Slow-release fertilizers represent a possible large-scale application for plant polymers. Here we show a facile way to stabilize urea in fertilizer systems by lignin. Chelation of kraft black liquor with calcium acetate at pH 13 precipitated lignin as a calcium complex (Ca-lignin), which offered beneficial effects if compared to those from lignin obtained by precipitation at low pH (Acid-lignin). The reduced affinity of water to Ca-lignin was exploited in the formulation of slow release fertilizers comprising wheat straw sections impregnated with Ca-lignin in molten urea. Compared to the case of Acid-lignin, immersion in water was slowed down more extensively by Ca-lignin. After 24 h incubation at low moisture conditions, the highest proportion of urea retained in the Ca-lignin/straw fertilizer system was 58%. The water resistance of Ca-lignin was explained by a lower aqueous solubility that differed from the typical pH-dependent solubility of Acid-lignin. Electron microscopy, infrared spectroscopy, and accessible surface areas suggested that Ca-lignin consisted of less densely packed molecules organized as calcium-chelated chains. Overall, the controlled water-solubility of lignin precipitated by metal cations is greatly beneficial in fertilizer systems and can open new opportunities in material development (permeable films and others).