Calcium Carbonate Reduces the Effectiveness of Soil-Added Monosilicic Acid in Cucumber Plants

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 (CaCO 3 ), 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 CaCO 3 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 CaCO 3 -dependent. In the –CaCO 3 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 CaCO 3 -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 CaCO 3 -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 CaCO 3 excess.