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Soil pH; having the right number is important for your plants


Question. My soil (in the Poconos by the Tannersville Cranberry Bog) is, to say the least, acidic. This year, I switched from using lime to raise the pH to Dolomitic Limestone. The bag reads, "For professional use only," but no indication of how much to use. So I attempted a search on your website for "acid soil," "alkaline soil," "pH," "lime," and "dolomitic limestone," but found nothing. How about you revisit the basics for us?

---Ken in Bartonsville, PA

Answer. Ken is right that there hadn't been entries under 'acid' or 'alkaline' in our A to Z archives, and this little missive will correct that. But there are links under 'lime' and 'pH', both of which lead to an excellent article on using wood ashes—which are highly alkaline—to adjust an acidic soil. But before we start spreading ashes, let's go over those requested basics.

pH (small p, capital H, both letters in italics) is the symbol for an internationally recognized standard of judging the relative acidity or alkalinity of a substance. (It is universally agreed that the H stands for the element Hydrogen, and generally agreed that the little p stands for some form of power, as in the relative power of the attendant Hydrogen; take THAT, science geeks!)

pH is measured on a scale of 1 to 14; numbers below 7 are acidic; numbers higher than 7 are alkaline (or 'basic'). As in craps, 7 is the perfect number, often referred to in agriculture as 'neutral' (but never referred to as neutral in craps).

Most garden plants do best in soil whose pH is neutral to slightly acidic—the famous 6.5 to 7 range. But some plants require a highly acidic soil to thrive, including blueberries (probably the biggest acid heads at a range of 4 to 5), and azaleas, rhododendrons and gardenias (between 5 and 6). To put these numbers in perspective, a soil pH of 4 is so acidic you would feel the burn if you stuck a cut finger in it. Some naturally occurring peat bogs are this low, as are the peat-rich soils that support native blueberries.

Fewer plants require an alkaline soil. One classic example is boxwood, which suffers if the pH drops below 7, and prefers it around 7.5, which often blindsides gardeners, as almost all other evergreens want an acidic soil. Following the oft-seen advice to "give all evergreens an acidic soil" could send your boxwood off to its final reward. (See this full story on boxwoods for more info on their confusing care.)

There are many ways to test the pH of your soil. Old time farmers would simply taste it, knowing that 'sweet soil' was alkaline and bitter soil was acidic. (The possibility of ingesting parasites, however, makes this an unwise practice.) Many state agricultural extension programs offer soil testing services, often at very low cost. And some of these labs accept samples from other states—a huge help if your state doesn't have a lab or if their tests are way more expensive.

There are also devices, test strips and such you can use to measure your soil's pH at home, including "Universal indicator paper", whose color readouts match the official international standards, where a different hue has been assigned for every pH number from red (for the super-acidic 1) to a bluish-purple for the extremely alkaline end. There's also a really cool science project detailed online that shows you how to use the juice of a red cabbage to measure pH.

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