Urea is a highly efficient and concentrated source of nitrogen (46-0-0 NPK), which promotes vigorous leaf and stem growth in plants. Because it contains 46% nitrogen by weight, precise concentration per liter of water is necessary; even a small error in measurement can lead to plant damage. The required concentration depends significantly on the application method, such as spraying the leaves (foliar feeding) or applying it to the soil (soil drenching). Determining the correct amount of urea to dissolve ensures plants receive adequate nutrition without risk of harm.
Understanding Urea Concentration
The 46-0-0 ratio indicates urea’s high nitrogen content, necessitating careful dilution. When urea dissolves, it increases the solute concentration, affecting the osmotic potential of the solution. Applying a solution that is too concentrated can draw water out of the plant roots or leaves, causing “salt burn.”
Concentration must be tailored to the application method. Plant roots absorb nutrients efficiently from dilute solutions. Foliar feeding requires a much lower concentration than soil application because leaf tissue has a limited ability to absorb urea through its surface.
Urea is extremely water-soluble, though highly concentrated solutions are typically only used as industrial stock solutions. Even concentrations far below the solubility limit can cause phytotoxicity, or plant damage, especially in foliar applications. Biuret, a compound formed during manufacturing, is generally safe in agricultural-grade urea but can become toxic to sensitive plants if the overall concentration is too high.
Standard Application Rates by Use Case
The target concentration of urea per liter of water is highly dependent on whether the solution is being applied to the soil or directly to the foliage. The maximum concentration that can be safely used for foliar feeding is much lower than the concentration used for soil drenching.
General Lawn Maintenance
For liquid application to lawns, the goal is delivering a specific weight of nitrogen per area. A typical application aims to deliver 0.1 to 0.2 pounds of actual nitrogen per 1,000 square feet. This requires dissolving 110 to 220 grams of 46-0-0 urea into approximately 4 liters of water, which is then sprayed over the area.
The resulting solution is generally mild to minimize leaf tip burn. If a higher concentration is used, it is recommended to water the lawn immediately afterward. This washes the urea off the blades and into the soil.
High-Demand Crops/Vegetables
For high-demand crops, such as corn or leafy greens, the total nitrogen requirement is higher, but the concentration in the application water must still be managed. For small-scale soil drenching, a mild solution of 4 to 5 grams of urea per liter of water can be used for frequent feeding.
For larger fertigation systems, a concentrated stock solution, often up to 300 grams per liter, may be prepared. This stock solution is never applied directly; it is injected into the main irrigation line and diluted to a safe concentration before reaching the root zone. Frequent, low-concentration applications are preferred for soil feeding.
Foliar Feeding
Foliar feeding requires the lowest concentration because leaves are highly susceptible to burn. The safe and effective range is typically 0.5% to 2.0% urea solution, meaning 5 grams to a maximum of 20 grams of urea per liter of water.
For sensitive plants or repeated applications, use the lower end of this range (5 to 10 grams per liter) and a low-biuret grade of urea. Foliar feeding provides a supplemental boost absorbed quickly by the leaves, but it should not be the sole source of a plant’s nitrogen needs.
Safety and Practical Mixing Guidelines
When mixing urea, observe practical and safety protocols. Urea is an endothermic compound; as it dissolves, it draws heat from the water, causing the temperature to drop. Using lukewarm water helps the urea dissolve more quickly and completely.
Consistent agitation is required to ensure all solid particles fully integrate before application. Agitation prevents undissolved granules from clogging spray equipment. Always wear appropriate personal protective equipment, including gloves and eye protection, when handling dry granules or mixing the solution.
During application, avoid spraying urea solution during high heat or intense midday sunlight. High temperatures cause rapid water evaporation, concentrating the urea and risking severe leaf burn. Applying the solution in the early morning or late evening, when temperatures are cooler, is the recommended practice.