Next time we might not be so fortunate, warns Farm Journal Field Agronomist Ken Ferrie. The perfect storm that wasn’t shows how critical it is for farmers to be the best possible nitrogen stewards.
“The dangerous situation was caused by unusual weather from October 2015 through June 2016,” Ferrie explains. “Those conditions made it harder for some nitrogen inhibitors to do their job of holding nitrogen in the stable ammonium form. If ammonium converts to nitrate, it can move from fields to water supplies.”
Nitrate levels of 10 ppm or more in drinking water are hazardous for pregnant women and young children.
Here’s what happened: By the end of October 2015, many acres in the Midwest were abnormally dry or suffering moderate drought, according to the drought monitor maintained by the University of Nebraska-Lincoln, USDA and the National Oceanic and Atmospheric Administration.
Then soil temperatures refused to follow the usual pattern, such as 2014, of dropping below 50°F and staying there. “It was Nov. 22 before soil temperatures fell below 50°F in many areas,” Ferrie says. “By Nov. 26, they rose back up above 55°F. They continued to move up and down across that line until Dec. 27, when they finally stayed below 50°F.”
When soil temperature falls below 50°F and stays there it’s safe to apply anhydrous ammonia. At higher temperatures, soil microorganisms remain active and convert the stable ammonium form to nitrate, which can be leached out by water.
When soil temperatures reached 50°F, some farmers began to apply nitrogen. “No one anticipated soil temperatures above 50°F would return in November and December,” Ferrie says.
Responding to the warm soil temperature, microbes began converting the fall-applied ammonium into nitrate. The industrious microscopic creatures didn’t stop there—they also broke down and converted naturally-occurring organic nitrogen, which had been stored in the soil, into nitrate.
“Even fields where no nitrogen fertilizer had been applied showed an uptick in nitrate levels,” Ferrie says. “As we analyzed soil in various fields, we often found 7 ppm to 10 ppm nitrate levels in fields following either corn or soybeans.”
Continuing to set the stage for disaster, along came the wettest December in several years. “Rainfall ranged from 5" to 10" during this period,” Ferrie says. “Dry soil was now saturated. If the ground had been frozen, the water would simply have run off. But because it was able to infiltrate into the soil, it began flushing nitrate out through tile lines.”
At this point, nitrate levels in some Illinois lakes that serve as municipal water sources began to climb—at a time of the year when concentrations normally are low. “One city had to treat water for nitrate in January for the first time ever,” Ferrie says. “Others became concerned as they watched nitrate values climb.”
Farmers in those areas, who understood the municipalities’ concerns, became worried, too. Ferrie analyzed soil in the fields of clients who had applied fall nitrogen to determine how much of the stable ammonium had been converted to unstable nitrate.
“The good news was we could account for most of the nitrogen applied in November,” Ferrie says. “What had been lost was nitrate that had been mineralized from the organic form by soil microbes prior to December, when soil temperatures were still warm. Instead of finding the normal 7 ppm to 10 ppm in fields that had not been fertilized, we found 2 ppm.”
In other words, it was the naturally mineralized nitrogen showing up in municipal water supplies—not the nitrogen farmers had applied.
Testing conducted from December through February revealed fields contained far too much nitrate in December and January. He continued to test soil every two weeks from February until May 23 to see how much applied nitrogen remained and how much was in the ammonium and nitrate forms.
Ferrie also studied the amount of nitrogen applied and rainfall. “We found the amount of nitrogen remaining in the ammonium form was influenced by two factors: moisture content of the soil when anhydrous ammonia was applied and the rate it was applied,” he says.
Soil moisture content is important because, when ammonia gas is applied to soil, each ammonia molecule (NH3) must pick up a hydrogen ion to convert itself into stable ammonium (NH4). Those hydrogen ions come from water in the soil. The ammonia gas continues to move in the soil until it finds a sufficient quantity of hydrogen ions.
The effect of dry soil was evident to farmers who could smell ammonia in their fields a day after they applied it. With insufficient moisture, ammonia gas was escaping into the atmosphere. “I don’t think my nitrapyrin inhibitor is working,” farmers told Ferrie.
They were confused about what their nitrogen stabilizers were expected to do. “Nitrapyrin nitrogen stabilizers slow nitrification [conversion from ammonium to nitrate], but they have no effect on volatility,” Ferrie says. “Lack of soil moisture can reduce their effectiveness. Nitrapyrin is kind of like a cocklebur, in that it sticks to soil near the injection site. But, in dry soil, the ammonia gas travels away from the injection site, escaping from the inhibitor that was supposed to protect it. That resulted in higher conversion of ammonium to nitrate.
“In fields that got rain in October, before nitrogen fertilizer was applied, the inhibitor performed much better than it did in dry fields,” Ferrie summarizes. “In the dry fields, the more ammonia we applied, the farther it had to move to find enough water. The larger the diameter of the ammonia core, the farther away from the inhibitor the ammonia ended up.”
In previous years, with normal rainfall and cooler soil temperatures, the performance of nitrapyrin nitrogen stabilizers was excellent, Ferrie notes.
In early spring, all the factors were in place for a perfect storm: Nitrification inhibitors often were ineffective because of dry soil; warm weather caused nitrifying bacteria to convert stable ammonium fertilizer and organic nitrogen in the soil into nitrate; and rainfall had moved nitrate deeper in the soil profile. All that was needed for it to enter tile lines and water supplies was rain in April, May and June.
“The crews of municipal water plants and farmers who understood the situation were on pins and needles,” he says. “But spring rains didn’t fall; by the end of June, we were worried about drought. It rained in July but by then the crops had taken up the nitrate in the soil and disaster was averted.”
- Nitrapyrin inhibitors attach to soil particles at the point of injection. “They work well as long as the ammonia doesn’t move too far away before converting from gas to ammonium. That depends on the ammonia rate applied and the amount of soil moisture,” Ferrie explains.
- “If you want to apply nitrapyrin inhibitors on the soil surface, you must use an encapsulated form,” Ferrie says. “Without it, the inhibitor will volatilize and be lost. The encapsulated form will volatilize eventually, but it will give you time to incorporate. You still need to incorporate urea sources because encapsulating nitrapyrin prevents the inhibitor from volatilizing, but it doesn’t prevent the urea from being lost through volatilization.”
- Another popular nitrification inhibitor is the DCD-type, which can be applied to the soil surface because they’re not volatile, Ferrie says. “But a heavy rain can carry the inhibitor deeper into the soil than the ammonium nitrogen because the inhibitor is slightly more water soluble than ammonium,” he adds.
- With urea, the risk is not only nitrification but also volatilization. Preventing volatilization requires a product that inhibits the urease enzyme. “We still occasionally find farmers using the wrong type of inhibitor,” Ferrie says. “Using the wrong one can be a waste of money and a source of risk to the environment.”
- With fall application, always consider the relationship between the rate of fertilizer and soil moisture. The higher the rate applied, the more soil moisture is required to enable the nitrification inhibitor to work.
- Don’t apply anhydrous ammonia the first fall day the soil temperature drops below 50°F. Wait until you’re confident it’s going to stay below 50°F.
- Have a backup plan in case the weather deteriorates before you can safely apply fall nitrogen. “If it gets wet and freezes we must wait until spring or sidedressing,” Ferrie says.
Industry Teams Up to Protect Water SuppliesThanks to a nitrate-monitoring program funded by Illinois farmers, municipalities that could have been affected by high nitrate levels described in the adjacent story had plenty of warning. The advance notice allowed them to gear up treatment plants and protect their citizens’ drinking water supplies.
The program, called N-Watch, is funded by a tax paid by farmers and retailers on every ton of fertilizer purchased in Illinois. The funds are used by the Illinois Nutrient Research and Education Council (NREC). A grant from NREC pays for the Illinois Fertilizer and Chemical Association (IFCA) to operate N-Watch.
“Through N-Watch, IFCA tracks soil conditions, weather and available nitrogen and its forms at locations throughout the state,” explains Isaac Ferrie of Crop-Tech Consulting. “Locations are sampled multiple times each year, down to the 2' depth, to determine ammonium and nitrate levels in the soil profile.”
IFCA also conducts nitrogen trials examining rates, timing and application practices to help farmers find the most efficient, effective and environmentally friendly ways to apply nitrogen, Ferrie adds.