Because of their high content in toxic metals, steel slag dumps are potential threats for the environment and public health. Among management methods that could mitigate their hazard, aided-phytostabilization is a relevant, though challenging, option. Indeed, steel slags are very unfavorable for plant growth, due to metal toxicity and very alkaline pH (>10). In this work, we investigated how composted sewage sludge could alleviate slag's toxicity while improving its nutritional status. A pot experiment was performed to study biomass production and leaf ionome composition of five herbaceous species (Achillea millefolium, Bromus erectus, Festuca arundinacea, Melilotus officinalis and Medicago sativa), in relation to soil pore water's pH, concentration of trace and major elements and their chemical speciation. Results showed that pH had a clear-cut effect on plant development. Above pH 8.6, plant biomass was severely affected, due to accumulation of Cr above toxic threshold and deficiencies in Mn, Zn and P. Below pH 8.6, biomass increased significantly, together with a decrease in leaf Cr below toxic level, and an increase in Mn, Zn and P above deficiency levels. Thus, these results bring new insights into the causes of slag phytotoxicity and allow considering aided-pytostabilization as a realistic and efficient approach for the remediation of steel slag dumps, provided soil pH is carefully monitored before seeding.