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."
Many farmers also are taking soil nitrate tests, monitoring nitrogen availability through the season, Ferrie notes.
That way, they can sidedress part of their application, even turning to high-clearance applicators in tall corn, as dictated by their test results. Applying more of their nitrogen during the use season, and less in the fall or before planting, reduces the risk of loss and requires fewer total pounds.
The nitrogen challenge. The next quantum leap in nitrogen efficiency will be widespread use of variable-rate application, based on the soil’s nitrogen-supplying power and weather conditions, Ferrie says. "We know how much nitrogen is required to produce a bushel of corn," he says. "The challenge is to figure out how much nitrogen the soil will provide through mineralization. That is even more complicated because most fields contain numerous soil types, which vary in their ability to mineralize nitrogen."
One of the most promising tools to do that, which cooperating farmers are helping Ferrie test on Illinois farms, is the Adapt-N program developed by Cornell University scientists.
Adapt-N is an online tool that calculates soil nitrogen (as well as water) status on a daily basis throughout the growing season for every field or management unit a farmer enrolls. That lets them precisely target sidedress and rescue nitrogen application rates. "It’s even possible to calculate variable-rate prescriptions by entering each soil type individually," Ferrie says.
Farmers input GPS coordinates for every field, using information available from the Internet. They also input nitrogen rates and placement; crop information, including planting date and population; soil information, including texture, slope, organic matter content and rooting depth; manure applications; sod in the rotation; and the prior crop.
The program uses high-end weather data to calculate how much nitrogen is being released by the soil, taken up by the crop or lost through leaching or denitrification. It is available now in northeastern Iowa, Illinois, Indiana, Minnesota and Wisconsin, and will be released soon to other states.
"This is the first time I’ve seen a program that deals with so many components of the nitrogen cycle," Ferrie says. "It even accounts for the carbon penalty—a function of crop rotation, soil texture and weather. You can rerun the assessment at any time, and you can use it at the end of the year to consider changes for the next season."
In 2013, Ferrie and cooperating farmers helped ground-truth the Adapt-N program by collecting their own rainfall data and comparing the program’s nitrogen recommendations to the ones Ferrie would have made using traditional procedures.
As press time, Ferrie was still analyzing the data, but he was optimistic. "It looks like the program recorded weather pretty accurately," he says. "That’s one of the most important factors. We can calibrate and refine nitrogen fertilizer recommendations, as long its weather data is accurate."
Like any new program, Adapt-N will require more on-farm research by farmers and the fertilizer industry. "The tool will help farmers think about all the variables they need to consider in nitrogen management," Ferrie says.
An Aggregate Role
There are even more interrelationships between nitrogen and soil health than are mentioned in the adjacent story. "For example, compacted soils will have more denitrification occurring during wet weather than noncompacted soils," says Bianca Moebius-Clune, an Extension associate on Cornell University’s Soil Health and Adapt-N Team. "This is because noncompacted soils with better aggregation will drain and become aerated more quickly following heavy rain. This will limit denitrification.
"Well-aggregated soils also allow more water infiltration and storage, so a field is better prepared for a potential drought and nitrogen uptake can continue. Cover crops play important roles—especially legumes that can fix their own nitrogen because the amount and kind of organic matter and residue available to soil microbes are factors in mineralizing nitrogen."
The Cornell soil health test helps farmers to evaluate the physical and biological health, as well as the chemical health, of their fields, Moebius-Clune adds. You can find more information by doing an Internet search for "Cornell University Soil Health."
Building on the Systems Approach, the Soil Health series will detail the chemical, physical and biological components of soil and how to give your crop a fighting chance. www.FarmJournal.com/soil_health
You can e-mail Darrell Smith at email@example.com.