Nitrate leaching is a big problem in Nebraska, and it’s a challenge that a team of researchers from the University of Nebraska-Lincoln looked into with their “Integrated Best Management Practices to Minimize Nitrate Leaching in Corn” study, which took place over a two-year period in 2022 and 2023.
The study was conducted by Javed Iqbal and Swetabh Patel, assistant professor, University of Minnesota; Leslie Johnson, extension educator, University of Nebraska-Lincoln; Amy Millmier Schmidt, professor and livestock bioenvironmental engineering specialist, University of Nebraska-Lincoln; and Michael Kurtzhals, graduate research assistant, University of Nebraska-Lincoln.
Their research has yielded a number of interesting findings, including the fact that solid manure can significantly reduce nitrate leaching, making it a better option than commercially-available synthetic fertilizers when nitrate leaching is a concern.
“In Nebraska, we have been using more [synthetic] fertilizer since about the 1960s or ‘70s, and that nitrate has been contaminating the ground water,” says Javed Iqbal, one of the authors of this study. Speaking to Manure Manager during the 2025 Waste to Worth conference in Boise, ID, where Millmier Schmidt presented a poster on the research.
He explains that a large number of townships in Nebraska have groundwater that has been contaminated with nitrate, which means that they also have a lot of drinking water that isn’t fit to drink.
The authors of this study reported that, “in Nebraska, approximately 117 out of nearly 550 groundwater-based community public water systems are required to conduct quarterly sampling due to elevated nitrate-N levels, with ten systems having already implemented costly treatment measures such as reverse osmosis to mitigate this issue.”
The cause of this problem, as reported by the authors of this study, is clear: “The intensive production of row crops under irrigation in the state are a primary reason for elevated nitrate concentrations in groundwater.”
Moreover, nitrate leaching is a much bigger issue that affects water systems far beyond Nebraska’s borders. “The environmental impact of nitrate leaching from agricultural fields is not confined to Nebraska; it is a widespread issue across the U.S. Midwest, where intensive crop production is prevalent,” the authors of this study noted in their report.
Two-year study
The two-year study was conducted on drip-irrigated land where six treatments were randomly assigned to a total of 24 plots of land, employing a combination of two fertilizer sources (manure and inorganic fertilizer), two cover crops (rye cover crop and no cover crop), and two mulch treatments (woodchips of mixed species and no woodchips).
All the plots reeived the same total N rate: 30 percent of the total N was applied at planting in the form of Agrotain coated urea, and while the manure plots received the remaining 70 percent of N in the form of beef manure at planting, the inorganic plots received the remaining 70 percent of N in the form of UAN side-dressed at the V6 corn growth stage. In addition, researchers reported that, “the inorganic fertilizer plots received additional P, S, and Zn at the time of planting to balance the amount of these nutrients supplied by the manure.”
Researchers then collected data by analyzing soil samples, crop quality and yield, and water quality.
Key findings
The research resulted in a number of important findings, including the fact that solid manure significantly reduces nitrate leaching because it provides, “a slower, more synchronized N release compared to inorganic fertilizers.”
Solid manure was also beneficial when analyzing above-ground biomass N uptake, since manure improved biomass N uptake by 11 percent, compared with commercially available synthetic fertilizer.
“Unfortunately, many farmers still think of manure as a waste product,” says Iqbal, “so they just want to get rid of it and apply it on the surface. On top of that, they still apply nitrogen through synthetic fertilizers, which leads to excessive nitrogen input, and to nitrogen losses through gaseous emissions into the atmosphere and into the groundwater.”
Another key finding: “Woodchip mulch initially delayed N availability and biomass N uptake, but ultimately helped reduce nitrate leaching by improving soil moisture retention and temperature moderation.” Unfortunately, using mulch comes at a price. “Mulch decreased crop growth early in the season and reduced crop yield,” Iqbal says.
So there’s a tradeoff, Iqbal adds. “If you’re going to use mulch, it can immobilize some of the nitrogen on the soil surface and reduce nitrate leaching, but it will [negatively] affect crop yield.”
Researchers noted that although synthetic fertilizers boosted corn yields by 9%, compared to manure treatments, they also increased the risk of nitrate leaching. That said, there’s an important caveat to consider. “Compared with inorganic fertilizer, manure decreased crop yield by nine percent in year one, while in the second year there was not effect,” says Iqbal. “Manure had the same yield as the inorganic fertilizer [in the second year].”
So while the synthetic fertilizer improved yield in the first year, that difference was no longer there in the second year. “And in both years, manure decreased nitrate leaching by up to 80 percent, compared with the inorganic fertilizer,” says Iqbal. “That’s because manure releases nitrogen more slowly, and synchronizes nitrogen availability with the crop nitrogen uptake during the growing season.”
In other words, solid manure acts like a slow-release fertilizer, whereas synthetic fertilizer does not.
“Once they are applied to the soil,” says Iqbal, “synthetic fertilizers nitrify quickly into nitrate-nitrogen, and if we get early season rain, that nitrate will move through the soil profile into the ground water. That’s the risk you take with synthetic nitrogen fertilizer.”
Iqbal adds that farmers can invest in nitrogen stabilizers that will help synthetic fertilizers release nitrogen more slowly over time. Manure, on the other hand, doesn’t need a stabilizer, “because manure itself is a natural nitrogen stabilizer. It releases nitrogen slowly over time and takes care of any implications from rainfall,” he says. (For more on how manure functions with nitrogen inhibitors, see Page 14).
Having weighed all the pros and cons Iqbal concludes that, “Manure is your best option to keep up your yield and reduce nitrate leaching.”
Iqbal stops short of declaring manure a panacea for the US Midwest’s nitrate leaching problems. Even if every farmer in the Midwest stopped using synthetic fertilizer, and switched to solid manure, Iqbal says that certain conditions would have to be met in order for manure to fix the problem completely.
“If everybody uses manure the right way, then we would see a big difference,” he says. “If they apply manure to keep up with the crop’s nitrogen needs, if they apply it at the right time, and if they do not over apply it, then we could eliminate the nitrate leaching problem.”
The key to success, Iqbal adds, “is the four R’s of manure. The right source, the right time, the right place and the right rate. If we follow these four R’s of manure then we can make a big difference.”
Cover crops
Although cover crops were initially part of this research, according to the authors of this study, “cover crop failed to establish in 2023 spring due to dry conditions. Therefore, cover crop data and its effects are not reported in this paper.”
That said, Iqbal notes that other research projects have concluded that the right cover crop can reduce nitrate leaching significantly. However, researchers aren’t sure which combination works best. “We don’t know yet how a cover crop would perform with manure and with synthetic fertilizer,” he says. That will have to be the focus of a future research project.•
