeister says.

Portions of Western Canada have seen consecutive dry growing seasons and/or very dry falls the last few years, and the Birch Effect is resulting in a large release of nitrogen from fields that have been in zero-till for 15 to over 30 years, he says.

“If farmers are not accounting for this plant-available nitrogen, they could easily be over-fertilizing their crops to the detriment of yield, profitability, and the environment,” Hammermeister says. BF