The initial products of controlled-nitrogen-release fertilisers contained 60% of total nitrogen that was cold-water insoluble, with less than 15% being unreacted (quick-release). However, in the 1970s, this problem was solved by using methylene urea and having 25% and 60% of the nitrogen as cold-water insoluble, respectively, and 15%-30% of unreacted urea nitrogen. The National Fertilizer Development Center developed Sulfur Coated Urea fertilisers, with sulphur serving as the primary coating material due to its value as a secondary nutrient and low cost. Polymers were used as the sealant in sulphur for developing and improving slow-release properties based on secondary sealant degradation by mechanical imperfections as well as soil microbes in the sulphur.
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