Soil pH measures the active acidity, while the buffer pH indicates the potential acidity. The amount of potential acidity for any given soil pH will depend upon the amount and type of clay and the level of organic matter in that soil. Therefore, it is possible to have two soils with the same soil pH but with different buffer pH's. A lower buffer pH represents a larger amount of potential acidity and thus more limestone is needed to increase the soil pH to a given level
 
Two buffer test that most labs use.

SMP Buffer Test (pHSMP ).
This test measures the total soluble and exchangeable hydrogen and aluminum. It is reliable for soils with a greater than 1 Ton/acre lime requirement and it is also well adapted for acid soils with a pH below 5.8 containing less than 10% organic matter and having appreciable amounts of aluminum. If the soil pH is greater than 6.5, the SMP buffer test is not made, since lime is not needed for most crops.

Adams-Evans Buffer Test
This buffer method is primarily an adaptation of the SMP buffer, but it is specifically designed for low organic matter, sandy soils where amounts of lime are needed in small quantities and the possibility of over-liming exists. The chemistry of the Adams-Evans buffer solution works in the same manner as the SMP buffer solution. The pH of the Adams-Evans buffer solution is 8.0. When the buffer solution is added to an acid soil, the original pH of the buffer will be lowered. Since it is known how much acid is required to lower the buffer solution pH to any given level, the total acidity of the soil can be determined.

The buffer pH is the sample pH after the laboratory has added a liming material. The laboratory adds the buffering solution, which acts like an extremely fast-acting lime. Each soil sample receives the same amount of buffering solution; therefore the resulting pH is different for each sample.

Too determine a lime recommendation, the laboratory looks at the difference between the original soil pH and the ending pH after the buffering solution has reacted with the soil. If the difference between the two pH measurements is large, it means that the soil pH is easily changed, and a low rate of lime will be sufficient. If the soil pH changes only a little after the buffering solution has reacted, it means that the soil pH is difficult to change and a larger lime addition is needed to reach the desired pH for the crop.

The reasons that a soil may require differing amounts of lime to change the soil pH relates to the soil CEC and the "reserve" acidity that is contained by the soil. Soil acidity is controlled by the amount of hydrogen (H+) and the aluminum (Al+++) that is either contained in, or generated by the soil and soil components. Soils with a high CEC have a greater capacity to contain or generate these sources of acidity. Therefore, at a given soil pH, a soil with a higher CEC (thus a lower buffered pH) will normally require more lime to reach a given target pH than a soil with a lower CEC.