Many growers automatically equate higher nitrogen rates with higher corn yields, but that’s not necessarily the case.

“Corn is very responsive to nitrogen, but high nitrogen rates may not always be the most profitable thing to look at for your program,” says Erick Larson, Mississippi State University Extension small grains agronomist.

The response to nitrogen will vary from year to year, especially in a Southern environment, he adds. “There are several unique things about the environment in the South that can make managing nitrogen very difficult. It’s certainly different from what growers in the Corn Belt of the Midwest deal with,” he says.

The traditional thinking is that growers who are making exceptional corn yields are using very high nitrogen rates, says Larson.

“But a grower I know in southwest Kansas still applies only about 225 pounds of nitrogen over the course of a season and makes up to 300 bushels per acre. A definite advantage they have is their unique environment, but they’re also doing some things to increase their nitrogen efficiency. They’re producing more than a bushel in yield with each added pound of nitrogen,” he says.

Nitrogen loss in the South is associated with conditions such as excessive moisture in the spring, says Larson.

“From the day we put corn seed in the ground until it reaches physiological maturity at the end of the season and is no longer using nitrogen may be as much as 140 days or more. We’ve got a long season to work with on corn. If we have dry conditions – a season when we don’t have 35 to 40 inches of rainfall – we can do just about whatever we want without seeing a response to nitrogen.”

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.

How much N and when

It’s important, says Larson, that growers know how much nitrogen the plant utilizes and when it utilizes it over the course of an entire season.

“We start off with very slow uptake, especially during the first 30 days. When the plant hits the rapid growth state – the V6 growth stage – which is about 18 inches tall, you need to have your nitrogen applied because that’s when the most rapid nitrogen uptake occurs during the season.

“You need to have a little bit of nitrogen applied early, either at planting or right after it comes up, using a number of different sources and application methods to apply it. You can even rely on a starter fertilizer to supply this first split because the corn plant utilizes very little nitrogen early, especially in the first 20 to 30 days.”

Just because you apply nitrogen doesn’t mean the soil will take it up, he warns. It’ll be in the soil and subject to loss. “You do need a little bit of nitrogen early, but you don’t need very much. You can even wait until the corn is up to a few inches tall.”

If you’re growing irrigated corn or applying more than 200 pounds of seasonal nitrogen, says Larson, you need to apply about 75 percent with the second split or sidedress application, at about the V6 stage.

“Timing might not be such a big issue, especially if we have a dry spring, or if you can get over your acreage fairly quickly. If it’ll take you 10 days or more to get over your acreage, timing can become difficult.”

Larson says he considers the pre-tassel nitrogen application as an additional split. “It won’t produce a yield response every year, and that’ll depend a lot on what happens earlier in the year.”

If you have extremely wet conditions during the season, this particular nitrogen application could be used to correct a deficit situation, he says.

“In that case, you may need to apply even more with this split. De-nitrification rates in the South can range from 2 percent per day when soil temperatures are less than 60 degrees to 4 to 5 percent per day when soil temperatures are greater than 60 degrees.”

Nitrogen deficiency is relatively easy to identify, says Larson. It will first show up on the leaf tip, with yellowing followed by browning.

“It’ll occur on the lower leaves in the crop canopy. The issue with using tissue analysis to determine nitrogen rates is that if you do tissue analysis at the vegetative stage or even up to tasseling, you’ve utilized less than 65 to 70 percent of your seasonal nitrogen, and you’d have to be in an extreme deficiency to show that the plant didn’t have sufficient nitrogen at that time. Even at tasseling, you’ve used only about 60 to 70 percent of your seasonal nitrogen.

Tissue sampling to me is more important for identifying a problem at the end of the year.”

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