Can silicon in glacial rock flour enhance phosphorus availability in acidic tropical soil?

Background and aim: Plant-available silicon (Si) is limited in strongly weathered tropical soils. The aim of the study was to evaluate Si fertilisation as a strategy to improve phosphorus (P) fertiliser use efficiency on acidic tropical soil, and the potential of using glacial rock flour (GRF) as a source of Si. Methods: Wheat (Triticum aestivum cv. Benchmark) plants were grown in a pot experiment in an oxisol soil. Si was supplied as either GRF or silicic acid (SiA) at low and sufficient P levels. A soil incubation study and a compartmented pot experiment were also included to assess how soil chemistry was affected by the Si sources both with and without plants. Results: Addition of Si as either GRF or SiA doubled shoot biomass under limited P conditions, and the combined treatment quadrupled it. SiA led to increased P uptake under P-limited conditions, while GRF did not. Soil incubation showed that only SiA increased P availability in bulk soil. The addition of SiA in combination with GRF led to increased root exudation of citric and oxalic acid. Localised weathering of GRF resulted in higher rhizosphere pH, but did not affect rhizosphere P availability or organic acid exudation. Conclusion: Si released from GRF can rejuvenate strongly weathered agricultural soils, improve stress tolerance in plants and ultimately increase yields but GRF does not improve P availability on acidic soils.