Two summers ago, the worst Midwestern drought in a century reduced the Gulf of Mexico dead zone to one of the smallest in at least 50 years, due to reduced nitrogen runoff. But by May 2013, heavy Upper Midwest spring rains had delivered an estimated 153,000 metric tons of nutrients to the Gulf, nearly twice the previous year's level (NOAA data). By August, the dead zone had doubled in size to 5,800 square miles from 2012, according to U.S. Geological Survey estimates.
To many environmentalists, the numbers argue the need for regulatory restrictions on nitrogen. To Cliff Snyder, nitrogen director, International Plant Nutrition Institute (IPNI), it represents wasted resources economically and environmentally. With margins tight to nonexistent, the time may never be better to make needed changes. "We need to look at lost value at the farm gate and environmentally," he says.
Finding the win-win economic solution for growers and society is the goal behind long-term nitrogen studies at Purdue University and at the University of Minnesota's Southern Research and Outreach Center (SROC). Both express the same reality: Corn and soybean production are leaky systems. No system has yet been devised to produce complete utilization, nor is it likely. "No one system will work across the Corn Belt," says Jeff Vetsch, assistant scientist, SROC. "There is no easy answer or solution. "
Purdue University researchers have gathered eight years of data from field scale trials at more than 200 sites across Indiana. As in the 15-year Minnesota research effort, they emphasize understanding the Agronomic Optimum nitrogen Rate (AONR) and the Economic Optimum nitrogen Rates (EONR).
"Whenever corn prices drop and fertilizer doesn't follow as quickly, growers pay more attention to economically optimum rates,” says Bob Nielsen, Purdue Extension corn specialist and agronomy professor.
Of course, identifying the optimum rate lies in the conversion of ammonium and nitrates from organic matter through mineralization, rainfall and from applied inorganic fertilizer. Managing the process to get the greatest economic value from the investment at the lowest risk of loss and environmental degradation depends on soils, organic matter, rainfall distribution, temperature range, tillage and cropping management systems.
Even within a particular region or farm, agronomically optimal nitrogen rates can vary substantially. At a West Lafayette, Ind., site, the average is 184 pounds per acre. However, it has varied from 130 to 221 pounds per acre in individual years, depending on soil-N supply, fertilizer nitrogen loss and growing season weather.
Both the agronomic and economic optimum nitrogen rates can vary considerably depending on corn prices and nitrogen costs. In the matrix for northeast, east-central and central Indiana with a 217-pound agronomically optimum nitrogen rate, the EONR for $4 corn ranges from 206 pounds per acre at 20¢ per pound of N, down to 161 pounds per acre nitrogen at $1 per pound of N.
Use of microbial inhibitors
The University of Minnesota research clearly defines the benefit of using microbial inhibitors with anhydrous ammonia applications in the fall and the advisability of spring application when possible. With Minnesota's frozen winter soils, adding a nitrification inhibitor to fall-applied anhydrous increased corn yield and N-use efficiency. Over the 15 years of research, fall application with an inhibitor averaged equal yields to spring application without. However, when spring was better, it was significantly better. "In seven (wet springs) of those 15 years, spring application without an inhibitor averaged a 12-bushel increase over fall application with an inhibitor," says Vetsch.
The writing is on the wall for more nutrient regulation, Nielsen thinks. "We hope this kind of research helps identify ballpark ranges of optimum nitrogen rates for optimum yield, while reducing over-fertilization," he says.
If growers wait for concerns about nitrate movement into groundwater from California to the Chesapeake Bay, we may face a "broad-brush" approach that doesn't reflect real-world variability, says IPNI’s Snyder. “Now is the time to get ahead of the game.
"If we work locally to minimize nitrogen losses and increase utilization, we'll get benefits downstream and the dead zone and other concerns will take care of themselves," Snyder says.
Tools to tame N-response variability
Purdue researchers recommend these management tools to calibrate nitrogen rates that are the most profitable and agronomically optimal:
- Apply nitrogen shortly before planting or in a sidedress program, closer to nitrogen uptake/
- The most effective application is to inject nitrogen before rapid crop uptake at the V6 stage.
- Use the pre-sidedress nitrate test to estimate soil nitrogen supply in manured fields or in high-organic-matter soils
- Use a chlorophyll meter or active sensor in conjunction with a high-N reference strip during the growing season to evaluate crop nitrogen status.
- Nitrification and urease inhibitors with early spring application, before planting or anytime it will be two months or more before the crop will use the N, are effective.
- An end-of-season stalk-nitrate test to determine if the "right" rate of nitrogen was applied for the season.
These tools, combined with current best management practices, can only improve nitrogen management. Important as that is in the short term economically, it may be even more important in the long term.