Legacy Phosphorus Mobilization by Silicon Amendments: From Laboratory Mechanisms to Agronomic Effectiveness in Winter Wheat

Background and Aims Historical manure applications on the Delmarva Peninsula have 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 silicon (Si) amendments can mobilize legacy P across three scales: laboratory chemical desorption, in-field pot plant uptake, and on-farm agronomic 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, an on-farm trial compared Ca-Mg silicate slag against standard lime and starter P practices. Results In the incubation, silicic acid and slag increased water-extractable P (WEP), while switchgrass char reduced it. In the pot study, soluble Si increased soil WEP and tissue Si concentrations, but did not enhance biomass or P uptake, likely due to Ca-P precipitation and low ambient stress. Conversely, the field trial showed that while slag did not significantly alter soil P availability, it significantly increased wheat yield compared to standard management, driven by high slag solubility and Si-mediated plant health benefits. Conclusion Silicon amendments can chemically mobilize legacy P, but the mechanism is constrained by soil buffering, Ca-interference, and plant homeostasis. Silicate slag improves yield through stress resilience rather than P-mobilization. Due to variable heavy metal content, caution is advised for its use in forage systems.