Influence of phosphorus application rate and forage species on yield and phosphorus balance in a three-year Ontario trial

Perennial forage species, including grasses and legumes, are often included in the same fertility recommendation category, despite the possibility of differing phosphorus (P) response between species. There is also evidence in Ontario, Canada of perennial forage stands performing well despite very low soil test P (STP) concentrations, indicating that some forage stands may require less P fertilization. This study examined the effect of P fertilizer application rate and species on forage yield and P balance over the course of a three-year (2022-2024) P fertility trial conducted at an Ontario field site in a calcareous silt-loam soil. Mixtures and monocultures of common Ontario forage species were grown under varying P application rates (0, 30, 60 and 90 kg ha-1). Three-year cumulative yield across all species was not significantly affected by P application rate, despite soil test P concentrations being low (mean of 4.46 mg Olsen P kg-1 at the end of the trial). Alfalfa-grass mixtures displayed the highest three-year cumulative yield (37 230 kg DM ha-1) and highest cumulative P removal through harvest (89 kg ha-1). Curiously, STP declined in the highest fertility treatment (from 10.9 in 2023 to 5.6 mg kg-1 in 2024) despite a positive three-year cumulative P balance of 30 kg ha-1. These results indicate that forages may not be sensitive to soil P deficiencies in all cases and that perhaps P application in forages can be reduced. However, differences in P removal between species may necessitate adjustments to avoid unwanted soil P accumulation or depletion over time.