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Photo Credit: Lane Greer

A small unit like this one can be used to test the pH of water and other solutions.

There are a lot of questions about soil pH. The main one I hear is, “Why does it have a little ‘p’ and a big ‘H’?” (My computer has finally stopped automatically “correcting” this.) I also get, “As a gardener, why should I care about soil pH?” Or, “When you see a bottle of shampoo with ‘pH balanced’ on the label, what does that mean?” Here’s my attempt to answer those questions (and more).

“pH” stands for potenz Hydrogen, a phrase meaning the “power of hydrogen” or the “potential of hydrogen,” and it’s just easier to abbreviate it as pH. H is capitalized because the chemical name abbreviation for hydrogen is H, as in H₂O for water.

pH is essentially a measure of acidity, and it uses a scale ranging from 0-14. Neutral pH, which is neither acid nor basic, is 7.0. A pH below 7.0 is considered acid, with lower numbers being more acidic (for example, 3 is more acidic than 6). A pH above 7.0 is basic (or alkaline), and 13 is far more basic than 10. The scale is logarithmic, meaning that a pH of 5.0 is 10 times more acidic than a pH of 6.0 – not just 1 time more acidic. (This could also be interpreted to mean that pH 6.0 is 10 times more basic than 5.0, even though both are considered acidic.)

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Photo Credit: Pennsylvania State University

Soil pH affects nutrient availability. Most nutrients are available when pH is between 5.5 and 6.5.

Most soils in the world range from pH 3.0-10.0. And most soils in the US range from 5.0-8.0. A soil pH of 4.5 would be considered very low, but this is typical for peat moss, the most common ingredient in potting mixes.

So why is pH important? All plants have an optimal pH range, with the most common being about 5.5-6.5. The soil pH affects the nutrient availability and form, which is vital to plants. In other words, if you’re constantly having trouble with plants not growing or thriving, don’t blame your lack of a “green thumb” – your soil pH may actually be responsible. It may be impossible for nutrients to reach your plant’s roots because they’re tied up by the soil.