Urea (CO(NH2)2) is the most commonly used nitrogen (N) fertilizer. The high N content per unit material (46%), lower cost, ease of storage, and solid or liquid formulation options make it a popular choice for farmers. However, if urea is not incorporated quickly after surface application, a large percentage of its N can be lost in the form of ammonia (NH3) gas. Along with reducing a farmer’s bottom line, ammonia emissions negatively impact the environment.
Conversion of Urea
Urea reacts with the water in the soil immediately after application through a biological process called hydrolysis. The urea is converted to ammonium carbonate which can quickly transform into ammonia gas. If the ammonia is not captured and converted to ammonium (NH4+) quickly after it is formed, ammonia will be lost into the air.
The conversion of ammonium to ammonia gas (NH4+ to NH3) is controlled by pH. As hydrolysis occurs the initial pH around the granule will increase. Once the pH exceeds 7.5, volatilization losses increase drastically. Additionally, soils with a high pH are more susceptible to ammonia volatilization than acidic soils.
Ammonia gas loss increases greatly over 60°F. At temperatures below 45°F, ammonia loss is limited. Early and late-season applications of urea will have a reduced risk of loss from temperature, but may have an increased risk for volatilization losses due to soil moisture.
High-moisture conditions common during early spring and late fall can increase the volatilization risk for surface-applied urea. Hydrolysis approaches zero when the soil is dry, as the conversion requires water. Urea applied to fine sandy loam and silt loam soils faces the greatest risk for losses, compared with urea applied to heavier clay soils. These losses can be reduced by changing the urea fertilizer placement.
Urea fertilizers are typically either surface-applied (broadcast application), with or without post-application incorporation, or surface- or subsurface-banded. Surface application of urea fertilizer without incorporation can lead to substantial loss of N in the form of ammonia gas. Subsurface and surface banding reduce the potential for ammonia gas volatilization; however, surface banding relies on rainfall or irrigation to move the nitrogen to the roots or rooting zone. Application in a localized band allows for more efficient use of N. Lower application rates can be used than would be needed if the urea was broadcast on the surface and not incorporated.
In row crop production, when urea is used as a band-applied starter, the planter should be carefully checked to ensure placement is not closer than 2" beside and below the seed, and be calibrated to apply no more than 65 lb. urea per acre (30 lb. of actual N from urea).
Ammonia gas losses from surface-applied urea increase in windy conditions. Wind lowers the ammonia concentration above the field to levels lower than what is found at the surface of the urea, causing additional volatilization.
Soils with a large cation exchange capacity (CEC) can retain more ammonium, reducing the chance of loss of N through volatilization. Ammonia volatilization risk is highest on sandy soils, which have low CEC and a low pH buffer capacity. Muck soils with high organic matter levels, and large amounts of residue on any soil, can also increase volatilization due to increased microbial activity. When urea sits on top of residue it is further exposed to volatilization.
If no rain occurs after application and the urea is not incorporated, N loss can be significant. As rainfall or irrigation increases, more urea from the surface moves into the soil profile. Typically, 0.5" of rain is sufficient to reduce N losses. Higher levels of rainfall can result in additional N loss.
Source: Fox, R.H. and L.D. Hoffman. The effect of N fertilizer source on grain yield, N uptake, soil pH, and lime requirement in no-till corn. Agronomy Journal. 73:891–895.
Management to reduce losses
Many different tactics can be used to reduce the potential of volatilization:
- Time surface applications for when soil temperatures are low and there is a chance of some rainfall. Do not apply urea to saturated soil.
- Apply urea before a light rainfall or irrigation. Avoid applications when more than 0.5" of rain is forecasted.
- Incorporate urea by irrigation or tillage.
- Apply urea in the starter band (no more than 30 lb. of actual N from urea, liquid or granular form), or as a surface or sub-surface band using a liquid formulation.
- Use urea treated with a urease inhibitor, or a coated urea controlled-release fertilizer. Urease inhibitors typically reduce volatilization for 10 to 14 days. Controlled-release fertilizers can help extend N release over an entire growing season.