Vertisols are clayey, shrink-swell soils that are widely recognized in the rock record, thus generating the need to better understand the dynamics of elemental concentrations on the development of weathering indexes for climate interpretations. We assessed the weathering performance of the four major base-forming oxides (CaO, MgO, Na2O, K2O) along a modern Vertisol climosequence spanning a strong precipitation gradient, and discovered that the concentration of bulk soil CaO and MgO yields the strongest correlation to mean annual precipitation (MAP). Based on this finding, we introduce the CALMAG weathering index, defined as Al2O3 /(Al2O3 + CaO + MgO) × 100, which improves rainfall estimates for Vertisols relative to the well-established CIA-K (chemical index of alteration minus potassium) weathering index. Rather than documenting the hydrolysis of weatherable minerals common in many other soil orders, in Vertisols CALMAG principally tracks the flux of calcium and magnesium sourced from calcium carbonate, detrital clay, and exchangeable Ca2+ and Mg2+. Application to two Mesozoic paleosols reveals that in drier climates CIA-K yields higher MAP estimates than CALMAG, but that the reverse is true in wetter climates. This work improves paleorainfall estimates from Vertisols and suggests that a family of weathering indexes is needed for different paleosol types.