Search

Considerable amounts of residual fertilizer phosphorus (P) have accumulated in the agricultural soils of Finland since the 1960s, and the P fertilizer application recommendations have been lowered. It is unknown how much P intensively managed silage grass can obtain from the accumulated reserves without a loss of yield. In two field experiments on sandy loam conducted in 2003-2020, four consecutive grass (70% timothy, 30% fescue) rotations were performed (4 or 5 years each, including the establishment year). The grass received mineral P fertilizers (PF; 16 kg P ha-1 year-1), cattle slurry (PS; 11 kg P ha-1 year-1) or no P (P0). The organic P (Po) and inorganic P (Pi) pools in 2003 and 2020 samples were determined following the Hedley procedure using H2O, NaHCO3, NaOH and HCl as sequential extractants. Soil test P (STP) was monitored annually using ammonium acetate extraction. The results showed that the cumulative P balance (P0: -344 to -412 kg ha-1; PF and PS: -101 to -198 kg ha-1) was highly negative, resulting in declining STP. Still, after 18 years, the grass showed no consistent yield response to P fertilizer application. The most significant Pi decline occurred in the Pi-NaHCO3 (~30%) and Pi-NaOH (~50%) pools, while the changes in Po were negligible. This study and international comparisons, Mehlich-3, degree of P saturation and the result of Hedley in other studies, suggest that these soils, initially above the critical STP level, contain plenty of legacy P and can provide perennial grass with sufficient P for a long time. [ABSTRACT FROM AUTHOR]