Particle size and calcium carbonate equivalent characterize agricultural limestone. When agricultural limestone is used for horticultural container substrates, variations in pH among batches of substrate produced with the same limestone rate, as well as pH drift from initial pH, occur. Horticultural endeavors require more detailed characterization of limestone neutralization capacity than is provided for agricultural limestones. Reactivity of twenty limestones encompassing a broad range of calcium and magnesium carbonate were compared. Limestones were wet‐sieved into three particle‐diameter fractions and reacted with ammonium chloride in the presence of steam. Fine particles reacted faster than coarse; however, there was a point beyond which degree of fineness of calcitic limestone no longer affected reactivity. Dolomitic limestones were slower reacting than calcitic. Reactivity within particle fractions of dolomitic and coarse calcitic limestones varied significantly. These results indicate that particle diameter and calcium carbonate equivalent are not the only factors influencing limestone neutralization capacity.