Monitoring 28% and UAN Solutions

UAN (urea-ammonium-nitrate) solutions are routinely applied in the late spring and early summer to deliver nitrogen (N) to young crops. Because UAN is a nonpressurized solution, it can be used without the hazards associated with anhydrous ammonia and can be spread more uniformly than granular fertilizer. Certain pesticides may also be added, eliminating an extra pass through the field. UAN solutions are usually manufactured with a 32% N analysis, transported nearer the point of use, and then diluted (“cut”) to 28% or 30% N with water or a nutrient rich solution such as ammonium sulfate. The N analysis of UAN solutions is monitored throughout the supply chain to assure quality and consistency. A hydrometer is commonly used to check the N analysis of solutions, estimating the N content by solution density. A hydrometer reading will vary with the temperature of the solution. Nitrogen and density values will differ by source of 32% UAN solution.

The density of a cutting solution should also be considered when making dilutions. The density of ammonium sulfate (AMS) solution is usually higher than water, and can vary by source. For example, if a hydrometer reading is 1.28 g/cc for a UAN solution that was diluted with water to 28% N, diluting the same UAN solution to that density with AMS would result in a product with only 26% N. The UAN density corresponding to a given N content must be adjusted when diluting with a solution other than water. A hydrometer calibration (chart) can be developed using laboratory analyses of N, S and specific gravity on the various solutions (UAN, AMS, etc.) and mixtures that might be used. Once a calibration is developed, hydrometer readings, adjusted for temperature, can approximate the N content of the UAN solution. Periodic laboratory analyses should be performed to update the hydrometer calibration since the density of new UAN and cutting solutions can change.