The pH of a solution is a measurement of hydrogen ion concentration, which indicates levels of acidity or alkalinity. Soils can have both active acidity and reserve acidity. The reserve acidity determines how much lime must be applied to raise pH.
Active acidity is the hydrogen ion concentration in soil water or moisture; these hydrogen ions are in solution. Reserve acidity refers to hydrogen ions bound up in soil particles and not dissolved in solution. The sites on soil particles that hold the hydrogen ions and other positively charged ions are called cation exchange sites.
When labs measure soil pH, they are measuring active acidity. Reserve acidity, however, is the main source of active acidity, so to raise soil pH it's ultimately necessary not only to neutralize active acidity but to neutralize some of the reserve acidity as well. Consequently, while active acidity determines whether soil pH needs to be raised, reserve acidity determines how much lime is needed to raise it.
Testing the pH buffer capacity of the soil can determine the reserve acidity, which varies depending on the kind of soil. Soils with a high clay content or high organic matter content generally have higher reserve acidity, so raising pH in these kinds of soils may require more lime than in other types of soil.