Slow-release fertilizer has been proven to be more effective than traditional fertilizer for providing a long-term stable nutrient supply. Although such fertilizers have been widely investigated, their water-retention properties and biodegradability have not been fully analysed. Composites of fertilizers and polymers provide opportunities to prepare new types of fertilizer with enhanced properties for real applications. Chicken feather protein-graft-poly(potassium acrylate)-polyvinyl alcohol semi-interpenetrating networks forming a super absorbent resin combined with nitrogen (N) and phosphorus (P) (CFP-g-PKA/PVA/NP semi-IPNs SAR) was prepared. The chemically bonded or physically embedded fertilizer compound could be released form the resin matrix to the surrounding soil under irrigation. The synthesis mechanism, morphology, and chemical and mechanical structure of the synthesized composites were investigated. The reactant doses were optimized through response surface methodology (RSM). A 30-day field trial of the prepared SAR was applied to detect the influence of sample particle size, soil salinity, pH, and moisture content on the slow-release behaviour of N and P. The maximum release values of N and P from the composites were 69.46% N and 65.23% P. A 120-day soil burying experiment and 30-day Aspergillus niger (A. niger) inoculation were performed, and the biodegradability and change in microstructure were monitored. The addition of SAR to soil could also improve the water-retention ability of the soil.