Starch wall of urea: Facile starch modification to residue-free stable urea coating for sustained release and crop productivity.

Quick leaching of urea fertilizer encourages different coatings, but achieving a stable coating without toxic linkers is still challenging. Here, the naturally abundant bio-polymer, i.e., starch, has been groomed to form a stable coating through phosphate modification and the support of eggshell nanoparticles (ESN) as a reinforcement agent. The ESN offers a calcium ion binding site for the phosphate to cause bio-mimetic folding. This coating retains hydrophilic ends in the core and gives an excellent hydrophobic surface (water contact angle 123°). Further, the phosphorylated starch+ESN led the coating to release only ∼30 % of the nutrient in the initial ten days and sustained for up to 60 days to show ∼90 % release. The stability of the coating has been attributed to its resistance to major soil factors viz., acidity and amylase degradation. The ESN also increases elasticity, cracking control, and self-repairing capacity by serving as buffer micro-bots. The coated urea enhanced the yield of rice grain by ∼10%.