Silicon (Si) in plants confers a number of benefits, including resistance to herbivores and water or nutrient stress. However, the dynamics of Si during long-term ecosystem development remain poorly documented, especially the changes in soils in terms of plant availability. We studied a 2-million-year soil chronosequence to examine how long-term changes in soil properties influence soil Si pools. The chronosequence exhibits extreme mineralogical changes-from carbonate-rich to quartz-rich soils-where a carbonate weathering domain is succeeded by a silicate weathering domain. Plant-available Si concentrations were lowest in young soils (Holocene, <6.5 ka), increased in intermediate soils (Middle Pleistocene, 120 ka), and finally decreased toward the oldest, quartz-rich soil (Early Pleistocene, 2 Ma). Silicon availability is likely low and relatively constant in the young soils because (1) carbonate weathering consumes protons and therefore reduces weathering of silicate minerals and (2) Si adsorption by secondary minerals is high in alkaline soils. In the middle-aged sites, Si availability rises with the loss of carbonates and the formation of kaolinite that appears to drive its concentration, and then falls in the oldest sites with quartz enrichment. The increasing accumulation of biogenic silica following carbonate depletion indicates stronger soil-plant Si cycling as ecosystem development proceeds. A literature analysis confirms the shift in processes controlling Si availability between the carbonate and silicate weathering domains. Overall, our results show a nonlinear response of plant-available Si to long-term pedogenesis, with likely important implications for the Si-related functioning of terrestrial ecosystems.