Anhydrous ammonia (\(NH_3\)) is a primary source of nitrogen fertilizer used across large-scale agriculture to support intensive crop production. This compound is a colorless, highly compressed gas that is stored and transported as a liquid under high pressure. Determining the precise amount of this product to apply per acre requires a careful calculation that begins with establishing the crop’s exact need for elemental nitrogen.
The Role of Anhydrous Ammonia in Crop Nutrition
Anhydrous ammonia is a highly efficient nitrogen source because it contains approximately 82% nitrogen by weight, making it the most concentrated fertilizer available. Once the liquid ammonia is injected into the soil, it immediately converts into a gas and reacts with soil moisture to form ammonium (\(NH_4^+\)). This positively charged ammonium ion is readily held by negatively charged clay and organic matter particles, which makes it stable and resistant to leaching or volatilization loss.
Soil microorganisms then gradually convert the ammonium to nitrate (\(NO_3^-\)) through a process called nitrification. Plants can absorb nitrogen in both the ammonium and nitrate forms, ensuring a sustained release of the nutrient throughout the growing season.
Factors Dictating Nitrogen Requirements
The actual amount of anhydrous ammonia needed per acre is entirely dependent on the required pounds of elemental nitrogen (N) a crop needs to reach its yield goal. A soil analysis is the most reliable way to determine the existing residual nitrogen present in the field before planting. Growers use this information to credit the N already available, preventing over-application.
The previous crop grown also contributes to the N budget; legumes like soybean or alfalfa leave behind residual nitrogen that reduces the fertilizer requirement for the following season. Crop-specific needs vary significantly and are directly tied to the target yield. For example, a high-demand crop like corn requires significantly more N per acre than a moderate-demand crop like wheat or cotton.
For a corn crop targeting high yields, the nitrogen recommendation might range from 140 to over 200 pounds of elemental N per acre, while a wheat crop might require a lower rate, perhaps 80 to 120 pounds of N per acre. The rate at which soil organic matter mineralizes and releases nitrogen also contributes to the final calculation of the net N requirement.
Calculating the Anhydrous Ammonia Application Rate
Once the precise pounds of elemental nitrogen (N) required per acre have been determined, the final step is to convert this figure into the actual weight of anhydrous ammonia product needed. This conversion is necessary because only 82% of the total product weight is the nutrient itself. The calculation uses a simple division: The pounds of anhydrous ammonia needed per acre equals the target pounds of nitrogen divided by 0.82.
For a corn field requiring 150 pounds of elemental N per acre, the calculation would be 150 divided by 0.82, which results in approximately 183 pounds of anhydrous ammonia product per acre. Similarly, a wheat field requiring 100 pounds of N would need about 122 pounds of the product per acre.
This calculation provides the target application rate, which is then controlled by calibrating the application equipment. Flow meters and controllers must be set to deliver this specific weight of product across the field. To prevent the loss of the gaseous product, the anhydrous ammonia must be injected at a depth of six to eight inches below the soil surface. The soil must properly seal the injection knife track to ensure the ammonia is trapped and reacts with the soil moisture.