Bioavailability and Plant Growth Stimulatory Activity of Exogenous Silicon in Agricultural Soils

Silicon (Si) has been proposed as a crop biostimulant owing to its ability to mitigate biotic and abiotic stresses, but its efficacy depends on its soil bioavailability. This study linked Si bioavailability with plant growth responses upon Si supplementation in agricultural soils. Silicon sorption was quantified in seven agricultural soils and model soil constituents, and bioavailable Si was determined in soil solutions from non-supplemented and Si-supplemented soils. Short-term plant bioassays with Lactuca sativa and Eruca sativa assessed early growth responses in water, soil extracts, and pot experiments. Endogenous bioavailable Si was related to pH, sand, and carbonate content, whereas exogenous Si sorption was associated with pH, silt, and oxalate-extractable Fe. The biostimulatory effect of Si in water was species-dependent: Lactuca sativa responded through increased secondary root formation, whereas Eruca sativa was highly sensitive in shoot biomass, with half-maximal effective concentration (EC50) values in the range of 8–12 mg Si L−1. In extracts of soils with low available Si, responses were species- and soil-dependent, and in soil pot bioassays, exogenous Si enhanced Eruca sativa biomass, consistent with the fraction of Si remaining bioavailable in the soil solution. These findings identify key factors governing Si bioavailability, illustrate how different soil–species combinations influence early Si responses, and provide a basis for the broader field studies required to confirm their agronomic relevance.