What is in this article?:
- Higher nitrogen rates don't always equal higher corn yields in the South
- Several ways to lose nitrogen
- How much N and when
- High nitrogen rates might not be the best strategy for your corn crop.
- Southern corn growers must consider their environment when applying nitrogen.
- It's important to know how much nitrogen to apply and when to apply it.
HIGH NITROGEN RATES don’t always equal high corn yields, especially in the South where weather conditions can be variable.
Several ways to lose nitrogen
There are several different ways to lose nitrogen during the growing season, notes Larson. Volatility is when nitrogen is applied to the surface and then volatilized and lost to the atmosphere.
“It’s normally associated with urea-based nitrogen sources. The sandier the soil, the less likely you are to see de-nitrification because de-nitrification happens when you have saturated soils for an extended time period," he said.
The other type of nitrogen loss, he says, is leaching, and that’s more likely in sandier soils, when nitrogen moves downward vertically through the soil profile and gets out of the rooting Growers need to understand these limitations and manage around them, says Larson.
A number of things can affect nitrogen response, he continues. “A lot of focus is on nitrogen rate, but other factors such as fertilizer source, application method and timing and probably every bit as important as rate. These things must be integrated to produce the most profitable crop possible.”
It’s no secret, says Larson, that corn is responsive to nitrogen rate. “We’ve looked at rates from zero to 400 pounds of soil-applied nitrogen. We see a plateau when we get the rate up to a certain point. You’ll reach a threshold level where you’re not likely to see much of a response above that.”
A producer wants to apply the most conservative amount of nitrogen he can and still be at maximum yield or profitability, says Larson.
“In two out of five years of research, we made just as much corn with 160 pounds per acre as we did with a higher nitrogen rate. There was no difference probably 40 percent of the time. You’ll see a lot of variability from year to year in response to nitrogen.”
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Soil type is another significant consideration in terms of response to nitrogen, says Larson.
“We saw the most efficient use of nitrogen in lighter-textured soils, he says.
Mississippi State University’s nitrogen rate recommendations for corn call for 240 pounds per acre for clay soils that are producing optimal yields – about 175 bushels per acre.
“Two-hundred and forty pounds of nitrogen divided by 175 bushels per acre will tell you that your nitrogen rate should be about 1.3 pounds of nitrogen per bushel of yield potential. Using this same calculation with lighter soils, you can knock about 10 to 15 percent off your nitrogen rate.”
Nitrogen sources are another important consideration. “Every nitrogen source has strengths and weaknesses, and we need to know what those are to manage around them. Urea sources will volatilize in the soil surface, and there are a number of ways we can manage that," he said.
"We can inject UAN solutions. Urea will require some type of incorporation – either rainfall or tillage to do that. We also can minimize the amount of urea applied to minimize the risks associated with that product. We also can put out a urease inhibitor that will help to slow volatility, particularly during the two to three-week period after we apply it.”
Normally, any rainfall received to incorporate, especially early during spring, isn’t much of an issue, says Larson. “As we get into May and June and our rainfall events become longer, the threat of volatility certainly increases. We want to make sure that these nitrogen sources get into the soil and are available to the crop.”
The crop will respond to these nitrogen sources pretty much the same, he says, but growers must make sure that the nitrogen is available for the crop to utilize.
“Ammonium nitrate does not have the same volatility issues as urea, so it’s preferred if you want to apply it as a mid-season broadcast application or for dryland crops. If you don’t have irrigation to water in the crop, ammonium nitrate may be a much better option compared to granular urea.”
The nitrate form of nitrogen is much more subject to de-nitrification than urea, says Larson.