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This document specifies a method for the determination of the biuret content by high-performance liquid chromatography (HPLC) with ultraviolet (UV) detector. This document is applicable to organic and organo-mineral fertilizers containing urea. This document is applicable to fertilizing products blends where a blend is a mix of at least two of the following products: fertilizers, liming materials, soil improvers, growing media, inhibitors, plant biostimulants and where organic matter is present in at least one of the products in the blend. In case a fertilizing product blend is composed only of inorganic products, the European Standard for inorganic fertilizers applies. Variations in analytical methods for fertilizing product blends can lead to differing results as some components or matrix interactions can affect the outcome. Validation procedures have shown that developed standard methods are robust and reliable across diverse product compositions, but possible interferences and unexpected results when analysing fertilizing product blends are possible. NOTE 1 This method was tested only for one fertilizing product blend during the inter-laboratory study. The product tested was a liquid organo-mineral mixture of a liquid organic fertilizer, a growing medium and a non-microbial plant biostimulant. NOTE 2 Nowadays, there is a method standardized as EN 15479 that allows the determination of biuret in urea by spectrophotometric detection. Organic and organo-mineral fertilizers contain organic matter and other compounds apart from urea that would interfere in a spectrophotometric method. HPLC allows an accurate determination of biuret by separating it from possible interfering compounds [2] [5].