Development of calcium hydrogen phosphate coated urea and its release characteristics

A series of poly-nutrient and slow release/controlled novel urea,which were harmless to soil,were made by using fertilizer-grade calcium hydrogen phosphate as coating material and PVA water solution as adhesive.The release characteristics of coated urea were determined by means of water marinating and soil incubation.Surface and cross-section topography of the urea were detected by scanning electronic microscopy.The experimental results indicate that coating proportion was the domain factor controlling the release rate of the urea.The results of regression of release data showed that the N-release process of CaHPO4 coated urea could be fitted by the first-order release kinetics;it will take 5.36 d and 12.53 d to release 80% N in water for CU-25 and CU-30 fertilizers respectively,while in the soil it will take over 110 d;the SEM results for the coated urea showed that the aperture in the membrane was less than 1 μm.