The Nitrogen Balancing Act

January 31, 2009 01:18 PM

If you needed proof that growing corn is an art as well as a science, nitrogen (N) management proves it.

Making part of your nitrogen application in the fall and/or spring, when you want to accelerate residue decomposition, helps keep soil microbes from outcompeting corn plants for nitrogen.

"Figuring out how much nitrogen the crop requires, based on how many bushels you expect to produce, is the easy part," says Farm Journal Field Agronomist Ken Ferrie. "Figuring out how much nitrogen you need to feed microbes, and when to apply it, so they don't tie up all your nitrogen while they decompose the old-crop residue, is much more challenging.

"We would like to open a book and pick out one static nitrogen rate," Ferrie continues. "But nitrogen is so tied to environmental conditions, we have to make a game plan but be prepared to change it based on conditions."

Demonstrations conducted by Ferrie at the Farm Journal Corn College site dramatically show how much the timing and placement of N and carbon affect the microbes' activities. On a given farm, the factors that influence microbial activity include weather, if and when tillage is done, when and where N is applied and the type and amount of carbon that is present in the field.

To lay the groundwork, let's review the N cycle—the process by which tiny creatures decompose residue, making nutrients available to future crops. To break down residue, the microbes need (among other things) inorganic N, which they obtain from the soil. While that N is in their bodies, it is "immobilized" into a form that plants cannot use. Eventually, through mineralization, this N—as well as N tied up in the crop residue—will become available again, some in the coming crop year and some as much as three years in the future.

Immobilization and mineralization occur simultaneously. Sometimes, as conditions change, there is net immobilization; sometimes there is net mineralization; and sometimes the rates are equal. (For more about immobilization and mineralization, see "Make Corn the Winner in Nitrogen Tug of War" on page 32.)

Fast Facts

  • Corn on corn needs at least 100 lb. per acre of N surface-applied.
  • Corn on beans needs at least 60 lb. per acre of N surface-applied.
  • If soil stays warm in the fall and you want decomposition, 30 lb. to 40 lb.of that N should be broadcast in the fall, in the form of ammonium nitrogen (NH4). The remainder should be applied in the spring.
  • This N is to feed microbes that are decomposing crop residue—but it counts as part of the total N required to grow the corn crop.

You might think of it this way: Carbon immobilizes N because carbon, wherever it is placed in the soil, causes populations of microbes to grow. The microbes then tie up available N in that part of the soil. So if lots of carbon (residue) is being decomposed in the top few inches of soil where the young plant is growing and too much net immobilization is occurring, that plant will go hungry.

The carbon to nitrogen ratio. Microbe buildup is greater following corn than following soybeans because of higher amounts of carbon and corn's carbon to nitrogen ratio. The higher the ratio, the more N is required to break it down. Corn residue has a carbon to nitrogen ratio of 60:1, and soybeans have a ratio of 30:1. For comparison, hairy vetch is 10:1, manure 20:1 or less and wheat straw 80:1.

To offset tie-up of nutrients by microbes, as a general rule Ferrie recommends applying at least 100 lb. per acre of your total N on the surface, shallowly incorporated or applied with the planter. That's for continuous corn, which produces more residue; for corn after soybeans, he recommends 60 lb.

If you have a warm fall, the microbes will likely stay active enough to decompose some of the residue. In that case, only 30 lb. to 40 lb. of that 100-lb. or 60-lb. total should be broadcasted in the fall. Moreover, it should be an ammonium form of N, such as ammonium sulfate, diammonium phosphate (DAP) or monoammonium phosphate (MAP).

This advice assumes that you want to stimulate fall decomposition of crop residue, which typically is the case from central Illinois north. Farther south, with warmer autumn weather, the microbes remain active so long that you may not want to help stimulate fall decomposition.

Ferrie's demonstration shows how critical it is to deal with N tie-up—and how many factors must be taken into account for the N equation.

"Your goal is to keep nitrate levels in the top 1' of soil above 10 ppm [parts per million] when the plants are transitioning from living on seed starch to a true root system—the V4 to V6 range," Ferrie says. "Below 10 ppm, corn plants ‘lock up' and stop growing. The result is delayed maturity and potential yield loss due to early season stress."

The 100-lb. Nitrogen Rule

Here's why we call it nitrogen (N) management and why there's no substitute for knowing your soils:
"Our recommendation to surface-apply 60 lb. to 100 lb. of nitrogen per acre, to keep soil nitrate levels from falling below 10 ppm at a critical stage in corn plants' growth [see adjacent story], is simply a rule of thumb," explains Farm Journal Field Agronomist Ken Ferrie, who has decades of experience fine-tuning N applications. "It is based on our research and on what we think soil nitrate levels will be.
"However, on some farms, 40 lb. to 50 lb. per acre will be enough. That might be because the soil carries higher levels of nitrogen or the farm has a history of manure application. We adjust the 60-lb. or 100-lb. recommendation up or down, based on what we know about each field."

How much N you need depends on the source of carbon—whether it is corn, bean or wheat residue—and what you did with it, Ferrie says. The photos of plants growing in fall- and spring-chiseled conditions in early June show how much difference N timing and tillage can make (see below).

Where 31 lb. of N was applied in the fall as ammonium sulfate, followed by 80 lb. of N in a spring weed and feed application, soil nitrate levels remained above the critical 10 ppm level for both fall and spring chiseling.

Tillage timing. "The pictures show that the more time you have to decompose carbon before plants reach the critical stage, the easier it is to manage net immobilization in the field," Ferrie says. "Incorporating large amounts of carbon just before planting brings bigger challenges."

Where 220 lb. of N was applied in the fall as anhydrous ammonia, where spring rains leached N below the corn roots and while soil microbes were immobilizing N, nitrate levels fell below 10 ppm in the top 1' of soil—almost as low as the check plot—and growth stopped. The ammonia applied in the fall was concentrated in a band that was too far away from the young plants' roots.

In contrast, with fall chiseling, decomposing some residue in the fall created less competition for N between microbes and corn plants in the spring. "This created better growth conditions, allowing deeper-rooted plants to access higher nitrate values in the second foot of soil," Ferrie says.

"Sometimes, in a situation like this, you will see green streaks 30" apart, when

Make Corn the Winner in the Nitrogen Tug of War

There's no better illustration of the concept of "living soil" than the tug of war that occurs between various types of soil organisms when old crop residue is being decomposed. Some organisms immobilize soil nitrogen (N) and some mineralize it, while some will do both. The two processes occur simultaneously.

As a result—and as Farm Journal Field Agronomist Ken Ferrie has documented with multiple soil nitrate tests—the amount of N available to a crop varies throughout the year.

"When conditions are right for the microbes that decompose crop residue, we can get net immobilization," Ferrie says. "More nitrogen is being immobilized than is being mineralized, so soil nitrate levels fall. At other times, we get net mineralization, and soil nitrate levels rise. At still other times, the rate of immobilization and the rate of mineralization are equal, and soil nitrate levels remain steady."

Keep tabs on nitrates. The challenge is to prevent soil nitrate levels from falling below a critical level while the young plants are in a transition stage, between living on the starch stored in the seed and nitrate in the soil.

"We do this by applying nitrogen, which feeds both the microbes and the plants," Ferrie explains. "The nitrogen we apply also stimulates the microbes. That's part of our goal—to speed the process of
decomposition of a large amount of carbon in the old-crop residue.

"We can't stop the microbes from tying up soil nitrogen as they decompose residue. But we can respect that the process is going on. So the second objective of our nitrogen application is to keep
nitrogen available to the corn plants by preventing soil nitrate levels from falling below a critical level."

Regardless of the tug of war going on in the soil, you always want your corn to win.

some plants finally reach the ammonia and other plants are still suffering," he says.

Now consider all the other tillage treatments you might have used. The graph on page 28 shows that with fall ammonia application and no other N, only moldboard plowing, strip-till and no-till were in the 10 ppm range in early June.

(Where residue is concerned, plowing functions much like strip-till and no-till by not incorporating residue into the top few inches of soil. The plow essentially flips the stalks between the soil surface and deep in the soil where there are no aerobic bacteria to decompose them.)

"A considerable amount of decomposition eventually takes place on the soil surface," Ferrie notes. "But when we incorporate residue, we speed the process."

The bottom line: If there is residue in top 12" of the soil profile, you must make N available to the microbes that will decompose it, as well as to the growing plants. That will keep young plants growing until their roots reach deeper in the soil and net mineralization begins.

Complicating factors. But here's where things get a bit more complicated—and where you become an artist as well as a craftsman. The recipe we just gave you—100 lb. (or 60 lb.) per acre of N on the surface or shallowly incorporated, split between fall and spring—works in latitudes such as central Illinois, where weather remains warm enough after harvest and tillage for the microbes do some decomposition.

"In Minnesota, it may not do as much good to apply small amounts of ammonium N in the fall," Ferrie says, "because by the time harvest is over and tillage done (if it is done), the soil will be so cold the microbes are no longer active. Farther south, such as in Kentucky and Tennessee, you may want to preserve residue longer, rather than accelerate its decomposition. In that case, you wouldn't need the fall surface application."

Farmers in northern latitudes will want to make the entire 100-lb. or 60-lb. surface application in the spring. "If high amounts of residue are being incorporated in the same week as planting, some of that nitrogen application should be in the nitrate form—UAN solution, rather than urea—if possible," Ferrie says. "Soil microbes tend to feed on the ammonium first, leaving the nitrates for the corn plants."

Weather Woes in 2008

With nitrogen (N) management, the best-laid plans often get waylaid by weather. For many farmers, says Farm Journal Field Agronomist Ken Ferrie, 2008 brought the "perfect storm."

"Some farmers had applied surface nitrogen to feed the soil microbes as they decomposed the old-crop residue," he says. "The plants were up and growing, the microbes were consuming nitrogen and then heavy rain began to fall. The nitrogen that had been applied leached away and the microbes immobilized what was left, so the corn plants had nothing to feed on. Microbes digesting nitrogen, a growing crop in the V4 to V6 range, transitioning from seed starch to true roots, and nitrogen leaching away is the perfect storm."

For some corn producers, it was even worse. "Some people applied all their nitrogen the previous fall, as anhydrous ammonia, because that was the cheapest way to go in the face of high nitrogen prices," Ferrie says.

"Even fall-applied nitrification inhibitors couldn't help against late-spring rains—they had done their job last fall. In the fall, we saw 100-bu. swings in yield, in the same field, because of nitrogen loss," he adds.

Even soybeans younger than the third trifoliate turned pale green and stopped growing for a time because all the soil nitrate was flushed away by the rain or immobilized, Ferrie notes. "Soybeans don't start producing their own nitrogen until about the third trifoliate," he says. "Some growers switched fields from corn to soybeans, as the season got too late for corn. In those fields, where nitrogen had been applied for the corn, those soybeans grew like crazy."

But soybeans cope better with nitrate loss than corn does, Ferrie adds. "With soybeans, there didn't seem to be much difference in the final yield," he says.

Central-latitude farmers also need to keep their eye on the ball. In fall 2008, central Illinois conditions were more like Minnesota—tillage wasn't complete until Thanksgiving, the soil got cold and little fall decomposition took place.

"Most of the residue will still be out there next spring," Ferrie says. "This would make fall chiseling perform like spring chiseling. You may need to shift more than 100 lb. (or 60 lb., if corn follows soybeans) of your total nitrogen package to your spring surface application because so much decomposition will be occurring, triggering net immobilization. In this situation, nitrogen placement becomes very important."

Even if you make your fall N application, complete your tillage and are blessed with several weeks of warm weather, you're not free to take off on a fishing trip and forget about your crop.

"If you get heavy rains in the spring, the nitrogen in the top few inches of soil, which was supposed to feed the microbes, may be leached downward, away from them and from the young, shallow-rooted corn plants," Ferrie says. "You might want to consider pulling soil nitrate samples to see how much nitrogen remains."

Quick reaction. "In that situation, if you haven't yet planted, you can move some of your nitrogen from side-dressing to a weed and feed application," Ferrie says. "If you have already planted, start sidedressing as quickly as possible to get ahead of the situation and make sure nitrogen is available to the plants as they transition from seed starch to true roots."
Here are things to keep in mind as you plan your N program:

  • Consider how much carbon must be decomposed. Corn creates more residue than beans, for example.
  • When does carbon breakdown begin—in the fall or the spring? The answer depends on your fall weather.
  • Where is the carbon located—on the surface or in the soil? How deep?
  • How much N was present when you put the carbon in the soil?
  • Was N lost due to heavy rainfall?

A simple test can show how much N you need to apply, under normal weather conditions, to keep microbes from immobilizing too much N.

"On a field that represents your farm, make surface applications of 0 lb. to 120 lb. of nitrogen per acre," Ferrie says. "Watch the crop, especially from the V3 to V10 stage. Wherever the corn stops visually responding to additional nitrogen, that's how much nitrogen you need to apply at the surface to offset immobilization."

For More Information
"Fields That Never Catch Up" September 2007 Farm Journal

You can e-mail Darrell Smith at

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