Feeding Corn When It’s Hungry

Corn takes up four to eight pounds of nitrogen per acre each day from the V8 leaf stage through tasseling

A wet spring probably washed away unstabilized nitrogen (N) from many fields in the Corn Belt. While the rain caused some farmers to reapply up to 100 percent of what they lost, others approached the season with a split-application strategy that focused on feeding the crop when it needs nitrogen the most.

Corn takes up about 70 percent of its N between the V4 to R1 growth stages (figure 1). From the V8 leaf stage through tasseling, N uptake is about four to eight pounds per acre per day. Most sidedress applications occur at the V6 to V8 growth stages, which allows a standard spray rig to safely make a pass through the field without damaging corn stalks, and provides N just before the corn begins its rapid uptake.

Seasonal Nitrogen
(Figure 1) University of Illinois

Right Rate for Sidedress Applications

Best practices for sidedress applications vary by soil and climate type. Close-to-ideal soils with deep topsoil and high organic matter (OM) in the central part of the Corn Belt may allow farmers to get by with putting down 75 percent of their nitrogen in a pre-plant application and the rest in a sidedress. For sandy soils, soils with lower OM, lower cation exchange capacity (CEC) or a shallow A horizon, farmers should apply half their N in a pre-plant application and the rest in a sidedress application.

In any soil, don’t apply more than 10 lbs. of N per unit of CEC. For example, in the Midwest a lot of soils are in the 15-25 CEC range, so you could put on 150-250 lbs. of N, which closely matches application rates in much of the Corn Belt. It’s important to regularly conduct soil tests so you know your soil’s CEC and percent organic matter, which translates into its ability to hold N.

Environmental Considerations

In-season application minimizes the time N sits in the field and is susceptible to loss to the environment. However, that doesn’t mean that the N applied as a sidedress isn’t at risk to environmental loss.

Each source of nitrogen more susceptible to a specific form of loss:

  • Urea can very quickly volatilize by way of the urease enzyme that causes gaseous loss of ammonia, so if you surface apply a lot of urea, it will be susceptible to volatilization loss, especially under warm  and dry conditions.
  • UAN is often surface applied, but because it is a liquid it generally moves down into the soil an inch or two more easily than urea. Knifing in, banding near the row, or injecting below the surface is recommended.
  • And while most of the sidedress applications are made with urea or UAN, some farmers may inject anhydrous ammonia nine to 12 inches deep into the soil profile, making N less susceptible to loss, provided it is injected correctly.

Weather and soil properties are the largest determining factors when it comes to the three forms of N loss:

  • Volatilization will occur in warm and dry conditions.
  • Nitrogen is susceptible to leaching when there is water moving through the soil in the presence of nitrates.
  • In water-saturated fields, denitrification changes nitrate nitrogen into unavailable gaseous nitrogen.

Protecting Nutrients from Loss

Farmers cannot predict the weather, and would be wise to protect their N investment with a nitrogen fertilizer manager. If applied N is untreated, farmers risk losing their investment to the environment.

It takes about 1 lb. of N per acre to produce 1 Bu./A of grain, so if you lose 10 lbs. of N to the environment, that’s 10 lbs. not making grain. Farmers are paid based on how much grain they produce, so an extra lb. of N adds another Bu./A of corn yield, and a few more dollars in the bank at the end of the season. Similarly, if a fertilizer additive results in 10 more bu per acre, that’s 10 pounds of N no longer in the soil that could be potentially lost.

And it’s not just lost yields that come with N loss, but there can be environmental considerations as well. When N converts from ammonium to nitrate, otherwise known as nitrate leaching, it can flow through the soil profile into the water, especially in tiled units.

The Environmental Protection Agency has standards limiting nitrates in the water supply, particularly in rural areas where many drink from well water. There are also ecological concerns of nitrates in rivers, streams and ponds that can cause an imbalance in biology where aquatic life can be negatively impacted.

Some on-farm data shows that treating applied N with a nitrogen manager can reduce nitrates coming through the tile line by as much as 40 to 45 percent, across urea, urea ammonium nitrate and anhydrous ammonia applications.

Another environmental impact to consider is the loss of N in gaseous form. Denitrification means nitrous oxide (NO) gas is emitted into the atmosphere, and volatilization emits ammonia (NH3) gas. Both NO and NH3 are considered greenhouse gases that could impact climate change.

Farmers care about environmental stewardship, and protecting applied N, as well as applying nitrogen when the crop needs it most, is good for the environment and good for maximizing yield potential.