This research aimed at using fiber Bragg grating (FBG) sensors for monitoring the slow release of urea. Urea granules were coated with ethyl cellulose for controlled-nitrogen release. The FBGs were fabricated using a 10 mm single-mode glass fiber optic, and silver nanoparticle coating was deposited on the partially removed cladding of the FBG using the electroless technique. The silver-coated FBGs at three operating wavelengths (FBG1: 1534 nm, FBG2: 1559 nm, and FBG3: 1565 nm) were used to monitor the slow release of urea in water and soil. The Bragg wavelength shift of FBG 1, 2, and 3 to the controlled-release rate in water was 0.028, 0.12, and 0.20 nm, respectively. In soil, the coated fertilizer responded to the FBG 1, 2, and 3 showing wavelength shifts of 0.028, 0.10, and 0.12 nm, respectively. This study demonstrates the potential of FBG sensors in agricultural applications. © 2023 Elsevier Ltd