When looking at my soil test, what is the difference between soil pH and buffer pH?

Cristie Preston, Ph.D.

Cristie Preston

Cristie Preston, Ph.D.


Senior Agronomist

Agriculture has always been an integral part of Dr. Cristie Preston’s life. She grew up in southwest Virginia and had interest in crop and animal agriculture since an early age. Once she began college, she initially chose to study animal science but switched to soil science. Dr. Preston attributes her decision partly to an influential professor who told her, “You can’t understand animals until you understand what they eat.” She received a Bachelor of Science degree in animal science and a Master of Science degree in crop and soil environmental science from Virginia Tech. Dr. Preston holds a Ph.D. in agronomy, focusing on soil fertility from Kansas State University. While completing her advanced degrees, Dr. Preston conducted more than six years of field and lab research. Dr. Preston has experience in laboratory research measuring volatility loss from urea-based fertilizers. Her field research has focused on phosphorus availability and the interactions with tillage and placement. She also has extensive experience in working with large data sets and analysis. Her main priority is helping growers to identify yield-limiting factors and fix those issues as cost efficiently as possible.

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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.

Soil vs Buffer pH comparision

Cristie Preston, Ph.D.
Nutrien | Senior Agronomist