Relative Agronomic Effectiveness of Struvite Phosphorus for Perennial Ryegrass ( Lolium perenne ) in Contrasting Grassland Soils

New Zealand grassland systems rely heavily on imported phosphorus (P), yet conventional water‐soluble fertilizers such as single superphosphate are used inefficiently and contribute to legacy P accumulation and water quality risks. Recovered P from wastewater, such as sparingly soluble struvite (MgNH 4 PO 4 ·6H 2 O), offers a potentially more sustainable alternative. This study evaluated the agronomic effectiveness of struvite compared with superphosphate in two acidic grassland soils with contrasting P retention capacities (12% and 96%). Perennial ryegrass (Lolium perenne L.) was grown for 3 months under controlled conditions. Treatments included two P sources (struvite and superphosphate) applied at 0, 15, 30, and 60 mg P kg −1 . Measurements included dry matter yield, plant P uptake, relative agronomic effectiveness, and residual soil Olsen‐P. Fertilizer application significantly increased plant growth and P uptake. While dry matter yields were similar between fertilizers, struvite increased plant P uptake by 49% compared with superphosphate across soils and rates. Relative agronomic effectiveness of struvite was greater in the low P retention soil. Olsen‐P concentrations were consistently higher under struvite in both soils. These results indicate that struvite is a viable alternative to superphosphate in acidic grassland soils, particularly in those where high P‐retention limits P availability.