Soil test to determine how much residual nitrogen remains in your fields

GRAND FORKS, N.D. — Soil tests in the northern Plains in the fall of 2022 showed that fields had lower amounts of nitrogen left over from that year’s growing season than there was the previous year's fall, a soil scientist says.

“It’s a big difference from the drought of 2021,” John Breker, Agvise Laboratories Inc., soil scientist, told farmers at the International Crops Expo in Grand Forks, North Dakota, on Feb. 22, 2023.

Agvise Laboratories Inc ., which has laboratories in Northwood, North Dakota, and Benson, Minnesota, and a receiving station in Canada, does soil testing and provides other agronomic services and support to customers including farmers, crop consultants and agronomy retailers.

Nitrogen levels in 2022, a year that had both wet and dry weather, were more typical, in contrast to 2021, when it was extremely dry, Breker said.

The average amount of nitrogen left over after grain and row crops were harvested in 2022 was from 25 to 30 pounds per acre. That's significantly less than in 2021 when there was as much as 80 pounds per acre of residual nitrogen in fields, Breker said.

The differences in the amount of residual nitrogen in 2021 and 2022 illustrates the need for farmers to soil test so they buy the proper amount of nitrogen fertilizer — not over-applying the fertilizer in some fields and under applying in others, he said.

"Rules of thumb show that if a zero- 24- inch soil profile has 30- to 60-pounds of nitrogen (per acre) after wheat, then the crop likely had enough of the fertilizer to meet crop yield and grain quality," Breker said. "Residual soil nitrate of less than 30 pounds likely had limited grain yields or impacted grain quality."

"If the amount of residual nitrate was more than 60 pounds an acre, the crop likely attained maximum yield, but also left more nitrogen in the soil profile vulnerable to loss," he said.

While fall application of nitrogen is most effective, dry conditions in the fall of 2022 made that difficult, Breker said.

Growers who are applying nitrogen in 2023 may want to consider a split application during the growing season instead of applying the entire amount in the spring. The split applications are one of the ways to insure against nitrogen loss and to make optimal use of the fertilizer, Breker said.

Subsurface or band placement is preferable to surface application, he said. The band application reduces ammonia volatilization. Volatilization is the loss of nitrogen through the conversion of ammonium to ammonia gas, which is released to the atmosphere. Meanwhile, the depth at which the fertilizer is incorporated also is important.

Nitrogen loss inhibitors have potential to reduce nitrogen loss, some products decreasing the losses by days and some by weeks.

For example, urease inhibitors, which can slow the conversion of urea to ammonia, reducing the amount of ammonia loss after surface application, is effective for days. Nitrification inhibitors which slow the conversion of ammonium to nitrate, reducing nitrate loss to leaching, is effective for weeks.

Similar to herbicides, the level of effectiveness of the nitrogen loss inhibitors is dependent on which brand is used, so farmers should do thorough research on them before they purchase them, Breker said.