Legacy phosphorus mobilization by reactive silica amendments: from laboratory mechanisms to agronomic effectiveness in winter wheat

Abstract Background and Aims Historical manure applications on the Delmarva Peninsula created legacy soil phosphorus (P) accumulation, yet winter wheat ( Triticum aestivum L.) often faces early-season P deficiency due to fixation in acidic soils. This study evaluated whether reactive silica (Si) amendments mobilize legacy P across three scales: laboratory chemical desorption, in-field pot plant uptake, and field trial yield. Methods A tiered approach used three legacy P soils. A 154-day incubation screened the desorption potential of silicic acid, Ca-Mg silicate slag, and switchgrass char. An in-field pot study isolated Si effects from liming effects using pH-balanced applications of silicic acid, silica gel, and slag. Finally, a field trial compared Ca-Mg silicate slag against standard lime and starter P practices. Results In the incubation, silicic acid and slag chemically mobilized P, while switchgrass char reduced water extractable P (WEP). In the pot study, reactive Si increased soil WEP by up to 70% and nearly doubled tissue Si, but did not enhance biomass or P uptake. Similarly, slag applied in-field did not alter soil P availability, though it significantly increased grain yield compared to standard management (8.25 vs. 7.58 Mg ha −1 ). Conclusion We hypothesize that pH-induced secondary Ca-P precipitation overrode Si-driven P mobilization. Furthermore, because yield improved without altering P nutrition, we hypothesize this field benefit was driven by Si-mediated stress resilience. Ultimately, while reactive silica chemically mobilizes legacy P, it cannot reliably replace starter P fertilizers. Heavy metal testing is recommended before applying slag in forage systems.