The Challenge of Nitrogen

December 8, 2012 04:23 PM
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In your quest for high yields, nothing is more crucial, or more difficult, than managing corn’s most important nutrient

After coping with the worst drought in decades, most farmers couldn’t wait to get whatever corn  they had harvested off their hands. While some of their frustration was sent down the line with the grain, complications still linger.

Because parched plants can’t take up nutrients, there’s a gold mine of nitrogen fertilizer buried in corn fields. The kicker, though: it is in the highly mobile nitrate form, and only the best crop managers will be able to carry these valuable nutrients over to the next corn crop. Mother Nature will play a large role in the success rate.

"You might be able to underapply phosphorus or potassium and still grow a good crop," says Farm Journal Field Agronomist Ken Ferrie. "That’s not so with nitrogen. It’s not just applying the right total amount—it’s having nitrogen present at crucial times in the crop’s life, all season long. Nitrogen management requires timing, placement and managing risk.

"From emergence to knee-high, you need to ensure that nitrogen doesn’t fall below 10 ppm [parts per million] in the top 12" of soil," Ferrie says. "From knee-high on, after the corn roots have some depth, you want to maintain 20 ppm of nitrogen in the top 2'."

That requires a basic understanding of nitrogen processes and the products you apply. Knowing your product will help ensure nitrogen is used by corn plants and not lost as a gas or leached away by water, downward into groundwater or laterally through tile lines into drainage ditches. Lost nitrogen not only costs yield, but nitrogen in water sources causes environmental problems as well.

There are a variety of nitrogen products available. "Anhydrous ammonia is the basic ingredient of all these products," Ferrie says. "It can be applied directly to the soil or used to create other products."

Besides anhydrous ammonia (a gas, shipped under pressure in the liquid form), the most common nitrogen fertilizers are urea (a solid), ammonium nitrate (a solid) and urea-ammonium nitrate (UAN) solutions. Other solid products include potassium nitrate, calcium nitrate and ammonium sulfate.

Corn production requires ammonium and nitrate, the only forms of nitrogen plants can use. However, Ferrie says, "plants need larger amounts of nitrate. Inside the plant, nitrate is stored in bulk and translocated to help with ear fill."

Calcium nitrate and potassium nitrate fertilizers are in the nitrate form, Ferrie explains. Ammonium nitrate is half ammonium and half nitrate. But anhydrous ammonia and urea must be converted to ammonium and nitrate after you apply them.

Prevent nitrogen loss. Nitrogen management is a challenge because the ammonium form of nitrogen is fairly stable in the soil, but the nitrate form is easily carried away in water. That’s because the ion carries a negative charge, which repels negatively charged soil particles.

You need to make nitrogen available to your corn plants all season long, but if you provide nitrate too far ahead of time, it might be lost before they can use it.

After ammonium is converted to nitrate, it can be lost through denitrification and leaching. Denitrification occurs when soil is saturated with water. The water drives out oxygen, creating an  anaerobic environment for denitrifying bacteria, which convert nitrate into a gas. The most denitrification occurs when soils are saturated during warm temperatures.

Leaching occurs when water carries nitrate downward through the soil or out through tile lines. Besides reducing your yield, that lost nitrogen creates environmental problems.

You can combat denitrification and leaching by applying nitrification inhibitors and stabilizers. They keep nitrogen in the stable ammonium form for a longer period by reducing the populations of bacteria that convert it to nitrate or by slowing the bacteria’s activity. Slowing the conversion process keeps more nitrogen in the stable ammonium form (which plants can use, just like nitrate) and less in the nitrate form, which can be lost.

With urea, there’s another hazard: it can be volatilized into a gas and lost into the atmosphere. Catching a rain shortly after application, or tilling urea into the soil, eliminates this danger.

If you must apply urea on the surface, as in no-till, you can reduce the risk of volatilization by applying an encapsulated form of urea; or you can apply a urease inhibitor, which controls a soil enzyme that starts the urea breakdown process. Either product can help prevent urea from  volatilizing before rain can wash it into the soil.

When you choose an inhibitor or stabilizer, remember that 28% and 32% UAN solutions contain 50% urea, 25% ammonium and 25% nitrate. So you might need two products: one for the urea and one for the ammonium.

We’ll help select the right inhibitor or stabilizer in a future installment of Nutrient Navigator. Meanwhile, let’s look at a few more reasons why nitrogen management is a challenge.

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The choice of a cover crop depends on whether you’ll plant corn or soybeans in 2013. How you manage the cover crop come spring will affect corn yield.

The carbon penalty. The carbon penalty can wreck a corn producer’s goal of making sure plants never go hungry, even for a day. It results from the nitrogen cycle.

Making nitrogen available to plants requires that soil microorganisms decompose old crop residue and release its nutrients (finishing a cycle that began when the plants took up nitrogen from the soil). As one source of energy, the organisms use nitrogen that is available in the soil itself. For a while, that nitrogen is immobilized in the microorganisms’ bodies, in the organic form that plants cannot use.

Eventually, after the microorganisms die and decompose, this nitrogen (along with the nitrogen they released from crop residue) is mineralized back into the inorganic form that crops can use. This can happen in the next crop season, but it could also take years.

Immobilization and mineralization occur simultaneously. Sometimes there is more of one or the other, and sometimes the rates are equal.

What Ferrie calls the carbon penalty results from the abundance of old crop residue in the top few inches of soil in early spring. The carbon in the residue provides food for the microorganisms that decompose it. All that crop residue causes microorganism populations to explode. The microbes draw so much nitrogen from the soil that there is none left for young corn plants.

The carbon penalty occurs when a corn crop follows high volumes of crop residue with a high carbon/nitrogen ratio. Examples of such crops include corn and wheat; soybeans, in contrast, have a low carbon/nitrogen ratio. The penalty is highest when large volumes of residue with a high carbon/nitrogen ratio are incorporated in the top few inches of soil (for example, continuous corn with chisel-type tillage).

To pay the carbon penalty, apply at least 100 lb. (out of your total nitrogen application) on the surface, shallowly incorporated or with the planter.

"The carbon penalty is significant from central Illinois northward," Ferrie points out. "As you go south, the warmer fall weather provides more time for microorganisms to decompose crop residue."

High and low risk. Identifying areas at risk for nitrogen loss involves using yield maps, Normalized Difference Vegetation Index (NDVI) maps, aerial photos and ground-truthing. It’s a season-long process.

"Maps and photos show you where high and low yields are coming from," Ferrie says. "Say you visit those sites early in the season. In the highly stressed or low-yielding spots, the corn is showing nitrogen deficiency symptoms and it’s struggling, but other parts of the field are lush and green. The struggling area makes 180 bu. per acre, and the lush area yields 240 bu.

"You have isolated and confirmed a high-nitrogen-risk area. Now you must figure out the cause. If the low yield is in a coarse soil with a high infiltration rate, the cause might be leaching. A low area could be losing nitrogen through denitrification, resulting from poor drainage and saturated soil.

"Perhaps you can solve the problem by tiling the low area, changing application timing on the coarse soil or applying nitrification inhibitors or nitrogen stabilizers."

Soil-supplied nitrogen. Another factor in efficient nitrogen management is the supplying power of the soil. It can account for yield differences in a field and affect how much nitrogen you need to apply to various areas.

"A 200-bu. corn crop requires more than 300 lb. of nitrogen per acre," Ferrie says. "Some soils will supply you with 70 lb. or 80 lb., and some will supply 200 lb. through mineralization. There are soil tests, such as the Illinois Soil Nitrogen Test and Cornell University’s test for potentially mineralizable nitrogen, that can help you identify high- and low-supplying areas.

"Typically, the soil’s nitrogen-supplying capacity is correlated to cropping and fertility history and to organic matter content," Ferrie continues. "In our example, the low-yielding area may need 100 lb. more nitrogen and the high-yielding area 100 lb. less."

Soil with good nitrogen-supplying power can also be a high risk for loss—if it’s poorly drained and subject to denitrification, for example. "The weather is also a factor," Ferrie says.

That brings us back to 2012 and all the nitrate lingering underneath corn fields. Can you save that nitrogen?

"If it rains too soon and too much this fall, probably not," Ferrie says. "You could plant wheat this fall and not need to apply any nitrogen next spring. If weather permits, and you plan to grow corn, you could plant a cover crop such as oats, rye or radishes to help take up the nitrogen and hold it for next spring’s corn crop.

"If you plan to grow soybeans next spring, you could attempt to pull last year’s nitrate through 2013 and keep it available for your 2014 corn crop. To do that, plant a cover crop of rye this fall. Let it get some size next spring to boost the carbon/nitrogen ratio. Then kill it late, and no-till your 2013 soybeans into it."

If you plant a cover crop followed by corn next spring, Ferrie offers these precautions: "The species of cover crop, when you kill it and whether or not you till it in will have a big effect on the carbon penalty. If you don’t manage your cover crop correctly, it could have a serious impact on your 2013 corn yield."

Producers in some locations apply a small amount of ammonium nitrogen in the fall, before chiseling cornstalks, to help the stalks decompose faster. "In 2012, with large amounts of nitrogen left over in the soil and high nitrate levels in the cornstalks, this won’t be necessary," Ferrie says.

With nitrogen, nothing is simple. We’ll give you more management tools in the next installment of this series.

A Basic Understanding of Nitrogen


Nitrate is stored in bulk inside the plant, then translocated to help with ear fill.

Managing nitrogen starts with knowing what products are available to apply—and then understanding how those forms are used by the corn plant. Corn production requires ammonium and nitrate, but the plant needs larger amounts of nitrate. One reason nitrogen management is so challenging is that the ammonium form of nitrogen is fairly stable in the soil, but the nitrate form is easily carried away in water.

Nitrogen products include:

  • Anhydrous ammonia: a gas, shipped under pressure in the liquid form; it can be applied directly to the soil but must be converted to ammonium and nitrate after applied; also used to create other products

  • Urea: a solid that must be converted to ammonium and nitrate after applied; can be volatilized into a gas and lost into the atmosphere

  • Ammonium nitrate: a solid that is half ammonium and half nitrate

  • Urea-ammonium nitrate (UAN): a solution of urea and ammonium nitrate in water

  • Calcium nitrate: a solid in the nitrate form

  • Potassium nitrate: a solid in the nitrate form

  • Ammonium sulfate: a solid

Learn and Profit from Nutrient Navigator


The Nutrient Navigator series focuses on efficient, environmentally sound management of nutrients. The goal is to provide practical knowledge that helps drive yields and profits higher.

You can e-mail Darrell Smith at

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