New slow release nitrogen (N), phosphorous (P), and potassium (K) (NPK) fertilizer formulation was synthesized by in situ free-radical graft copolymerization of sodium alginate (NaAlg), acrylic acid (AA), and acrylamide (AAm) in the presence of rice husk ash (RHA) and fertilizer compound solution. FTIR results confirmed that NaAlg chains have been grafted onto acrylate monomers successfully. The water absorbency of hydrogel sample was affected greatly by different salt and pH solutions. The swelling ratio of the hydrogel sample was increased from 830 to 1070 g/g by the addition of RHA up to 15 wt%. The experimental findings indicated that incorporation of RHA into the hydrogel network not only improved its water retention capability, but also caused the system to liberate fertilizer in a more controlled manner compared with the neat hydrogel. Consequently, this new environment-friendly product with good slow release property and high water retention capacity can be useful in agricultural and horticultural applications.