Bio-based polyurethane coatings for controlled-release phosphorus fertilizers: Synthesis, characterization and effect on P use efficiency

The use of petroleum-based coatings in controlled-release fertilizers (CRFs) raises environmental concerns and it is unclear if, and to what extent, CRFs can improve P use efficiency. This study explored the synthesis of bio-based polyurethane (PU) fertilizer coatings from castor oil and liquefied wheat stover and their effect on P use efficiency. The castor oil-based PU effectively delayed P release, achieving a 75 % release time (longevity) of 209 d at a 6 % coating rate on monoammonium phosphate (MAP). In contrast, wheat stover-based PU was less effective, likely due to incomplete polymerization. Three coated MAPs with longevities of 7, 37 and 209 d (7d-, 37d-, and 209d-MAP) were tested in an incubation trial using three contrasting soils and in a pot trial using two of those soils. The 37d-MAP reduced P fixation and increased wheat P uptake by 50 % compared to uncoated MAP in a highly P-fixing Oxisol, but had no effect on biomass, likely due to early P deficiency. The CRFs did not reduce P fixation in the other two soils with lower P-fixing capacities, and no improvement in P uptake was observed. The 209d-MAP performed poorly in both soils in the pot trial, as most P was still retained in the granule after 70 d. Overall, these results indicate that CRFs with suitable release rate can enhance P fertilizer efficiency in highly P-fixing soils, but effects are likely to be small or non-existent in most other soils.