Farmers use nitrogen fertilizers in their fields to boost crop yields, but too much can be a waste of money. Up to a certain point, more nitrogen produces more crops and more dollars in profit. However, as crop plants approach their maximum productivity, each extra pound of fertilizer is less helpful. The sweet spot, where farmers balance fertilizer expenses against crop profit margins, is called the economically optimal nitrogen rate, or EONR. The EONR is dependent on the maximum nitrogen usage capacity for each specific type of crop.
Farmers also want to avoid using excessive fertilizer because it can damage the environment. For example, leftover nitrogen that plants don’t take up can be converted by soil microbes into soluble nitrate, which can be easily washed away by rain or irrigation. In the US Midwest Corn Belt, high nitrate levels contaminate surface waters and disrupt underwater ecosystems, killing thousands of fish and other wildlife.
Farmers maximize their profits by estimating the EONR for their fields so they don’t waste money on extra nitrogen fertilizer. However, the exact EONR is difficult to predict ahead of time. Many factors can influence it, including fertilizer costs, crop market prices, soil type, and precipitation. Because of this uncertainty, researchers wanted to know if the EONR guideline restricts fertilizer use enough to protect the environment as well as profits.
Scientists and farmers across 8 Corn Belt states, including Wisconsin, Indiana, South Dakota, North Dakota, Nebraska, Minnesota, Illinois, and Iowa, compiled data on cornfield fertilization to address this question. They also tested whether it matters if farmers apply nitrogen fertilizer all at once or split it into 2 doses over the growing season. Some farmers use this strategy to deliver fertilizer to the crops closer to when they need it, which shortens the amount of time nitrogen is left sitting in the soil.
The scientists monitored 2 to 3 field sites in each state for 3 years. They tested a range of yearly fertilizer levels, from 0 kilograms of nitrogen per hectare (kg N/ha) to 315 kg N/ha. At the end of each growing season, the scientists analyzed soil samples and whole corn plants collected at each plot. They measured how much nitrogen was taken up by the plants, how much leftover nitrogen remained in the soil, and how much was missing and unaccounted for at the end of the season, which could be released into the environment.
The researchers found that the more nitrogen fertilizer they applied, the more nitrogen the corn plants absorbed—at least up to the EONR. Above the EONR, more fertilizer corresponded to more leftover nitrate in the soil and more nitrogen unaccounted for without any increase in plant uptake. They suggested that this extra nitrogen poses a risk of environmental contamination.
However, the researchers also found that soil nitrates didn’t increase until they pushed fertilizer levels to about 70 kg N/ha above the EONR. At nitrogen rates below that, even those below the EONR, the amount of leftover soil nitrates stayed around the same. The authors concluded that staying at or near the EONR is generally sufficient to minimize excess soil nitrate.
While the plants took up similar amounts of nitrogen whether farmers applied fertilizer in 1 or 2 doses, the fate of the leftover nitrogen was different. More nitrates remained in the soil when researchers applied the nitrogen fertilizer in 2 doses (62 kg N/ha) than when they applied it all at once (42 kg N/ha). At the same time, more than twice as much nitrogen was unaccounted for after 1 dose (46 kg N/ha) than after 2 doses (21 kg N/ha). They suggested that this nitrogen was already lost into the environment during the growing season. The team concluded that split-application fertilization doesn’t necessarily protect the environment or enhance crop nitrogen uptake. Instead, it simply shifts when losses are most likely.
The researchers acknowledged that their study looked for broad patterns across the US Midwest, so the results may not apply to every site. Overall, the team offered a cautiously optimistic message: farmers who apply fertilizer around EONR levels to maximize their profits are likely minimizing negative environmental impacts at the same time. Lost nitrogen is detrimental both to profit margins and to the environment—so optimizing to cut waste is a win-win.
