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).
The flowers of bigleaf hydrangea (Hydrangea macrophylla) often show variation in color depending upon the soil pH: pink if neutral/alkaline and blue if acidic.
Photo Credit: Felder Rushing
Soil pH affects nutrient availability. Most nutrients are available when pH is between 5.5 and 6.5.
Photo Credit: Pennsylvania State University
A small unit like this one can be used to test the pH of water and other solutions.
Photo Credit: Lane Greer
“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.)
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.
How can you “fix” soil pH? First, have your soil tested to find out what your pH really is. Some garden retailers sell at-home test kits, or you can have your soil tested through your local Cooperative Extension service, which will process a soil sample (taken by you), then send you the results.
Let’s say that you have your soil report, and your pH is low. To raise your soil’s pH, apply dolomitic limestone, available at any garden center or home-supply store. While this isn’t an easy process (you may have to apply hundreds of pounds of limestone – usually shortened to lime – over a period of several years), but it’s worth it since you’ll be able to grow more plants with less hassle. And that will save you lots of time and aggravation over the years.
If your soil pH is high and you want to bring it down (make it more acidic), you need to apply sulfur. Again, this process could take years.
Another way to change your soil pH is through fertilizer application. You may have seen Holly-tone® and other fertilizers recommended for acid-loving plants. Although these products won’t change your soil pH over the long term, they’re a good short-term approach that can help your plants look better quickly.
I hope this explanation has helped clear up some questions about pH.
And before I forget, I want to answer the question about “pH balanced” shampoo. That’s one phrase that really perplexed me when I first saw it in the grocery store. I had to do some Internet searching to get the scoop on this one. The expression should probably be “pH balanced for hair as determined by the shampoo industry,” which has done lots of research and found that shampoo with a pH of 4.5-5.5 is best for hair; shampoo with pH outside that range makes hair look dull or damaged. (Well, what do you know? You really do learn something new every day!)