There are two kind of acidity which are important for soil testing. There is the "active" acidity which is the acidity which the roots "see". The active acidity is measured with litmus paper, indicator solutions or a pH electrode. This is the acidit y people refer to when they want to know their soil pH value. The other kind of acidity is referred to as the reserve or stored acidity. Clay and organic matter in the soil tends to store acidity. It takes more lime to raise the pH value of a soil with clay or organic matter present than it would for a sandy soil with very little clay or organic matter. Therefore, soils with more clay or organic matter have a greater ability to resist pH changes.
The buffer pH is measured with a weak base that starts out at pH 8.00. Soils that will require lime and have an "active" acidity pH value of 5.8 or below are set aside in the lab to have this solution added. The more the solution decreases from pH 8. 00, the more stored acidity the soil sample has. The computer program will then recommend lime according to the determined stored acidity of the soil.
As an example, a sandy soil from the central part of South Carolina and a a soil from the Piedmont with plenty of clay each have a pH value of 5.5. This would be the "active" acidity mentioned earlier. The laboratory determination of the buffer pH va lue of the sandy soil, though, could be 7.80 while the buffer pH value of the soil with clay could easily be 7.30.
Since the sandy soil has decreased the buffer solution pH value of 8.00 less than the clay soil (7.80 vs. 7.30), the sandy soil will get a lower lime recommendation since it has a smaller amount of stored acidity. In this particular case, to bring these two soils up to a pH value of 6.5 the sandy soil will get a lime recommendation of 0.50 tons per acre while the lime recommendation for the clay soil will be 1.00 tons of lime per acre...twice as much lime, even though they both have the same pH value.