A common question get at the lab is, “Why is the buffer pH on my soil test report blank?” When the soil pH is 6.8 or higher, the buffer pH will be blank on your ALGL soil test report, and for good reason. The reason stems from what the buffer pH value is used for.
On a ALGL soil test there are two pH columns, one titled “Soil pH” and one tilted “Buffer pH”. The soil pH is measured by combining equal parts soil and water to create a slurry. Then the pH of the slurry is measured with a pH meter. This value is used to make management decisions and indicates how the soil chemistry will be affected. For example, this is the value used when referring to soil pH restrictions on pesticides and is the pH value that is managed in soil fertility. If the soil pH is below the desired level, usually 0.2 - 0.3, the buffer pH is used to determine the amount of lime to apply to correct low soil pH soils. In high organic matter soils and soils with a CEC of 7.0 or less, other methods to determine lime application rates are used.
The highest desirable soil pH is 6.8. If a soil has a pH is 6.8 or higher there no need for a lime application to increase soil pH and therefore there is no need to determine a lime rate. Thus, no need for a buffer pH value to determine a lime rate. In GIS software data sets a value of 7.2 is often used to ensure the software calculates a “zero” lime rate without causing an error due to missing data.