SummaryThe way pH changes in soil are propagated by movement of acids and bases is described. In acid soils the H3O+‐H2O acid‐base pair is most important, while in alkaline soils the H2CO3‐HCO3−pair is always dominant, its effect depending directly on the pressure of CO2. In neutral and slightly acid soils, soluble organic matter and the H2PO4−‐HPO24−pair may also contribute.A soil acidity diffusion coefficient is derived, and defined as:wherevl= the volume fraction of the soil solution,fl= the impedance factor for the liquid diffusion pathway,bHS= the pH buffer capacity of the soil,bHB= the pH buffer capacity of each mobile acid‐base pair,DlHB= the diffusion coefficient of each mobile acid‐base pair in free solution,and the sum is taken over all mobile acid‐base pairs.The soil acidity diffusion coefficient may be used to predict the course of pH equilibration in practical situations. It is high in acid and alkaline soil, and at a minimum in slightly acid soil. It is little affected by variation of the ionic strength of the soil solution at concentrations less than 0.01M. When the pH buffer capacity of the soil is constant, and only the H3O+‐H2O and H2CO3‐HCO3−pairs are important, the soil acidity diffusion coefficient varies as cosh{2.303(pH—pH0)}, where pH0is the pH at which the soil‐acidity diffu