What's up with pH?

By Paul James

At some point, all gardeners hear the term pH, perhaps most frequently when they’re trying to change the color of their hydrangeas from pink to blue or vice versa, because the only way to do that is by changing the pH of the soil. But what the heck is pH, anyway?

Without getting too technical, pH refers to the relative acidity or alkalinity of a substance and is measured on a scale of 0 to 14. Anything with a pH less than 7.0 is considered acidic, whereas anything with a pH above 7.0 is considered alkaline, and a substance with a pH of 7.0 is considered neutral. To understand the scale in more familiar terms, here are the pH values of some common household items.

pH Chart_v2-01.jpg

The ideal soil pH for the vast majority of plants is close to or just below neutral, with 6.5 to 6.8 being the sweet spot. Exceptions include so-called acid-loving plants such as pin oaks, azaleas, blueberries, and gardenias, all of which prefer a pH of slightly above or below 5.0. There are also plants that do well in alkaline soils, including burr oak, barberry, Spirea, junipers, Viburnums, yews, geraniums, yarrow, daylilies, and honeysuckle, just to name a few.

If a plant isn’t growing in a soil with the ideal pH, its roots may not be able to take up important nutrients, even if the soil is loaded with nutrients, and that can lead to everything from a lack of vigor to discoloring of leaves to the outright death of the plant. A pH that is either too low or too high can also cause a toxic buildup of certain elements, including manganese in low pH soils and aluminum in high pH soils.

Thankfully, raising or lowering pH is pretty simple. To make soils more acidic (to lower the pH) you add sulfur, and to make soils more alkaline (to raise the pH) you add limestone, preferably in the form of dolomitic limestone. Neither is soluble in water, so it’ll take at least a couple of weeks for them to have an effect on your soil’s pH.

And just how do you determine the pH of your soil? The most accurate test is one done by a lab, which technicians at OSU will perform for $10. You’ll also get measurements of the amount of nitrogen, phosphorous, and potassium in your soil. Just take a soil sample to a county extension office and they’ll mail it to the lab. (Click here to learn how to properly prepare a sample.) You can also buy inexpensive kits, which are nowhere near as accurate, but will at least let you know if you soil is within an acceptable range.

I suppose I could have started this blog by explaining that pH is the negative of the base 10 logarithm of the hydrogen ion activity at 25-degrees Celsius and measured in moles per liter. But then, I actually wanted you to read beyond the first paragraph.

And by the way, to change the color of your hydrangea flowers from pink to blue, you need to acidify the soil. To change them from blue to pink, you need to make the soil more alkaline. If you prefer a mix of pink and blue, shoot for a neutral pH.