Understanding the chemical reactions of urea in the soil

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With the current high prices of commercial fertilizers, it is even more important to implement management practices that minimize nutrient loss and maximize crop uptake. Some common nitrogen (N) containing fertilizers used in Kansas are composed of urea. This article explains some of the science behind what happens to urea when it is applied to soil.

Urea fertilizers range in composition from pure, dry, granular urea (46-0-0) to products that are mixtures of urea and other sources of N and/or phosphate and potash. The most common mixture of urea with other N fertilizers is the liquid urea-ammonium nitrate solution (UAN), which in Kansas is most often sold as a solution containing 28% N. It also may be sold as a 32% N solution. Approximately half of the N in UAN is urea.

While cost advantages favor increased use of urea, questions are often raised about its availability to crops compared to other N sources and its potential for loss when applied to the soil surface and not incorporated by tillage or irrigation. Chemical reactions of urea and ammoniacal N (ammonia and ammonium) in soil, and soil, climate, and management factors that affect the performance of urea need to be understood for proper use.

Reactions of Urea in Soil: How Urea Behaves

When urea is applied to the soil, it reacts with water and the soil enzyme urease and is swiftly transformed into ammonium. This process, known as urea hydrolysis, consumes hydrogen ions (H+) and causes the soil pH near the fertilizer to increase. If the pH rises above 7, a significant amount of gaseous ammonia can form in soil for a few days following urea application. Surface application can lead to ammonia loss, and banded application with seeds might cause plant damage due to ammonia toxicity. The severity of these processes largely depends on the concentration of ammonia formed in the soil.

The ammonia concentration from urea hydrolysis is influenced by several factors:

1. Amount of Urea Applied: Larger applications result in more hydrolysis and higher ammonia concentrations. Band applications concentrate urea in smaller soil volumes, increasing ammonia formation where the fertilizer is placed.
2. Soil pH After Application: Higher pH levels during the first few days after application lead to more ammonia formation. Soils rich in clay and organic matter with low initial pH have more hydrogen ions, leading to less ammonia formation. Conversely, sandy soils with low organic matter and high pH allow the formation of more ammonia from urea hydrolysis.
3. Rate of Urea Hydrolysis: Rapid hydrolysis reduces the time for urea and ammonium to diffuse deeper into the soil. Faster hydrolysis leaves more ammonium at the surface, raises pH, and enhances ammonia formation. Factors affecting hydrolysis speed include soil urease content, temperature, and moisture. Band application reduces contact between fertilizer and soil urease, thereby slowing hydrolysis.

Weather Conditions Influencing Urea's Behavior

Temperature and moisture significantly affect urea hydrolysis rates and ammonia loss from surface-applied urea. If conditions allow, urea fertilizers should be applied during cooler temperatures. For crops like wheat and cool-season grasses, late winter application is preferable over late spring. Early application is usually advantageous, but unusual winter rainstorms or rapid snowmelt on frozen soils can cause fertilizer loss. To minimize this risk, avoid applying fertilizer on frozen soils when rapid warming and runoff are likely.

Applying urea fertilizers under dry surface soil conditions is generally better than under wet conditions to avoid ammonia loss. Typically, well-drained soil surfaces dry quickly, but soils with high water tables may remain moist longer. In fields with wetter spots, higher application rates might counteract some nitrogen loss and improve production.

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Reference: Perin, V., Santos, E.A., Lollato, R., Ruiz Diaz, D.A., Kluitenberg, G.J. 2020. Impacts of ammonia volatilization from broadcast urea on winter wheat production. Agronomy Journal. 2020; 112: 3758–3772. https://doi.org/10.1002/agj2.20371

Dorivar Ruiz Diaz, Soil Fertility Specialist
ruizdiaz@ksu.edu

Tags: nitrogen fertilizer urea

Defining Urea Fertilizer
Urea fertilizer is known for providing nitrogen, an essential nutrient for vegetative growth. Unlike other fertilizers that offer a mix of nitrogen, phosphorous, and potassium (NPK), urea focuses solely on delivering nitrogen.

Why Urea Reigns Supreme
Urea fertilizer offers several advantages:

• First, it boasts a high nitrogen content of about 46%.
• Second, it is a crystalline, organic chemical compound that can be used on nearly any type of soil.
• Third, it serves dual purposes in agriculture: as a fertilizer and as an animal feed additive.

The fundamental function of urea fertilizer is to provide nitrogen to plants, promoting lush, green, leafy growth. However, it doesn't offer phosphorous or potassium.

Advantages of Urea Fertilizer

• Urea has the highest nitrogen content compared to other nitrogenous fertilizers on the market.
• The cost of production is relatively low.
• It does not pose fire or explosion hazards, making it safer for storage.
• It can be used for a wide variety of crops and soils without harming the soil.

Disadvantages of Urea Fertilizer

• Highly soluble in water, requiring better packaging.
• Less stable, with potential for decomposition even at room temperatures.
• If impurities exceed 2%, it becomes toxic to certain crops like citrus.

How to Effectively Use Urea Fertilizer

1. Apply urea at the time of sowing. It should not contact seeds directly.
2. Mix urea with earth or sand before application for even distribution.
3. Avoid application when soils are waterlogged or expected to remain wet for several days.

Tips for Blending Urea with Other Fertilizers
Urea can be readily blended with monoammonium phosphate or diammonium phosphate. However, it should not be mixed with superphosphate unless applied immediately, as the reaction between urea and superphosphate produces water, creating a damp mixture that's hard to store and apply.

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