Fly bioash (FBA) is a complex nutrient-rich and very alkaline (pH>12) co-product of biomass-fuelled plants. In organic farming under field conditions, characterized by a very acid (pHKCl 4.0) siltic Luvisol, a two-vegetationseason trial (March 2021-August 2022) was performed to study the influence of increasing FBA levels (0.0-17.2 t/ha) on: i) physicochemical pedovariables, ii) vegetative and iii) mineral performances of millet (Panicum miliaceum L.) and sunflower (Helianthus annuus L.). The scanning electron microscope - SEM coupled with an energy dispersive spectrometer - EDS footprint of the tested FBA confirmed a complex mixture of nano-micro particles, rich in Ca, Mg, P, K and Si, different in sizes/shapes and developed surface, while the Fouriertransform infrared spectroscopy - FTIR with attenuated total reflectance - ATR spectra of FBA reflected characteristic bands rich in alkali-activated oxides. Conditioning of Luvisol with FBA increased soil pH (by up to 3.6 units) and nutrients (bioavailable P and K, total Ca, Fe, Mo and Mn), without compromising soil from (in)organic (metals, PAHs, PCBs) contamination. Although soil electrical conductivity increased notably after the first vegetation (from 0.081 to 0.288 mS/cm), this trend was not observed after the second vegetation, suggesting salt leaching. Furthermore, FBA significantly enhanced the grain yield and shoot dry matter in both test plants, and root dry matter in millet, accompanied by increased concentration and accumulation of certain nutrients (Ca, Mg, Zn, Fe, Mo) in tissues. Correlation-based network analysis revealed numerous significant soil-plant interactions under FBA application, likely driven by alterations in soil pH. Further research is necessary to elucidate the underlying mechanisms governing these relationships and their long-term implications.
