Urea is a widely used synthetic nitrogen fertilizer in agriculture and gardening, prized for its high nitrogen content, typically around 46%. This high concentration makes it an efficient and cost-effective way to deliver nitrogen, a macronutrient essential for plant growth. Standard, uncoated urea is definitively not a slow-release product; its primary characteristic is a rapid transformation process in the soil that quickly makes its nitrogen available to plants.
Standard Urea: The Rapid Transformation Process
The speed at which standard urea releases its nitrogen is governed by hydrolysis, a chemical reaction that begins immediately after the urea granule dissolves in soil moisture. The reaction is catalyzed by the ubiquitous soil enzyme, urease, which is produced by soil microbes and plants.
The urease enzyme quickly converts the urea molecule into ammonium carbonate, which then breaks down into ammonium (\(\text{NH}_4^+\)). Under warm and moist conditions, this initial conversion (urea hydrolysis) can be completed within 48 to 96 hours. This rapid breakdown leads to a temporary, localized spike in alkalinity, increasing the soil pH immediately surrounding the fertilizer granule.
Once the nitrogen is in the ammonium form, a second biological process called nitrification begins. Soil-dwelling bacteria oxidize the ammonium, first into nitrite, and then into the plant-usable form of nitrate (\(\text{NO}_3^-\)). This entire conversion from urea to nitrate can occur in a matter of days to a week, depending on soil temperature and moisture levels. This quick sequence means the nitrogen is rapidly available for plant uptake.
The Consequences of Quick Nitrogen Release
The rapid transformation of standard urea provides a fast nutrient boost but creates challenges related to inefficiency and environmental impact. One major issue is ammonia volatilization, which occurs when the ammonium created by hydrolysis converts to ammonia gas (\(\text{NH}_3\)). This gaseous loss happens when the fertilizer is left on the soil surface, particularly in high-pH or warm conditions, and can result in significant nitrogen loss.
Another consequence is nutrient leaching, which primarily involves the nitrate form of nitrogen. Nitrate is an anion (negatively charged) and is not held tightly by the negatively charged soil particles. Because it is highly mobile and water-soluble, excess nitrate can easily be carried by rain or irrigation water below the plant’s root zone, contaminating groundwater.
The rapid concentration of nitrogen compounds near the roots can also cause phytotoxicity, commonly known as fertilizer burn. The sudden release of ammonium and the temporary rise in soil pH can damage or kill sensitive plant roots and foliage. This burn risk is pronounced when large amounts of urea are applied near established plants, such as turfgrass.
Engineered Slow-Release Urea Alternatives
To mitigate the rapid release and associated losses of standard urea, two primary categories of engineered products have been developed to achieve a true slow-release effect.
Coated Fertilizers
The first category involves physical barriers, such as coated fertilizers. These products, including Sulfur-Coated Urea (SCU) and Polymer-Coated Urea (PCU), feature a protective outer layer applied to the urea granule. The coating regulates the rate at which water can penetrate and dissolve the urea inside.
With SCU, the thickness and integrity of the sulfur coating determine the release rate. PCU relies on the polymer membrane to control the diffusion of dissolved nitrogen. The release of nitrogen is delayed and extended over weeks or even months, depending on the coating’s thickness and composition.
Nitrogen Stabilizers
The second category uses chemical additives known as nitrogen stabilizers. These stabilizers interfere with the enzymatic or microbial steps of the nitrogen cycle.
Urease inhibitors, such as N-(n-butyl) thiophosphoric triamide (NBPT), temporarily slow the urease enzyme. This delays the initial conversion of urea to ammonium and reduces volatilization.
Nitrification inhibitors, like dicyandiamide (DCD) or nitrapyrin, target the soil bacteria responsible for converting ammonium to nitrate. By slowing this second step, they keep the nitrogen in the less-mobile ammonium form for a longer period. This provides protection against leaching and denitrification, extending the nitrogen’s availability in the root zone.