Multilayer Silk Sericin-Based Coating for Controlled Release of Water and Nutrients in Soil: Development, Characterization, and Performance Evaluation in Agricultural Production Model

The use of fertilizers coated with insoluble organic compounds is a promising approach for enhancing fertilizer efficiency and crop yield. Silk sericin (SS) is a protein with a high potential for the development of materials oriented toward fertilizer coating and soil amendment because of its biodegradability and the fact that it represents an important source of nitrogen for plants. Thus, this study proposes the design and evaluation of a novel SS-based multilayer coating for fertilizer granules. A pan-coating process was applied to form two distinct layers on the granules: an inner layer made of silk sericin/polyvinyl alcohol, SS/PVA (50/50 w/w), which has low solubility and porosity, and an outer hydrogel layer of SS/PVA with carboxymethyl cellulose CMC (SS/CMC/PVA 45/25/30 w/w/w). Scanning electron microscopy (SEM) was employed for the morphological characterization of the coated fertilizer (CF), examining both the cross-section and surface, while SEM with energy-dispersive X-ray spectroscopy (SEM/EDS) was used to analyze the chemical composition of the surface. The ability of the coating to reduce the nutrient-release rate was studied using water- and soil-release tests. Furthermore, its performance was evaluated in in vivo assays using jalapeño bell pepper (Capsicum annum) plants. The results revealed that the structure and composition of the multilayer coating significantly influenced its ability to delay nutrient release in both water and soil. Moreover, the inclusion of SS in the coating potentially contributed to the increased nitrogen content in the soil, thereby improving plant growth rates.