A novel, slow‐release polymer containing phosphorous fertilizer was synthesized by the rapid coordination of iron ions with polymers after the microwave radiation polymerization of collagen, 2‐acrylamido‐2‐methyl‐1‐propanesulfonic acid, and acrylic acid. We obtained an optimized collagen‐g‐poly(acrylic acid‐co‐2‐acrylamido‐2‐methyl‐1‐propane sulfonic acid) collagen‐g‐p(AA‐co‐AMPS)–Fe(III) polymer with great equilibrium swelling capacities of 2595 g/g in distilled water and 121 g/g in 0.9 wt % NaCl solution and an excellent sustained fertilizer release of about 30 days. In addition, for the collagen‐g‐p(AA‐co‐AMPS)–Fe(III) polymer, the effect of cations on its swelling followed the order K+ > Na+ > Mg2+ > Ca2+. The structure and morphology were characterized by Fourier transform infrared spectroscopy, thermogravimetric analysis, and scanning electron microscopy. Moreover, the collagen‐g‐p(AA‐co‐AMPS)–Fe(III) polymers emancipated the nutrients in a more controlled manner than collagen‐g‐p(AA‐co‐AMPS). The thermal stability, water absorbency, and good slow‐release of fertilizer provide the collagen‐g‐p(AA‐co‐AMPS)–Fe(III) polymer with great potential as a fertilizer‐carrying vehicle and an aquasorb to be applied in agriculture and horticulture. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47178.