Sulfur-Modified Coated Slow-Release Fertilizer Based on Castor Oil: Synthesis and a Controlled-Release Model

There has been advancement in the preparation of coated slow-release fertilizer, but problems remain, such as a complex production process, high production costs, and uncontrollable nutrient release. Herein, a new interpenetrating polymer network (IPN) for coating urea was prepared with inexpensive castor oil via a one-pot method. Considering the low strength of oil-based materials, sulfur was used as a reinforcing material to synthesize the interpenetrating network. After prepolymerization, the material strength increased by a maximum of 455.9% and the antiswelling ability increased by 46.10%; however, the thermal stability decreased. The results show that the nutrient release longevity of coatings with only 5.0 wt % polymer could reach 210 days, which is probably the widest applicability range reported so far. Furthermore, a nutrient release model was established to provide guidance for the preparation of slow-release fertilizers suitable for the growth of different plants. Pot experiments show that different slow-release fertilizers have different promoting effects on plant growth, and selecting a suitable slow-release fertilizer according to the needs can maximize the nutrient utilization efficiency. This work offers a simple and effective method to prepare slow-release fertilizers and can guide their development.