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The Nitrogen Balancing Act

January 31, 2009
By: Darrell Smith, Farm Journal Conservation and Machinery Editor

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.

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FEATURED IN: Farm Journal - February 2009
RELATED TOPICS: Corn, Crops, Fertilizer

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