Traditional lanthanum-based powdered adsorbents for phosphate recovery often face challenges such as powder loss, low stability, and high material costs, while lacking agricultural applicability. To address these limitations and bridge water treatment with agricultural reuse, we developed a novel composite adsorbent, PFS-PVA-La, by incorporating La(OH)₃ nanoparticles onto a polyvinyl alcohol (PVA)-coated polyurethane foam sponge (PFS) matrix. The designed material serves dual functions: efficient phosphorus capture from water and subsequent utilization as a hydroponic growth substrate. The results demonstrate that the PFS-PVA-La configuration effectively mitigates the issue of powder loss typically associated with traditional lanthanum-based adsorbents, retaining 94% of the original adsorption capacity of La(OH)₃ nanoparticles. Moreover, the PFS-PVA-La exhibits a high phosphorus adsorption capacity of 39.66 mg_P/g, surpassing the performance of most existing composite adsorbents. La(OH)₃ nanoparticles are physically encapsulated within cross-linked PVA layers on the hydrophilic, three-dimensional pore structure of the PFS. The mechanism for phosphate recovery by PFS-PVA-La is attributed to inner-sphere complexation, pore filling, and electrostatic interactions, all of which are significantly enhanced by the incorporation of PVA and La(OH)₃ nanoparticles. Importantly, hydroponic experiments demonstrate the prepared adsorbent’s agricultural value: When used as growth substrate for lettuce, PFS-PVA-La increases fresh weight by 23% compared to control groups while maintaining optimal leaf chlorophyll and vitamin C levels. This work offers a stable, cost-effective material for phosphorus management while creating new value in hydroponic food production.