Soil pH – What Does it Mean?

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Lane Greer

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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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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.

Facts

Pure rain water has a pH of 7. However, very little pure rain is found in industrialized countries because of the higher levels of carbon dioxide present in the air. Commonly called “acid rain,” it has a pH of 5.5-6.0. Since few of us catch rainwater, the more important question is where do we get our water after it enters the aboveground or belowground river/reservoir system? The United States Environmental Protection Agency recommends that drinking water should be between pH 6.5 and 8.5. So almost all urban water has a pH around neutral or higher.
Old farmers I know often refer to acid soils as “sweet,” probably because most crops prefer slightly acid soil.

Tips

In addition to testing the amount of nutrients found in your soil, it’s also a good idea to conduct a percolation test to evaluate soil texture and determine how quickly your soil drains.
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Faqs

Q: What’s the pH of some other common items?
A: Battery acid has a 0 pH, lemon juice registers a 2.5, saliva comes in at 6, baking soda at 9, bleach at 13 and liquid drain cleaner at 14.

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