This document specifies a method for the determination of the cold and hot water insoluble nitrogen content in solid urea formaldehyde and methylene urea slow-release fertilizers and for the determination of the solubility of nutrient polymers in a phosphate buffer solution with a pH of 7,5 at 100 °C, in their pure form in inorganic fertilizers. This document is not applicable to the following nutrient polymers: liquid methylene urea.