Urea is a widely used nitrogen fertilizer, typically sold as 46-0-0, containing the highest nitrogen concentration of any solid form. The numbers represent the percentage of nitrogen (N), phosphate (P₂O₅), and potash (K₂O) by weight, meaning urea is 46% actual nitrogen. Applying the correct amount per acre requires converting the crop’s nutrient needs into a physical product volume. This guide provides a step-by-step method for accurately calculating the necessary application rate to maximize plant nutrition and minimize environmental risk.
Determining Nitrogen Needs
The first step in calculating the amount of urea required is to determine the precise amount of actual nitrogen (N) your specific crop or turf needs per acre. This target rate is highly variable and depends on the specific plant, its projected yield goal, and the nutrient status of the soil. The most accurate way to establish this requirement is through a comprehensive soil test, which provides a detailed analysis of existing nutrient levels and offers localized recommendations.
For commercial crops like corn, nitrogen recommendations often fall in the range of 130 to over 200 pounds of actual N per acre, based on expected yield and factors like the previous crop grown. Turfgrass requires less overall nitrogen and is typically managed with multiple, smaller applications throughout the growing season. For calculation purposes, a target of 100 pounds of actual nitrogen per acre provides a necessary baseline for the next step.
Calculating the Urea Application Rate
Once the target amount of actual nitrogen is determined, a straightforward calculation is used to convert that nutrient weight into the required weight of urea product. Since urea is 46% nitrogen, the formula requires dividing the desired pounds of N by the decimal equivalent of urea’s nitrogen percentage, which is 0.46. This calculation directly yields the total pounds of urea fertilizer needed to cover one acre.
For example, if the determined nitrogen requirement is 100 pounds of actual N per acre, the calculation would be 100 pounds of N divided by 0.46. This results in an application rate of approximately 217.4 pounds of urea product per acre. This conversion is necessary because the plant requires the nutrient, not the total weight of the fertilizer material itself. Accurately performing this division is the core action in determining the application rate.
Managing Environmental Loss
Despite its high nitrogen content, urea presents a risk of nitrogen loss through volatilization, where nitrogen converts into ammonia gas and escapes into the atmosphere. This loss is especially pronounced when urea is surface-applied to warm, moist soil with a high pH, as the enzyme urease quickly breaks down the urea molecule. The resulting loss reduces the fertilizer’s efficiency and negatively impacts the environment.
To mitigate this effect, immediate incorporation of the fertilizer into the soil is the most effective strategy, either through tillage or irrigation. Applying 0.4 to 0.6 inches (10 to 16 mm) of water shortly after application can move the urea deep enough to prevent the surface conversion to ammonia. Another common strategy is the use of urease inhibitors, often containing the active ingredient N-(n-butyl) thiophosphoric triamide (NBPT). These inhibitors temporarily block the urease enzyme, providing a window of several days for the urea to be naturally incorporated by rain or moisture.
Avoiding Fertilizer Burn
Urea carries a high salt index, meaning it has the potential to draw moisture out of plant tissues, a condition known as fertilizer burn or phytotoxicity. This damage occurs when the fertilizer granules dissolve and create a temporary zone of high salt concentration near the roots or on the leaves, leading to dehydration and cell death. The risk is highest on turf and when the application rate is too concentrated or immediately followed by dry conditions.
To prevent this burn, avoid applying high rates of soluble nitrogen in a single pass. A maximum safe rate for a single broadcast application of soluble nitrogen on turfgrass is generally no more than 1 pound of actual nitrogen per 1,000 square feet. Applying the urea when the foliage is dry and ensuring the application is followed by light irrigation is recommended. The water helps dissolve and disperse the urea, diluting the salt concentration and moving the nitrogen into the soil where it can be safely absorbed by the roots.