Soil is a continuum in geologic time. Like today, pedogenesis in the geologic past altered soils by transformations, translocations, additions, and losses as continents drifted and collided across global climatic zones. One of the major biogeochemical systems operating in soils across geologic time is the carbonic acid system, which includes gaseous CO_2(g), dissolved CO_2(aq), carbonic acid H₂CO₃(aq), bicarbonate HCO₃^- _(aq), carbonate CO₃ ^2- _(aq), and solid-phase calcium carbonate (mainly calcite). These chemical species, collectively referred to as inorganic carbon, increased in the Silurian when vascular plants colonized the continents and began pumping atmospheric CO₂ into soils via photosynthesis and root respiration, which accelerated mineral weathering and made soils “generators” of bicarbonate. In humid regions soils are “flushing” and bicarbonate enters groundwater, streams, and is transported to the oceans. In arid regions soils are “non-flushing” and bicarbonate is not linked to the hydrological system, but precipitates in soil as pedogenic carbonate, making arid soils sinks of inorganic carbon. Pedogenic carbonate is documented in paleosols throughout the Cenozoic, Mesozoic, and back into the Silurian of mid-Paleozoic time. Paleogeographic maps from Present to Precambrian are useful for revealing where arid regions were located in geologic time and how they expanded and contracted; thus, propounding the question of how (and if) a currently expanding “Aridosphere” will impact the global carbon cycle.