Dr. Dan Talks Agronomy

Is Fall Nitrogen Necessary?

Anhydrous tanks lie in wait for the fall nitrogen season, but it's time to take a hard look at this practice. (DTN photo by Jim Patrico)

The now-famous Clairol commercial used to ask: "Does she or doesn't she?" Today, agriculture's version of that theme could be applied to fall nitrogen applications: "Should you or shouldn't you?"

It's never been more important to better manage nitrogen (N). With commodity prices down, farmers should be talking with their local fertilizer dealers to make sure they are following the 4Rs -- right nutrient source applied at the right rate in the right place and at the right time.

The 4R phrase should be our mantra when buying and applying fertilizer regardless of season. Environmental pressures are also forcing us to change how we manage N and keep more for the crop and less out of the surface waters.

From an environmental perspective, eliminating fall N application would probably go along ways to reducing N losses. However, growers like applying N in fall because ammonia is cheaper, weather is more favorable, soil is in better condition for knifing in and closing the slot, and there's more time to apply.

Getting your fall N program right begins with establishing the N rate that supports your corn yield goals. There are a lot of ways to arrive at this from both a crop and soil perspective, but the general rule of thumb today is from 0.9 to 1.0 pound per bushel. From that point, think about what your soil can supply and how soil and weather conditions could affect the fate of the N applied.

In general, what's best for the plant and for the environment is to apply fertilizer as close as possible to when the plant needs it. For corn, that is usually around V6 to V8 growth stages and that's especially true for certain soil types like sands that can't hold N. Nitrogen is both a transformation and mobile nutrient. Ammonia will quickly convert to nitrate, which is vulnerable to leaching and denitrification. The problem is the ideal application time is not always logistically possible.

With all the nutrient loss-reduction strategies being promoted today, we are continually reminded that a split application is more efficient use of N. This is nothing new, but as farms got larger, one-and-done applications have become more common. We know that applying some N in fall and some in spring or in spring followed by early summer is more efficient than one application during fall. Corn only needs a small amount of N in the spring to get the corn plant growing early, and a weed and feed or starter can easily fulfill those needs.

If you are determined to apply N in the fall, consider splitting it with a spring application and only apply about 50% of your total N budget now. Apply ammonia and only when soil temperatures are consistently below 50 degrees Fahrenheit at a 4-inch depth. It should be placed 8 inches deep into soil that is in a condition that the mole knife slot closes. Ammonia is the most stable form of N in the soil because it attaches to the soil's exchange site. When soil temperatures are below 50 F, the bacterial conversion to nitrate grinds to a halt.

Today it is common to add a stabilizer to N, regardless of when is applied. It keeps ammonia in the stable ammonia form longer in case of a weather warm-up. NServe (nitrapyrin) has been on the market for decades and inhibits the bacteria that convert ammonia to nitrate. Today there are a number of commercial nitrapyrin knockoffs as well as Verdesian's Nutrisphere, a polymer that surrounds and protects the ammonia and slows conversation to nitrate.

Farmers who apply N in the fall should also be sure they are far enough north so that the soil stays consistently cold over the winter.

For more information on decisions on using nitrification inhibitors, go to: http://bit.ly/…

Dan Davidson can be reached at AskDrDan@dtn.com