Source: Angie Peltier, UMN Extension Educator, Crops - Crookston Regional Office, 218-281-8686 a email@example.com
Climate plays a huge role in the fate of applied nitrogen fertilizer. The two primary loss processes of nitrogen in the soil are both water-based:
Leaching, when water percolates through the soil
Denitrification, when the soil is saturated
The conversion of nitrogen to nitrate, the form necessary for loss to happen, is a bacterial process that is dependent on temperature and moisture. With increasing temperature comes faster rates of nitrification. “Microbes respond to temperature … biological activity slows greatly when the temperature gets below 50 degrees,” Carlson said in the webinar. This is also why fall nitrogen application is not recommended when daily high soil temperatures at a 6-inch depth are greater than 50 degrees. Similarly, the process of denitrification, which converts nitrate into inert nitrogen gas, becomes significantly more important as soils warm during the growing season.
Extra water on the landscape is going to impact the fate of both naturally occurring (mineralized) nitrogen in the soil and applied nitrogen fertilizer. When precipitation falls in relation to crop growth and development is also important. During the early growing season, there is often more moisture in the soil than the newly seeded crop can use. This means that more water is likely to leave the field through drainage than evapotranspiration. The nitrate form of nitrogen is easily leached through the soil profile where it can also be lost through drainage tile. As the crop grows and develops, it uses increasingly more water, decreasing the amount of nitrate that ends up in drain tile.
The typical design criteria for an artificial drainage system in southern Minnesota is to use a ½ inch drainage coefficient, meaning that a ½ inch of water can be drained in 24 hours. As a general rule of thumb, nitrate will move 5-6 inches in silt or clay-loam soils for every inch of drainage. This means that a drainage system flowing at full capacity will remove an excess inch of water in two days, moving nitrate 6 inches lower in the soil profile. If you monitor water flow through the drainage outlet (in addition to knowing how much excess precipitation was received), you can estimate the amount of drainage. This can also help you figure out whether significant amounts of applied N have been lost. For example, if drainage tile with a ½ inch drainage coefficient is 3-feet deep and nitrogen was applied at a depth of 6 inches, after receiving 5 excess inches of rain, it would take approximately 10 days at full tile capacity for that nitrate to be lost to tile drainage. On average, water that is at the top of the soil profile will get to the tile by the end of the year.
The past two very dry years that some parts of Minnesota experienced have led to nitrate carrying over from one growing season to the next. This is due to unused fertilizer that the drought-stressed crop was unable to use, and because mineralized nitrate that accumulates after the growing season may still be present next year. One can see this in soil test data courtesy of Minnesota Valley Testing Labs from the fall of 2021 and 2022. More than 72% of samples in 2021 and 78% of samples in 2022 had 6 ppm or more nitrate, translating to a significant nitrogen credit that can be used to reduce fertilizer application rates for the next year. The pre-plant soil nitrate test is recommended this coming spring in areas that experienced drought where corn will follow corn, or where there is a manure application history.
Nitrogen guidelines changing with changing water dynamics
When the U of M’s corn fertilizer guidelines were developed in 1994, nitrogen management was different for different regions in southern Minnesota due to differences in excess precipitation compared with the evaporative demand of the crop. Western Minnesota (roughly west of US Highway 71) typically saw a water deficit during the growing season, making the likelihood of N loss minor. However, with the trend of recent, wetter growing seasons, the U of M is now suggesting that nitrogen be managed similarly in southwest and south-central Minnesota. To avoid significant losses, the data indicates that fall urea is no longer a recommended practice, and a nitrification inhibitor is suggested with fall anhydrous ammonia application.
The supply chain is pushing farmers to make N management decisions far enough in advance so as to allow for efficient movement and storage of fertilizer. Unfortunately, this is at odds with climate conditions that have been trending wetter, making it difficult to know what the best management is far enough in advance. Farmers are encouraged to know how conditions affect applied fertilizer as well as other factors such as crop growth and the ability to get into the field. Knowing your options and having the ability to switch to alternatives based on conditions will allow you to change management from one year to the next in order to maximize profit while minimizing the impact on the environment.