Fertilizer decisions haven’t gotten any simpler as farmers work to balance input costs and nutrient stewardship without skimping on yields.
Iowa State University Extension Soil Fertility Specialist John Sawyer said farmers can use the Iowa State University Department of Agronomy Corn Nitrogen Rate Calculator to determine the most profitable N rate, adjusted for their nitrogen and corn prices.
Sawyer said the calculator, developed in 2005, is based on nitrogen response trials from seven land grant universities across the Midwest.
“We update the database for Iowa almost every year. If there are any changes, it’s a reflection of how the research results have changed.”
The suggested nitrogen rates from the calculator, both the Maximum Return to Nitrogen (MRTN) and most profitable range, are specific to each state or sub-state region. Users pick the state they are in and their specific region of the state. For Iowa, rates are based on research from Iowa.
These economic rates also produce near maximum yield, he said.
After several wet seasons in recent years, the suggested nitrogen rates for Iowa have gone up a little, but not drastically, Sawyer said. In addition, the southeast region of Iowa was broken out as a separate region in the calculator.
The N rate calculator, available at cnrc.agron.iastate.edu, works for both fall and spring applications as well as various nitrogen products, including anhydrous, UAN and urea, Sawyer said.
Despite a dry growing season in many parts of Iowa this summer, most producers should still use the MRTN rate or profitable range next year. If conditions stay dry, corn fields with depressed yields can have increased residual nitrate and more carryover than normal for the 2018 corn crop. Therefore, some producers may be able to use the lower bound of recommended range, or account further for the additional carryover nitrate, he said.
However, it is still too early to tell if spring rains will reduce any residual N, Sawyer said. After dry conditions in 2012, spring rains leached residual N so it was unavailable for the 2013 crop.
Farmers interested in reducing N rates due to dryness this summer could wait until spring to decide on applications or can apply a lower rate this fall and adjust rates with side dress applications next spring based on estimated N carryover, Sawyer said.
Profile soil sampling for soil nitrate in the spring is the best way to determine carryover N amounts. For a quick estimate, farmers can subtract the 2017 corn yield in bushels per acre from the total N applied to the 2017 crop and divide by 2. The remainder can be subtracted from the 2018 recommended N rate.
For more details on estimating residual N, Sawyer’s latest Integrated Crop Management post delves into this topic at https://tinyurl.com/yca7cu7y.
For fall applications, farmers may want to check that dry soils are sealing properly over ammonia bands during and after injection. While this typically isn’t a major issue, farmers may need to evaluate injection depth and soil coverage on a site-to-site basis, Sawyer said.
As always, farmers applying fall anhydrous are recommended to wait until soil temperatures are 50°F at a 4-inch soil depth and falling, he said.
Nitrification, the oxidation of ammonium (NH4+) to nitrate (NO3-), occurs at temperatures above 32°F and is maximized near 86°F.
Nitrification inhibitors, such as N-Serve, which utilize various nitrapyrin formulations to inhibit an enzyme pathway in the bacteria responsible for nitrification, thereby slowing the loss of ammonium, were discussed at Dow AgroSciences’ Aug. 10 Field Day in Cambridge, Iowa.
Mike Wright, who farms around 2,500 acres with his father, Steve, said he participated in a 2014 N-Serve strip trial with Key Cooperative and Dow AgroSciences. Three fields with three strips each had soil samples taken and ammonium and nitrate levels collected. His yields that fall showed a 5 to 6 bushel per acre advantage with N-Serve across the three trial locations on his farm, Wright said.
Wright also uses cover crops for erosion control. He said his farmland is characterized by a mix of heavy river bottom soils, lighter terraced soils and sandy loam ground. Now, any fall N applications he makes “get N-Serve for sure,” he said.
Wright uses a Raven Sidekick unit on his anhydrous bar and variably adjusts N rates through the field.
“I think technology has brought us a long way to be able to do stuff like this,” Wright said.
He said being able to variably apply with the Sidekick unit and check georeferenced yields in the combine allows him to make his own comparisons.
Eric Scherder, Dow AgroSciences field scientist, also discussed the use of Instinct HC as a nitrification inhibitor in manure applications. Scherder said the product can be added directly into manure pits just prior to agitation. He explained a “good, complete agitation” is needed for uniformity in the pit so the product’s value proposition is realized.
Sawyer said nitrification inhibitors have shown positive responses in some university trials, but not always. It’s important for farmers to understand that without N loss conditions, the nitrification inhibitor won’t make a difference.
These products should be considered along with late fall applications to give farmers an increased chance for success with fall nitrogen applications, he said.