Nitrogen is a fundamental building block for plant life, driving processes like photosynthesis and protein formation. When discussing liquid nitrogen in agriculture, the term almost universally refers to Urea Ammonium Nitrate (UAN) solution, a popular liquid fertilizer. Applying the correct amount of this nutrient is directly tied to maximizing crop yield and minimizing waste. Determining the application rate requires a precise calculation that converts the crop’s nutritional requirement into a specific volume of liquid product per acre, ensuring the exact amount of elemental nitrogen is delivered.
Understanding Urea Ammonium Nitrate (UAN) Solution
Urea Ammonium Nitrate solution is a clear, liquid fertilizer combining three distinct forms of nitrogen: urea, ammonium nitrate, and water. This blend provides nitrogen in three different chemical forms, offering both immediate availability and sustained release to the plant roots. Approximately 50% of the total nitrogen is derived from urea, while the remaining 50% is split equally between the fast-acting nitrate form and the slower-converting ammonium form.
The most common UAN concentrations available in the market are UAN-28 and UAN-32, where the number indicates the percentage of actual elemental nitrogen (N) by weight in the solution. For instance, UAN-32 is 32% nitrogen by weight, with the remainder being primarily water and other components. The concentration chosen often depends on the local climate, as a higher concentration like UAN-32 has a higher “salt out” temperature, the point at which fertilizer components begin to crystallize out of the solution. This crystallization risk means that UAN-28 is generally preferred in regions experiencing colder storage temperatures.
Determining Required Nitrogen Pounds
The foundational step in this calculation is determining the crop’s actual nutrient requirement, measured in pounds of elemental nitrogen per acre (lbs N/acre). This requirement is not a fixed number but is highly dependent on a variety of site-specific agronomic factors. Soil testing is a primary tool used to establish the baseline of existing available nitrogen in the field, helping to avoid over-application.
A major factor influencing the required rate is the farmer’s yield goal. For corn, a common rule of thumb is to calculate between 1.0 to 1.2 pounds of nitrogen for every bushel of expected yield. This calculation must be adjusted based on the field’s rotation history, as planting corn after a nitrogen-fixing crop like soybeans requires less applied nitrogen. Furthermore, university extension offices and regional bodies often provide updated recommendations based on long-term field research, accounting for local soil types and climate conditions.
Calculating Application Volume Per Acre
Once the required pounds of elemental nitrogen per acre have been determined, the next step is converting this mass requirement into the liquid volume needed. This conversion relies entirely on the specific concentration of the UAN product being used. The first variable needed is the weight of actual nitrogen contained in a single gallon of the liquid fertilizer.
To find this value, the total weight of one gallon of the UAN solution is multiplied by its nitrogen concentration percentage. For example, a gallon of UAN-28 weighs approximately 10.65 pounds, meaning it contains about 2.98 pounds of actual nitrogen (10.65 lbs \(\times\) 0.28 = 2.98 lbs N/gallon). A gallon of UAN-32 is denser, weighing about 11.06 pounds, which translates to approximately 3.54 pounds of nitrogen per gallon (11.06 lbs \(\times\) 0.32 = 3.54 lbs N/gallon).
The final volume calculation is performed by dividing the total required pounds of nitrogen by the pounds of nitrogen per gallon. For instance, if the goal is to apply 150 pounds of nitrogen using UAN-32, the required volume would be 42.3 gallons per acre (150 lbs N \(\div\) 3.54 lbs N/gallon \(\approx\) 42.3 gallons per acre). This arithmetic ensures that the correct mass of the nutrient is delivered to the field, regardless of the UAN concentration used.
Application Techniques and Timing
The calculated volume of UAN solution must be delivered to the crop using a method and timing that maximizes nitrogen uptake and minimizes environmental loss. Split application timing, where the total nitrogen is applied in two or more smaller doses, is a common strategy that increases efficiency by supplying the nutrient closer to the crop’s peak demand period. This approach often involves an initial pre-plant or at-planting application followed by a “sidedress” application later in the growing season.
The physical method of application is also a factor in nutrient efficiency. Broadcasting the liquid fertilizer across the soil surface can lead to the loss of the urea component to the atmosphere as ammonia gas (volatilization). To combat this loss, UAN is often applied through injection or banding, placing the liquid directly below the soil surface or concentrated near the plant row. When surface application is necessary, a urease inhibitor is used to temporarily slow the conversion of urea, allowing rainfall or irrigation to move the nitrogen into the soil before volatilization occurs.