Legacy Phosphorus in Agricultural Soils: Accessibility, Mobilization, Diagnostics, and Management Implications—a Review

Legacy phosphorus (P), accumulated through long-term fertilizer and manure surpluses, represents both a potential nutrient reserve and an environmental concern in agricultural soils. This review synthesizes current understanding of the mechanisms controlling legacy P accessibility, evidence from long-term field studies, and implications for soil diagnostics and adaptive nutrient management strategies. Accessibility of legacy P is regulated by interacting chemical, biological, and physical processes governing phosphate retention and rhizosphere resupply. Evidence from long-term studies indicate that legacy P primarily functions as a finite buffering reserve rather than a fully accessible substitute for fertilizer inputs, with responses varying among soils, climates, and management systems. Current evidence also suggests that crop accessibility to legacy P is governed more strongly by rhizosphere-scale resupply dynamics and soil buffering behavior than by total soil P stocks alone. Conventional soil P tests often inadequately predict crop responses during prolonged fertilizer drawdown because static extractants incompletely represent soil P resupply dynamics. Dynamic and plant-based diagnostic methods may improve assessment of functional P availability under legacy-P conditions, although practical and calibration limitations remain. Effective management of legacy-P-rich soils therefore requires adaptive, site-specific strategies integrating gradual fertilizer drawdown, biologically supportive practices, diagnostic-guided management, and environmental safeguards.