Urea fertilizer is the most widely utilized nitrogen source in global agriculture, prized for its high nutrient concentration and ease of handling. Containing 46% nitrogen by weight, it has the highest concentration among all solid nitrogen fertilizers. Its standard NPK grade is listed as 46-0-0, signifying its role as a pure nitrogen supplier. This high nitrogen percentage makes urea an economical choice for transportation and application, establishing it as the dominant nitrogen fertilizer worldwide.
Chemical Identity and Synthesis
The chemical identity of urea is represented by the formula CO(NH2)2, which is chemically known as carbamide. This structure is distinguished by two amine groups attached to a central carbonyl group. The high nitrogen content is directly related to the small molecular weight of the compound, with nearly half of the molecule’s mass derived from nitrogen atoms.
Industrially, urea is synthesized through a two-stage process using ammonia (NH3) and carbon dioxide (CO2) as raw materials. These materials are reacted under high pressure (140 to 180 bar) and elevated temperatures (170 and 210 degrees Celsius). The initial reaction forms an intermediate compound called ammonium carbamate, which then decomposes to yield urea and water. This manufacturing process often occurs adjacent to ammonia production facilities, utilizing the carbon dioxide byproduct from ammonia synthesis.
The Process of Nitrogen Availability in Soil
When applied to the soil, the nitrogen in urea is not immediately available for plant uptake and must first undergo a transformation process. The initial step is called hydrolysis, a rapid chemical reaction catalyzed by the enzyme urease, which is produced by soil microorganisms and is present in nearly all soils. This reaction converts the water-soluble urea molecule into ammonium carbonate. Hydrolysis is a fast process, often completing within hours, especially in warm, moist soil conditions.
The ammonium carbonate immediately breaks down further to release ammonium ions (NH4+) and carbon dioxide. This rapid release also produces hydroxyl ions, which causes a temporary and localized spike in the soil’s pH immediately surrounding the fertilizer granules. While plants can absorb some nitrogen in the ammonium form, the temporary high pH environment increases the risk of the ammonium converting to ammonia gas (NH3), which can escape into the atmosphere through a process called volatilization.
The final and slower stage of conversion is nitrification, a two-step microbial process performed by specialized soil bacteria. First, bacteria convert the ammonium (NH4+) to nitrite (NO2-), and then other bacteria convert the nitrite into nitrate (NO3-). This nitrification process takes longer, ranging from several days to a week depending on soil temperature. The resulting nitrate is the form of nitrogen most readily absorbed by crops, but its negatively charged nature means it moves freely with water, making it susceptible to leaching and denitrification loss.
Practical Guidelines for Use
To maximize the efficiency of urea fertilizer, users must manage the risk of ammonia volatilization, which is the primary source of loss when urea is surface-applied. The volatilization risk is highest when urea is left on the soil surface in warm, moist conditions, especially on soils with a high pH or heavy surface residue. Under these conditions, the temporary pH rise caused by hydrolysis can lead to substantial nitrogen loss to the atmosphere.
The most effective strategy to prevent this gaseous loss is to physically incorporate the fertilizer into the soil profile. This can be achieved through light tillage, subsurface banding, or injecting the urea at least two inches deep. If mechanical incorporation is not feasible, the application should be immediately followed by irrigation or rainfall. Research indicates that at least 0.5 inches of water is necessary to move the dissolved urea deep enough into the soil to prevent the loss.
When surface application is unavoidable, a chemical additive known as a urease inhibitor can be mixed with the fertilizer. The inhibitor temporarily blocks the urease enzyme, delaying the onset of hydrolysis. This delay extends the window of time, often by 7 to 14 days, allowing for a timely rain or irrigation event to move the urea into the soil before the conversion to volatile ammonia begins. Applying urea during periods of cooler soil temperatures (typically below 60 degrees Fahrenheit) also slows the hydrolysis reaction, providing a natural safeguard.