the desired degree of sphericity and their low crushing and impact strength, the performance of prills during bulk storage, handling and use is generally (with some exceptions) considered inferior to that of granules. High-quality compound fertilizers containing nitrogen co-granulated with other components such as phosphates have been produced routinely since the beginnings of the modern fertilizer industry, but on account of the low melting point and hygroscopic nature of urea it took courage to apply the same kind of technology to granulate urea on its own. But at the end of the 1970s three companies began to develop fluidized-bed granulation. UAN solutions In admixture, the combined solubility of ammonium nitrate and urea is so much higher than that of either component alone that it is possible to obtain a stable solution (known as UAN) with a total nitrogen content (32%) approaching that of solid ammonium nitrate (33.5%), though not, of course, that of urea itself (46%). Given the ongoing safety and security concerns surrounding fertilizer-grade solid ammonium nitrate, UAN provides a considerably safer alternative without entirely sacrificing the agronomic properties that make ammonium nitrate more attractive than urea as a fertilizer in areas with short growing seasons. It is also more convenient to store and handle than a solid product and easier to apply accurately to the land by mechanical means. Laboratory preparation Ureas in the more general sense can be accessed in the laboratory by reaction of phosgene with primary or secondary amines: These reactions proceed through an isocyanate intermediate. Non-symmetric ureas can be accessed by the reaction of primary or secondary amines with an isocyanate. Urea can also be produced by heating ammonium cyanate to 60 °C. See also Wöhler urea synthesis Thiourea References External links Nitrogen cycle Functional groups Excretion Soil improvers Fertilizers Urea cycle Lymph fluid Commodity chemicals Household chemicals E-number additives Organic compounds with 1 carbon atom