Long-term changes in soil phosphorus in response to fertilization and negative phosphorus balance under grass rotation in mineral soils in Nordic conditions

Considerable amounts of residual fertilizer phosphorus (P) have accumulated in the agricultural soils of Finland since the 1960s, and the P fertilization 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 (four or five 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– −412 kg ha⁻¹; PF and PS: −101– −198 kg ha⁻¹) was highly negative, resulting in declining STP. Still, after 18 years, the grass showed no consistent yield response to P fertilization. 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.