Smart pH-Responsive Controlled-Release Fertilizers Using Castor Oil Alkyd Resin-Based Polyurethane Coatings

Biobased controlled-release fertilizers (CRFs) contribute to sustainable agricultural development. However, their nutrient release often lacks sufficient responsiveness to environmental fluctuations, limiting overall fertilizer use efficiency. This study aimed to develop smart and green CRFs capable of modulating nutrient release in response to soil pH variations. A low-cost dehydrated castor oil (DCO)-based or castor oil (CO)-based alkyd resin (DCOCA/COCA) was used as the foundation for polyurethane (PU) reactions to prepare a DCOCA/COCA-coated CRF (DCOCAPF/COCAPF). Next, a one-step polymerization reaction was performed between HDI and differing amounts of 3-dimethylamino-1,2-propanediol (DMAPD), which contains a tertiary amine group with pH-responsive properties. The resulting products (DMAPDIs) were then subjected to a further polyurethane reaction with COCA to produce pH-responsive CRFs (DMAPDIPFs). The DMAPDIPFs exhibited excellent pH responsiveness with approximately 29% increase in the nitrogen release rate and 7-day reduction in the nitrogen release longevity under acidic conditions compared to that under neutral conditions, requiring only 1% of DMAPDI by total mass of the coating materials. This work provides valuable insights for developing smart CRFs that can autonomously adjust their nutrient release characteristics in response to environmental stimuli to increase the fertilizer utilization rate and thereby promote sustainable agricultural development.