Although the price of nitrogen fertilizer has fallen in the past year, the lower price of corn means that decisions about nitrogen management need to be made carefully, with an eye towards maximizing the return on investment for this important input.
The first question on nitrogen management is rate: How much nitrogen will the corn crop need, and how much of this will need to come from fertilizer?
“The generally accepted rule of thumb is that the crop will take up a total of about one pound of nitrogen for each bushel of yield. We’ve found a similar number in a few studies we’ve done,” says University of Illinois crop scientist Emerson Nafziger.
Not all of the nitrogen needed for the crop has to come from fertilizer, though; some of it will come from soil organic matter. How much the soil provides is related to soil depth and amount of organic matter, but it also varies by year, depending on weather and crop conditions. That makes the amount difficult to predict.
“In the deeper, higher organic matter soils in Illinois, we might see amounts of up to 200 pounds of nitrogen per acre available to the crop in a good year, while in shallower and lower organic-matter soils or in a year with cool, dry soil conditions this could be as little as 20 or 30 pounds,” Nafziger notes.
At current corn and nitrogen prices, studies over recent years have shown that corn following soybean in southern and central Illinois should be fertilized with about 175 pounds of nitrogen per acre, while in northern Illinois, where more nitrogen is present in the soil, this rate is about 150 pounds of nitrogen. For corn following corn, the rate that provides the maximum return to nitrogen is about 200 pounds of nitrogen per acre everywhere, but perhaps slightly less in southern Illinois.
Form, timing and placement of nitrogen fertilizer can affect nitrogen availability to the crop.
“Knowing the basics of how different fertilizer materials behave can only take us so far,” Nafziger says. “What happens to nitrogen in the soil that affects it availability to the crop is heavily dependent on weather. This means that our predictions regarding nitrogen form and timing are only about as good as our ability to predict the weather before the season starts.”
Still, nitrogen management can be improved with research over a range of sites and years. Nafziger and his research team initiated a large study in 2014 to look at the effect of nitrogen form, timing, and placement on corn yield. There were a total of 15 treatment variables in the study, but the nitrogen application rate was held constant at 150 pounds per acre.
Yield varied somewhat with the form of nitrogen applied. Dry forms of urea with Agrotain® and SuperU® applied at planting produced the highest yields, but yields obtained with urea ammonium nitrate (UAN) injected at planting and of anhydrous ammonia with N-Serve were also high.
The team also experimented with non-traditional application methods and timing, such as surface-banding UAN at planting and holding some of the nitrogen back until tasseling.
“While we saw some small differences among treatments, commonly used timing and forms of nitrogen all produced similar yields, even under what we would consider high-loss conditions with all the rain in June 2015,” Nafziger says.
Their results showed that both the risk of nitrogen loss and the benefit from delaying nitrogen application or using inhibitors were less substantial than expected.
“That provides some confidence that most of the nitrogen management systems in use today have good potential to provide the crop with adequate nitrogen. Adding costs by changing nitrogen management, for example by making another trip over the field to apply late nitrogen, may not provide a positive return compared to applying all of the nitrogen in one or two earlier trips,” Nafziger says.
Nafziger’s research is sponsored by the Nutrient Research and Education Council. More details and data are available.