Urea is a highly concentrated source of nitrogen fertilizer, designated as 46-0-0, meaning it contains 46% nitrogen by weight. This form of nitrogen is widely used in corn production to support the plant’s high nutrient demand and maximize grain yield potential. Determining the precise amount of urea to apply per acre is a calculated rate based on the specific needs of the crop and the conditions of the field. Applying the correct rate involves balancing the corn’s nutrient requirements with the existing nitrogen supply in the soil.
Understanding Corn’s Nitrogen Requirements
Corn requires substantial amounts of nitrogen (N) because it is a foundational component of proteins, enzymes, and chlorophyll, the molecule responsible for photosynthesis. Nitrogen availability directly influences stalk strength, leaf development, and the eventual size of the ear and grain fill. The total amount of N needed by the plant is accumulated throughout the growing season, but the demand is not constant.
The vegetative growth stage, particularly from the V6 leaf stage up to tasseling (VT), is the period of most rapid nutrient accumulation. During this phase, the corn plant takes up approximately 60% of its total nitrogen requirement. Supplying adequate nitrogen during this high-demand window is paramount because the plant is setting its potential kernel number and size, which determines the final yield.
Key Variables Determining Application Rates
The total nitrogen requirement for a corn crop falls within a range of 1.0 to 1.5 pounds of elemental nitrogen (N) for every bushel of expected yield. This baseline figure must be adjusted downward by subtracting all non-fertilizer nitrogen sources available in the soil. Soil testing is the most accurate way to quantify the nitrogen already present, serving as the most significant input for calculating the required fertilizer rate.
The target yield itself is a primary driver, as a higher yield goal necessitates a larger total N supply. Another adjustment comes from previous crop credits, where a preceding legume crop, such as soybeans, contributes a residual amount of nitrogen to the soil. This contribution, often estimated to be between 30 and 50 pounds of N per acre, effectively reduces the amount of fertilizer nitrogen that must be applied.
Soil characteristics and local weather patterns also modify the final application rate and strategy. Fields with sandy soils or those expecting high rainfall are more prone to nitrogen loss through leaching, which may reduce the efficiency of a single application. Conversely, fine-textured clay soils may fix a portion of applied ammonium-nitrogen, potentially requiring a slightly higher overall rate to ensure plant access.
Calculating the Urea Application Rate
The calculation for determining the pounds of urea product needed per acre begins by establishing the amount of elemental nitrogen (N) the crop requires. This is calculated by multiplying the target yield by the N required per bushel (e.g., 1.2 lbs N/bushel), and then subtracting any soil credits. For example, a 200-bushel yield goal requiring 1.2 lbs N/bushel suggests a total need of 240 pounds of N per acre; subtracting a 40-pound soybean credit reduces the required fertilizer nitrogen to 200 pounds of elemental N per acre. To convert this requirement into the physical amount of urea product, divide the pounds of elemental N by 0.46 (since urea is 46% nitrogen). Using the previous example, 200 pounds of elemental N divided by 0.46 equals approximately 435 pounds of urea product per acre.
Timing and Methods for Effective Urea Use
Once the correct rate is determined, application logistics focus on maximizing nitrogen use efficiency and minimizing loss. A common strategy is the use of split applications, where a portion of the urea is applied pre-plant or at planting and the remainder is sidedressed later in the season. Applying the second portion as a sidedress application, typically around the V4 to V8 growth stage, aligns the nutrient supply with the corn’s period of peak nitrogen demand.
Urea is highly susceptible to volatilization, the conversion to ammonia gas when left on the soil surface in warm, moist conditions. This process leads to significant nitrogen loss to the atmosphere. To mitigate this risk, the urea must be incorporated into the soil, either by light tillage or by receiving at least 0.5 inches of rain or irrigation shortly after application.
Alternatively, a urease inhibitor, such as an NBPT product, can be mixed with the urea before application. These products temporarily slow the enzymatic conversion of urea to ammonia, providing a protective window for the fertilizer to be incorporated by rainfall or mechanical means.