Compound granular fertilizers frequently cake during storage, making even distribution difficult. An investigation into this problem by X‐ray crystallographic and optical methods showed that interaction occurred between the ingredients during manufacture, with the formation of ammonium chloride from ammonium sulphate and potassium chloride. During drying the ammonium chloride is concentrated at the surface of the granules by sublimation. Small needle‐shaped crystals of ammonium chloride grow outwards from the granules in the presence of moisture, and this recrystallization is the cause of caking. To prevent caking, thorough drying is necessary, but high temperatures should be avoided as they would increase the surface concentration of ammonium chloride by sublimation.
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W. A. Mitchell (1954) studied this question.