Key Points:
- A central challenge in managing nitrogen fertility in corn production is the susceptibility of nitrogen to loss through leaching, denitrification, or volatilization.
- Nitrogen loss via volatilization can occur with manure and fertilizer products containing urea that are not incorporated into the soil by tillage or rainfall following application.
- Nitrogen in the nitrate form is susceptible to loss through leaching or denitrification.
- All three mechanisms of nitrogen loss can be influenced by weather and soil conditions.
- Nitrogen stabilizers can reduce the risk of nitrogen loss by slowing the rate of chemical reactions that occur in soil.
Leaching
Loss of nitrate that is carried by soil water downward below the crop root zone.
Situations at Risk of N Loss
- Leaching risk is typically greatest during April to June when intense rainfall events are most common and applied N is most susceptible to loss.
Processes and Reactions
- Nitrate is a negatively charged ion that is not attracted to soil particles and can move with soil water.
- Ammonium ions are converted to the nitrate form by the action of soil bacteria in a process known as nitrification.
- Nitrification is a two-step process: 1) oxidation of ammonia into nitrite, and 2) oxidation of nitrite into nitrate. Both steps are carried out by bacteria in the soil that use oxidation of chemical compounds as a source of energy.
Important Factors
- Rainfall – the more water that moves through the root zone, the greater the potential for leaching.
- Soil texture – Coarse-textured soils have a lower water-holding capacity and more potential for leaching.
- Rate of Nitrification - multiple factors influence the rate of nitrification, which affects the susceptibility of N to leaching.
- Soil temperature – Nitrification is a biological process, so it is highly affected by temperature. It is maximized at soil temperatures above 75°F, (24°C), slows at cooler temperatures, and essentially ceases below 40°F (4°C).
- Soil moisture – Both water and oxygen are required for nitrification, so it is favored by adequate but not excessive soil moisture. Nitrification is limited when saturation of soil pore space with water exceeds 60%.
- Soil pH – The optimal pH range for nitrification is between 6.5 and 8.8. Nitrification rates are reduced in more acidic soils.
Tactics to Reduce N Loss
- Application timing – Applying a portion of nitrogen in-season closer to the time of greatest crop demand reduces the opportunity for it to leach before being taken up.
- Nitrification inhibitors – Products such as N-Serve® and Instinct NXTGEN® slow the conversion of ammonium to nitrate, prolonging the period of time that nitrogen is in the ammonium form and reducing nitrogen loss from the soil.