Preparation and characterization of slow-release urea fertilizer encapsulated by a blend of starch derivative and polyvinyl alcohol with desirable biodegradability and availability

In this study, a novel double-layer slow-release fertilizer (SRF) was developed utilizing stearic acid (SA) as a hydrophobic inner coating and a blend of starch phosphate carbamate (abbreviated as SPC) and polyvinyl alcohol (PVA) as a hydrophilic outer coating (designated as SPCP). The mass ratios of SPC and PVA in the SPCP matrices were systematically optimized by comprehensively checking the water absorbency, water contact angle (WCA), water retention property (WR), and mechanical properties such as percentage elongation at break and tensile strength with FTIR, XRD, EDS, and XPS techniques, etc. Moreover, the optimal SPCP/5:5 demonstrated superior water absorbency with an 80.2 % increase for the total mass compared to natural starch/PVA(NSP), along with desirable water retention capacity in the soil, exhibiting a weight loss of only 48 % over 13 d. Relative to pure urea and SA/NSPU/5:5, SA/SPCPU/5:5 released 50.3 % of its nutrient within 15 h, leading to nearly complete release over 25 h in the aqueous phase, while only 46.6 % of urea was released within 20 d in soil, extending to approximately 30 d. The slow release performance of urea reveals that the diffusion rate of urea release shows a significant decrease with an increase in coating layers. Consequently, this work demonstrated a prospective technology for the exploration of environmentally friendly SRF by integrating biodegradable starch derivatives with other polymers.