Mineral dissolution is necessary for enhanced rock weathering (ERW), but it does not determine when weathering products become measurable beyond the soil profile. This study defines alkalinity breakthrough as the point at which weathering-derived base charge exceeds internal soil interception by proton demand, exchange storage, and other sinks sufficiently for export to emerge. This transition was evaluated using a daily charge-conserving model across four Ghanaian climates, four tropical soil archetypes, five feedstocks, three hydrologic structures, and 2,400 Latin hypercube simulations. After 20 years, median weathering reached 84.1% of the applied charge, but median export across the 1 m accounting boundary was only 0.0023%; 48.8% of simulations exported no charge, and 91.6% failed to reach 10% breakthrough. This divergence was captured by a dimensionless barrier ratio relating initial proton demand plus exchange headroom to the applied charge. The median barrier ratio was 2.27, and no simulation above 0.865 reached 10% breakthrough. The boundary changed little across a fourfold range of application rates or under an alternative charge-allocation structure but shifted over longer horizons as exchange-stored charge was released. Acid capacity, exchange capacity, and weathering rate were the dominant controls on breakthrough, while hydrologic structure modified export without overcoming large chemical barriers. Where export was measurable, feedstock rankings matched weathering-based rankings in all 342 resolved pairwise comparisons; however, 75% of matched contexts contained at least one feedstock pair with zero export, eliminating material discrimination under high-barrier conditions. The soil system therefore behaves as a chemical fill-and-spill system: mineral dissolution can proceed extensively while weathering-derived charge remains internally buffered. Alkalinity breakthrough provides a mechanistic diagnostic for determining when mineral weathering becomes measurable as CDR-relevant export.