The bicarbonate ion has been implicated as a causative agent in iron chlorosis induced by high moisture in calcareous soils. Our studies have demonstrated a correlation between soil solution bicarbonate and chlorosis in soybeans in fields in western Minnesota and under growth chamber conditions. In the growth chamber studies, we found that the severity of chlorosis increases with increasing soil moisture to near the saturated water content. At high moisture, soil pores were filled with water and the partial pressure of CO2in the soil air increased. Since the soil pH is strongly buffered by the high cation exchange capacity of the soils, soil pH remained constant and [HCO‐3] increased. In a system in which CaC03is in equilibrium with the solution phase, the [HCO‐3] should be controlled by CaCO3solubility. In soils, however, the soil solutions were as much as 20‐fold over‐saturated with respect to CaCO3solubility and [HCO‐3] was much greater than predicted by solubility calculations.