Urea fertilizer (46-0-0) is a widely adopted, cost-effective source of nitrogen used in both agriculture and home gardening. Its high nitrogen concentration makes it an efficient nutrient delivery system. Nitrogen drives vegetative growth and is a component of chlorophyll, so proper application can significantly enhance plant health and yield. However, the concentrated nature of urea requires careful management to ensure the nutrient is absorbed efficiently and safely.
Understanding the Composition of Urea
Urea is an organic compound synthesized industrially from ammonia and carbon dioxide. Containing approximately 46% nitrogen by weight, it is the most concentrated solid nitrogen fertilizer available. This high analysis is why it is listed as 46-0-0 in the standard NPK grading system.
The nitrogen is in an amide form, which plants cannot immediately use for growth. It must first undergo hydrolysis, facilitated by the urease enzyme in the soil, converting it into plant-available ammonium. This conversion process is critical because the intermediary ammonium can be easily lost to the atmosphere if not managed correctly.
Commercial urea is sold as small prills or larger, harder granules. Granular urea is preferred because its density and resistance to moisture help reduce nutrient loss compared to the prilled form.
Calculating the Required Application Rate
Determining the precise amount of urea to apply is a preparatory step that prevents both nutrient waste and plant damage. The general guideline is to apply no more than 1 pound of actual nitrogen per 1,000 square feet in a single application, particularly for turfgrass and other shallow-rooted plants. Applying more than this increases the risk of phytotoxicity, or plant burn, due to salt concentration.
To calculate the specific amount of urea product needed, divide the target nitrogen rate by the percentage of nitrogen in the fertilizer. For example, if the target is 1 pound of nitrogen and the product is 46% nitrogen, the calculation (1 / 0.46) yields 2.17 pounds of product per 1,000 square feet.
A soil test provides the most reliable way to determine the target nitrogen rate. The application rate should be adjusted based on the specific crop’s needs, as heavy feeders like corn require more nitrogen than light feeders.
Successful Application Methods
Urea can be applied in solid granular form or dissolved in water for liquid application. Granular urea is typically broadcast across the soil surface using a rotary or drop spreader. Equipment must be calibrated before application to ensure the correct output rate and even distribution.
For liquid application, urea is highly water-soluble and can be used as a foliar spray or a soil drench. Foliar spray concentration should be kept low (2% to 4%) to prevent scorching the foliage. Foliar feeding provides quick nutrient delivery, bypassing the soil conversion process.
In agricultural settings, mechanical incorporation is important when applying large amounts to bare soil. Tillage equipment mixes the fertilizer into the top few inches of soil immediately after broadcasting. This minimizes the fertilizer’s exposure to the atmosphere, reducing nitrogen loss.
Preventing Nitrogen Loss and Plant Burn
The two primary risks associated with urea application are volatilization, which is the loss of nitrogen to the air, and phytotoxicity, or plant burn. Volatilization occurs when ammonium converts into ammonia gas (NH3) and escapes from the soil surface. This process is accelerated by high temperatures, dry conditions, and alkaline soils.
To prevent volatilization, the fertilizer must be moved below the soil surface. This is accomplished by applying urea immediately before rain or by ensuring immediate irrigation after application. As little as 0.25 inches of water is sufficient to dissolve the urea and move it into the soil profile. If water is unavailable, the urea must be mechanically incorporated into the soil within 24 hours to prevent significant loss.
Phytotoxicity happens when high concentrations of soluble salts draw water out of the plant roots. For granular applications, the risk is mitigated by watering the product in immediately to dilute the concentration around the roots. For liquid applications, using the correct dilution ratio prevents scorching the leaf tissue.
An additional strategy is using enhanced efficiency fertilizers that incorporate urease inhibitors, such as N-(n-butyl) thiophosphoric triamide (NBPT). These inhibitors temporarily slow the urease enzyme, delaying hydrolysis and allowing more time for the urea to be moved safely below the surface by water or tillage.