What is the difference between soil pH and buffer pH?

Cristie Preston, Ph.D.

The soil pH is a measure of the active acidity or the hydrogen ion concentration in solution. Depending on the laboratory used, the soil pH solution can be distilled water or a low salt concentration solution like 0.01 M CaCl2 or 1 M KCl. In general, the soil pH tells a farmer if they have an acidic (pH < 7) or alkaline (pH > 7) soil and, ultimately, if they need to lime.

The buffer pH is a measure of the residual or reserve soil acidity — the soil acidity that is neutralized by lime in order to raise the pH. In general, the change in buffer pH determines how much lime is needed to change the pH to the desired level (based on the crops being grown). The more reserve acidity, the lower the buffer index and the more lime required to increase the soil pH.