Know Nitrogen Forms to Understand What's at Risk

With warmer temperatures and spring rains right around the corner, now is the perfect time to refresh our understanding of the forms of nitrogen and how to reduce the risk of nitrate loss. The process of moving from the applied form to nitrate is nitrification. So when we talk about nitrogen management, we are really talking about managing the conversion of the forms so the nitrate form is available to the plant at the right time.

Primary forms of nitrogen fertilizer

  • Anhydrous Ammonia the most prevalent and lowest cost form of nitrogen, is 82% nitrogen.
  • Urea has the highest nitrogen content of all solid fertilizers at 46% N.
  • UAN solutions, such as 28% and 32% liquid nitrogen, are made up of different forms of nitrogen. 28% liquid nitrogen is 50% Urea, 25% Ammonium and 25% Nitrate.

Ways we lose nitrogen

Denitrification is a biological process impacted by temperature, moisture, pH and more. As soil organisms become active, they need oxygen and search to the easiest source – if soils are saturated (limited oxygen in the pore spaces) they take oxygen from the nitrate molecule which coverts it to Nitrous Oxide, Di-nitrous Oxide or pure nitrogen gas. These gases are released into the atmosphere.

Volatilization or Urea Hydrolysis is a biological process that occurs when the urea molecule converts to ammonia gas and is released into the atmosphere.

Leaching is the movement of nitrogen through the soil profile. Both the soil and nitrates are negatively charged. So the nitrate molecule is not held by the soil and is carried by water. While this mass flow delivers nitrate to the roots, it is also moves nitrates into the subsoil.

Risks of Loss

Anhydrous Ammonia. Immediately after application, the NH3 molecule grabs a Hydrogen molecule from water and converts to ammonium (NH4+). The positively charged Ammonium is very stable. Soil is negatively charged. So the NH4+ is held tightly by the soil. Nitrosamonas bacteria convert the ammonium to nitrite and nitrite quickly converts to nitrate. The conversion to nitrate begins in warm, unsaturated soils. Inhibitors like N-Serve kill the nitrosomonas bacteria and slows the conversion to nitrite.

Urea. The urea molecule, when applied to the surface of the soil, combines with water to form ammonia. If the ammonia molecule does not come in contact with soil, it converts to a gas and goes off into the atmosphere. But in soil, the ammonia molecule grabs a hydrogen molecule and converts to ammonium. Soil contact is critical – whether by tillage or rainfall – to prevent urea hydrolysis. For each day that urea sits on top of the soil, about three percent of the urea-based nitrogen can be lost. This process is caused by the urease enzyme. Urease inhibitors kill this enzyme to slow the hydrolysis – providing about a 10-day cushion to move the molecule into soil.

UAN Solutions. Because UAN solutions contain three forms of nitrogen, they are affected by all three forms of loss. Half of 28% is urea and is subject to volatilization. Twenty-five percent is ammonium. And 25% is nitrate and is subject to leaching and denitrification risk.

Develop a plan to reduce loss

The common strategy for ensuring that corn has an adequate supply of nitrogen available during grain fill is to manage the nitrification process by managing the biological environment. This is a combination of managing pH, delaying application, using tillage and employing stabilizers and inhibitors. Corn needs both the ammonium and nitrate form for early development and determining maximum girth and length. So provide a stable base of nitrogen to get corn through to V10. Then measure nitrate levels and make a mid-season application to provide the nitrate push needed to efficiently reach your yield potential.

For more information about nitrogen forms, watch The Nitrogen Story on the YouTube video below.