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The percentage of cold water insoluble nitrogen that is soluble in hot water. Higher values typically mean a more efficient use (because it is more soluble) of the nitrogen from the applied controlled slow release fertiliser, being either urea formaldehyde or methylene urea products. The term is commonly shortened to AI in equations.

AI= (CWIN-HWIN)/CWIN x 100%

Note: CWIN = Cold Water Insoluble Nitrogen | HWIN = Hot Water Insoluble Nitrogen
(Source: ISO 19670:2017. Fertilizers and soil conditioners - Solid urea aldehyde slow release fertilizer - General requirements)