pH is used in two distinct contexts within our pages.
In a gardening context, pH refers to the measure of the acidity or alkalinity of the soil plants are grown in. The optimal pH range for most plants falls between 5.5 and 7.0, although many plants have adapted in ways which allow them to thrive at pH values outside of this range.
In food preservation involving the jarring of produce to reach a safe and shelf stable state, pH refers to the acidic level of the contents of a jar. In this context, if something has a pH of 4.6 or lower, it is considered high in acid and is safe for boiling water bath canning. If the pH is 4.7 or above, it is considered low in acid, in which case other preservation methods must be employed or the pH of the contents of the jar must somehow be lowered sufficiently. Fruits such as tomatoes, figs, asian pears, melons, papayas, persimmons, white peaches / nectarines, and bananas are often just slightly too low in acid content in their natural states to be safely canned. In such cases the pH can be lowered to a safe level by adding acid in the form of lemon juice, lime juice, or a powdered citric acid. Once the acid level is made high enough to inhibit botulism spores, the product is considered safe for boiling water bath canning.