Management Practices Affecting Nitrogen Loss from Urea

By Dorivar Ruiz Diaz

When urea fertilizers are top-dressed to winter wheat in late winter or broadcast and incorporated for production of any crop, studies have shown good crop performance and little or no loss of ammonia. Low losses from urea top-dressed to wheat are due largely to the low soil temperatures typical at the time when top-dressing is usually done. For other crops, incorporation of urea by tillage or by 1/2 inch or more of rain or irrigation water the day of application (ideally in rainfall withing 2-3 days) will generally eliminate ammonia loss. Even when conditions are considered ideal for ammonia loss (lots of urease, warm temperatures, and moist soil), losses are unlikely to exceed 20% of the surface-applied urea. Injection of urea-containing fertilizer solutions into irrigation water also results in little ammonia loss. The effectiveness of this method of fertilizer application depends on how uniformly the water (and therefore the urea) can be applied across the field. The following describes some cases in which ammonia loss from urea fertilizers may be a problem and suggests practices to reduce losses.

Ammonia loss from surface-applied urea is likely to be greater for no-tillage than for conventional tillage systems. Continued no-till crop production results in a layer of crop residue on the soil surface that can enhance ammonia loss from surface-applied urea or UAN solution. A layer of partially decomposed or undecomposed crop residue can increase loss because:

1. The urease activity of this residue layer is higher than in underlying soil;
2. Undecomposed crop residue may reduce diffusion of fertilizer into the soil;
3. Crop residue at the surface often increases the water content of the surface soil layer, which can increase ammonia loss as discussed earlier. The layer of partially decomposed crop residue can tie up N temporarily, making it less available to the crop as discussed below.

Cumulative N loss by volatilization can be significant even under low temperatures for top-dressed urea on wheat, as shown in a 2020 study in Kansas. These types of losses occur in smaller increments but extended over time if there is lack of precipitation to incorporate the nitrogen. The effect of tillage and the presence of heavy residue can have a significant effect on ammonia volatilization, with typically higher volatilization potential under no-till. Also, urease inhibitors (NBPT) reduce urease activity and minimize volatilization. Urease inhibitors can delay the process of urea hydrolysis, providing an opportunity for precipitation to incorporate the nitrogen into the soil.

In many cases, UAN solution may be mixed with herbicides and applied together preplant. The loss of ammonia from this mixture after application will not be affected by the herbicide and will still be determined by the factors discussed here for urea-containing fertilizers applied alone.

No-till row crops fertilized with surface-applied urea or UAN solution have sometimes yielded less than crops fertilized with other N sources, which do not lose ammonia when applied to neutral pH or acid soils. However, sufficient evidence has been collected to show that the differences in crop response to the various N sources are not always due to differences in ammonia loss from the various fertilizers. Decomposing crop residue can tie up surface-applied N (making it unavailable to crops), whereas N placed below the decomposing crop residue is not as susceptible to this problem. Therefore, N fertilizer banded below the soil surface will often be more available than surface-applied N, even with non-urea fertilizer sources.

An alternative method of applying liquid N sources with little or no tillage is to apply the fertilizer in surface bands (such as strimmer bars). When differences occur, this method of placement provides better N availability to row crops or small grains than surface broadcast applications, but not as good as fertilizer injected below the soil surface.

Take-home Message

Ammonia can form in soils following the application of urea fertilizers. If urea is surface-applied and not incorporated by tillage or does not receive 1/2 inch of rainfall or irrigation within 24 hours, there is some potential for ammonia loss. In Kansas, this potential is generally small for many surface-applied urea fertilizers.

Urea may be safely applied for the following conditions when tillage is not possible:

• Wheat top-dressed in winter when soil temperatures are cool or cold;
• Cool-season grasses fertilized during cool weather from November through March on well-drained soils;
• Early spring application for summer crops on clean-tilled fields.

Enough ammonia loss to reduce crop yields can sometimes occur under the following conditions:

• No-till crop production when urea is applied to a warm and moist soil surface heavily covered with crop residue.
• Soils kept wet by water tables near the soil surface. These are most likely to be lower-lying areas of fields following very wet weather.