Nitrogen is essential to plant growth and development. On average, 1.2 pounds of N is needed to produce a bushel of corn, with a corn crop of 200 bushels per acre needing 240 pounds of N per acre. This high demand is partially met through the application of N fertilizer.

Partial factor productivity of N (PFPN), a measure of how many pounds of grain are produced per pound of fertilizer N, can be used to inform N-use efficiency (NUE) at the farm level. On average, Nebraska corn farms have a PFPN of 65 pounds of grain per pound of N. Assuming a grain N concentration of 1.3%, this means that 0.84 pounds of N is exported in the grain for each pound of N applied as fertilizer. Under scenarios with high loss of N fertilizer, PFPN will decrease.

While a nutrient management plan can help decrease the chance for N loss, weather can play a major role. One way to decrease the impact of unpredictable weather is by using an N fertilizer stabilizer. The decision to use one, however, can be complicated, as the decision needs to be made before you know the effect weather will play in any given season. Because of that, it becomes important to understand your management practices, production environment, and the way various stabilizers work in order to select the most profitable strategy to reduce N losses, improve NUE, and improve or maintain yields.

Types of N stabilizers
Nitrogen fertilizer stabilizers can be classified into three major types:

• Nitrification inhibitors are compounds mixed with ammonium-forming N fertilizers to decrease the rate of transformation of ammonium (NH₄₊) to nitrate (NO_3-). Both N forms are plant-available, but nitrate is prone to leaching and denitrification. Despite loss potential with nitrate-N, nitrification inhibitors are only expected to have a positive impact on grain yield when weather patterns favor losses large enough that N becomes insufficient to meet crop needs.

• Urease inhibitors are compounds mixed with urea-based fertilizers to decrease the rate of urea hydrolysis by temporarily blocking the active site of urease enzyme. Urea inhibitors can be especially important when urea-based fertilizers are surface-applied on high-pH soil, and in high-residue conditions such as no-till — as crop residue contains high concentrations of urease. Urea inhibitors protect urea from being quickly hydrolyzed and potentially volatilized before it is incorporated (mechanically or via more than 0.25 inch of rainfall or irrigation) and becomes protected in the soil. Similar to nitrification inhibitors, the urease inhibitor effect on yield is weather-dependent and may only be beneficial if no rainfall or irrigation occurs in the first five days after fertilizer application on drying soil conditions.

• Slow-release coated fertilizers are conventional fertilizers like urea that are coated with sulfur, polymers or both. Release of fertilizer through coating is a function of coating characteristics affected by soil, water and temperature. The coating technology can provide a gradual supply of N for the developing crop. Soil and climatic conditions can alter the effectiveness of coatings.

Stabilizers, weather and yield
In a 28-year study near Clay Center, Neb., where the nitrification inhibitor nitrapyrin was evaluated along with spring-applied anhydrous ammonia on a silt-loam soil, a positive yield response was observed 36% of the time. However, a negative yield response from using nitrification inhibitors was observed 18% of the time, with the remaining 46% having no effect on yield. Nitrification inhibitors had a positive impact on yield (from 8 to 13 bushels per acre higher than fertilizer alone) when weather during the six weeks after N application included:

• moderate-volume, well-distributed rainfall, or

• higher-volume rainfall coupled with higher temperatures

Urease inhibitors were also tested in central Nebraska the last three years. Nitrogen fertilizer in the form of urea or urea-ammonium nitrate was surface-applied near corn planting time with and without urease inhibitors. Urease inhibitors only had a positive effect on yield at one site, where a urease inhibitor plus urea produced 20 bushels per acre more than urea alone. This site received 2.3 inches of rain five to 10 days after fertilizer application. (Also, at four and five days after application, it received rain of 0.03 inch and 0.09 inch.) Rainfall volumes of less than 0.25 inch are not enough to move fertilizer into the soil, yet allow for urea hydrolysis on the surface, increasing volatilization potential. In other years, urease inhibitors had no effect on yield. In all years, using them decreased potential ammonia volatilization losses from 3.6 to 13.3 pounds N-NH₃ per acre less than fertilizer alone, which meant 3% to 7% of applied N was not lost.

The three-year study in coarse-textured soil in south-central Nebraska demonstrated the benefit of using polymer-coated urea (ESN) irrespective of inter-annual climatic variation. ESN consistently improved corn yield, compared to UAN at various rates of N application. Averaged across N rates, corn yield with ESN was greater than with UAN by 49%. A laboratory study simulating those field conditions suggested that ESN reduced ammonia loss by 15% in a dry year; in a wet year, it reduced nitrate leaching by 60% compared to UAN.

Summary
Here are some recommendations for using nitrogen fertilizer stabilizers:

• N fertilizer stabilizer use will only have a positive impact on yield if weather conditions are conducive to N losses, to the point of N becoming limiting in relation to crop demand.

• Nitrification inhibitors may increase yield with a single application of ammonium-forming fertilizer on irrigated fields. Timing of nitrification inhibitor use should allow for high effectiveness in May and June, when greatest nitrate-N leaching commonly occurs.

• Urease inhibitors are more likely to have a positive impact on yield when urea-based fertilizer is surface-applied on high-pH (more than 7), and low-cation-exchange-capacity soil with high residue cover, where no incorporation (mechanical or rainfall) occurs in the first five days after application on a drying soil.

• ESN is more likely to have a positive impact on yield when field conditions represent extremes for weather risk of N loss to leaching, volatilization or denitrification.

Source: This report comes from the University of Nebraska-Lincoln CropWatch.