The amounts of each composition in obtained double-coated urea granules were measured as follows, and those data were average values. Granules only coated with EC were immersed into abundant water until the inner urea granules were dissolved totally and the EC ghost finally floated on the water. The average weight of pure urea granules could be calculated based on the mass lose of granules only coated with EC before and after water soak. The amount of EC coated on the surface of urea granules could be obtained by weighting the EC ghost. Also, the average content of SAP coated as second layer could be acquired by measuring the mass gain before and after coating process with SAP. Therefore, the slow-release fertilizer exhibited a core-shell structure, of which the urea granule was core (ca. 55% w/w) with EC (ca. 10%) as the inner shell and starch-SAP (ca. 35%) as the outer shell. Additionally, the fracture morphology of maize-SAP coated fertilizer granule is shown in Fig. S2. The fertilizer granule showed a typical core-sell structure, while the thicknesses of EC and starch-SAP layers were approximately 180 and 700 μm, respectively