This study aimed to determine the capacity of monosilicic acid [Si(OH)₄] to mitigate nutritional and metabolic disorders in plants induced by high levels of soil calcium carbonate (CaCO₃), which raises soil pH and lowers solubility of soil micronutrients. Plants (Cucumis sativus L.) were grown in a soil with increasing rates of Si(OH)₄ at two CaCO₃ doses. Biometrical parameters, leaf chlorophyll and leaf and stem nutrients as well as leaf metabolic responses, using a metabolomics approach, were evaluated. Additionally, extractable Si and Si plant availability were examined. Manifestations of Si-induced effects in cucumber were highly CaCO₃-dependent. In the –CaCO₃ plants, Si(OH)₄ addition induced distinct plant-beneficial effects, such as enhanced transport of iron (Fe) and manganese (Mn) to stems and/or leaves, accompanied with enhanced leaf abundance of metabolites (α-tocopherol, galactinol, threonic and ferulic acids) potentially involved in plant defence mechanisms against diverse environmental stresses. Biostimulant activity of Si(OH)₄ was not evident in plants grown in the CaCO₃-treated soil, characterized by reduced extractability and bioavailability of Si, compared with the untreated soils. The low physiological effectiveness of Si(OH)₄ on the CaCO₃-treated soil is likely due to a significant decrease in plant availability of Si, against a background of drastic plant performance impairment at high pH values induced by CaCO₃ excess.