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Managing Nitrogen is a Complex Partnership

January 4, 2014
By: Darrell Smith, Farm Journal Conservation and Machinery Editor
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Careful timing and placement of nitrogen, such as by sidedressing or applying with the planter, requires fewer total pounds of fertilizer. That benefits farmers, the environment and soil health.   

The more efficient you are with nitrogen fertilizer, the healthier your soil will become—and vice versa

Soil health and efficient nitrogen use go hand in hand, but the relationship is complicated. Fortunately, there are new tools being developed that will help farmers use nitrogen more efficiently. That will benefit both the soil and farmers, says Farm Journal Field Agronomist Ken Ferrie.

"A healthy soil is a recycling soil," Ferrie explains. "Nitrogen (as well as phosphorus and sulfur) constantly cycle from unavailable to available forms (mineralization) and back again (immobilization). Healthy soils provide nitrogen for plants because there is more microbial activity to mineralize organic nitrogen into the inorganic form that plants can use. The healthier your soil, the less nitrogen fertilizer you need to apply, the less chance of running out of nitrogen late in the season and the cheaper your nitrogen cost per bushel.

"One of the components of soil health that affects nitrogen cycling is acidity," Ferrie explains. "Acid soil decreases the microbial activity that makes nitrogen available."

Acid soil creates a need for more applied nitrogen fertilizer. But applying nitrogen creates more acidity. "It’s a cycle," Ferrie says, "but it’s not a good cycle, like the recycling of nitrogen, phosphorus and sulfur."


Because acid soil has poor mineralizing ability, operators are forced to apply more nitrogen fertilizer in order to compensate, Ferrie explains. The applied nitrogen stimulates all the microbes in the soil. That in turn causes the microbes to metabolize carbon stored in the soil (in organic matter). That process releases soil carbon to the atmosphere as carbon dioxide, a greenhouse gas, while depleting soil organic matter content.

"The same thing happens when we overapply nitrogen to a soil where the pH is optimum," Ferrie says. "It causes increased microbial activity in an already active soil. That microbial activity releases soil carbon as carbon dioxide. At the same time, it puts more inorganic nitrogen in the soil than we need for the crop. That nitrogen may be lost and become a pollutant in water sources."

How problems occur. All ammoniacal sources of nitrogen create acidity, while nitrate sources do not. "There’s a wide range in the amount of limestone needed to offset acidity from applied nitrogen fertilizers," Ferrie says. "As a rule of thumb (because it depends on limestone quality), it takes from zero pounds of limestone per pound of nitrogen for calcium nitrate to almost 8 lb. of limestone per pound of nitrogen for ammonium sulfate."

Failing to understand this can lead to problems. "If a grower chooses a more acidic form of nitrogen because it’s cheaper but fails to offset the acidity by applying lime, he may over time create an acid soil," Ferrie says. "That soil will run into trouble trying to mineralize organic nitrogen into the inorganic form. So the farmer’s quest for cheap nitrogen created a chain reaction that lowered soil health and eventually increased the need for nitrogen fertilizer."

We know more now. See what we meant by a complicated interaction? Every farmer’s goal is to apply exactly the right amount of nitrogen fertilizer to maximize yield, and no more. But that’s difficult to do—and let’s face it, corn growers can’t afford to underapply nitrogen; the financial risk is too great. So their tendency is to overapply, at least slightly.

During the past decade, farmers have adopted various practices to become more nitrogen efficient. One management technique is learning how to deal with the carbon penalty. The penalty occurs because high volumes of crop residue, as in continuous corn, stimulate soil microbe populations, causing them to temporarily deplete soil nitrogen supplies (by immobilizing the nitrogen) to the detriment of young corn plants.

"It used to be that, if a farmer saw yellow corn in the spring, he automatically increased his nitrogen fertilizer rate for the next crop," Ferrie says. "But we were looking at things backward—we thought corn following soybeans needed less applied nitrogen, and we called that the ‘soybean credit.’

"Now we understand that the issue is more crop residue produced by continuous corn. It doesn’t necessarily require higher nitrogen rates—it just requires different timing. The solution to spring immobilization, or the carbon penalty, is to apply 40 lb. to 50 lb. of your total nitrogen application in the spring, in the top couple inches of soil, to feed the young corn plants when the microbes temporarily immobilize soil nitrogen.

"By managing the carbon penalty early in the season using timing and placement, the young corn plants get the nitrogen they need while the microorganisms recycle the nitrogen in crop residue for use by plants later in the season or in future years."

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FEATURED IN: Farm Journal - January 2014
RELATED TOPICS: Agronomy, Fertilizer, Soil Health

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